One pound is 0.45359237 kilograms, exactly. The hard part isn't the multiplication. It's knowing which factor to trust, how many decimals to keep, and why the divide-by-2.2 shortcut quietly fails at the exact moments precision matters.
To convert pounds to kilograms, multiply the pound value by 0.45359237. That factor is exact, not an approximation — it has been the legal definition of the international avoirdupois pound since the 1959 International Yard and Pound Agreement. So 150 lb × 0.45359237 = 68.0388555 kg, which rounds to 68.04 kg.
Going the other way, divide kilograms by 0.45359237 — or multiply by its reciprocal, 2.20462262. The two operations are not independent conversions; they are the same relationship read in opposite directions.
Where 0.45359237 comes from — and why it has eight decimals
The pound is not defined independently. Since 1959, it is defined by the kilogram: one international avoirdupois pound equals exactly 0.45359237 kg. That number is not a measurement with error bars. It is a definition, agreed by Australia, Canada, New Zealand, South Africa, the United Kingdom and the United States, and it is the value adopted in NIST Handbook 44 and Handbook 130 for legal-for-trade weighing in the US.
The chain of definitions runs like this:
- 1 kilogram is the SI base unit of mass (redefined in 2019 via the Planck constant, with no change to its magnitude).
- 1 pound = 0.45359237 kg exactly.
- 1 pound = 16 avoirdupois ounces, so 1 ounce = 28.349523125 g exactly.
- 1 pound = 7,000 grains, so 1 grain = 64.79891 mg exactly.
- 1 stone = 14 pounds = 6.35029318 kg exactly.
Because the pound is derived from the kilogram rather than the other way around, the pound-to-kilogram factor is a terminating decimal. The reverse direction is not: dividing 1 by 0.45359237 produces 2.204622621848776…, a repeating decimal that cannot be written out in full. That asymmetry is the source of most rounding confusion on this topic.
1 lb = 0.45359237 kg exactly (International Yard and Pound Agreement, 1959). 1 kg = 2.2046226218… lb (repeating). 1 oz = 28.349523125 g. 1 st = 6.35029318 kg.
One common mix-up is worth flagging early: the troy pound is a different unit, used for precious metals, equal to 373.2417216 g. A troy ounce is 31.1034768 g — about 10% heavier than the avoirdupois ounce on a food label. Every conversion on this page uses the avoirdupois pound unless troy is explicitly named.
One relationship, three formulas — and why they disagree
Most converter pages pick one formula and present it as the formula. In practice three versions circulate, and they give slightly different answers because each rounds a different part of the expression.
kg = lb ÷ 2.20462262
kg = lb ÷ 2.2
| Form | Rounding loss | 150 lb gives | Error vs exact |
|---|---|---|---|
| × 0.45359237 | None (exact definition) | 68.0388555 kg | 0 g |
| × 0.4536 | Factor cut at 4 dp | 68.0400 kg | +1.1 g |
| ÷ 2.20462262 | Divisor rounded at 8 dp | 68.0389 kg | < 0.1 g |
| ÷ 2.2 | Divisor rounded at 1 dp | 68.1818 kg | +143 g |
The difference between the first three rows is invisible on a bathroom scale. The fourth row is a 0.21% error, which matters in medicine, freight and anything with a regulatory threshold.
The key insight competitors miss: rounding the divisor is not the same kind of error as rounding the multiplier. Dividing by 2.20462262 and multiplying by 0.45359237 are mathematically identical operations — 1/2.20462262 is 0.45359237. But if you round the divisor to 2.2, you are no longer performing the inverse of the exact multiplication. The error is baked into the operation, not just the display.
If you want a single number to memorise, memorise 0.45359237. If you want a mental approximation, use 0.45 and accept a 0.8% shortfall, or use 0.454 and accept a 0.09% overshoot — both are more predictable than 2.2.
Four worked examples, one for each input shape
Pounds arrive in four different shapes in real life: a plain decimal, a pounds-and-ounces pair, a stone-and-pounds pair, and a kilogram figure you need to reverse. Each route needs a different first step before the factor applies.
Plain pounds — 180 lb to kg
180 lb
180 × 0.45359237 = 81.6466266. Round once, at the end: 81.65 kg to two decimals, 81.6 kg to one, 82 kg to the nearest whole number.
Answer: 81.65 kg
Pounds and ounces — 7 lb 8 oz to kg
7 lb 8 oz
Convert the ounces to a decimal fraction of a pound first: 8 ÷ 16 = 0.5. Total = 7.5 lb. Then 7.5 × 0.45359237 = 3.401942775. Round: 3.40 kg.
Answer: 3.40 kg
Stone and pounds — 11 st 4 lb to kg
11 st 4 lb
One stone is 14 lb. 11 × 14 = 154 lb, plus 4 = 158 lb. Then 158 × 0.45359237 = 71.66759446. Round: 71.67 kg.
Answer: 71.67 kg
Reverse direction — 70 kg to lb
70 kg
Divide by the factor, or multiply by the reciprocal: 70 ÷ 0.45359237 = 154.3235835. Round: 154.32 lb. Multiplying instead by 2.2 gives 154.00 lb — 0.32 lb light, about 147 g.
Answer: 154.32 lb
Notice that in all four routes the rounding happens once, after the full-precision multiplication. The next section explains why that matters more than it sounds.
The rounding rule that prevents most conversion errors
Regulatory guidance on unit conversion is unusually blunt about this. The Uniform Unit Pricing Regulation, reproduced in NIST Handbook 130, states that packagers should not round conversion factors or intermediate quantities, and should round only the final result — and do so in a single step.
Carry the full factor through every intermediate step. Round once, at the end, to the precision the use case requires. Never round a factor, never round a converted value and then convert it again.
Here is what goes wrong when you break it. Suppose you convert 3 lb 4 oz, and you decide to round the pounds to one decimal before applying the factor:
- Correct: 4 ÷ 16 = 0.25 → 3.25 lb → 3.25 × 0.45359237 = 1.474175 kg.
- Rounded first: 3.25 lb → 3.3 lb → 3.3 × 0.45359237 = 1.496855 kg.
- Difference: 22.7 g — enough to fail a 5-gram tolerance on a food label.
The error compounds when you round-trip. Convert 180 lb to kg and round to one decimal: 81.6 kg. Convert that back: 81.6 ÷ 0.45359237 = 179.90 lb. You have lost 0.1 lb — about 45 g — and running the round-trip three times can lose several hundred grams. In a spreadsheet this happens silently if you format the intermediate cell to zero decimals; the displayed value is 82, but the stored value is still 81.6466266, which is why formatted cells and exported CSVs often disagree.
Number formatting does not change the underlying value; the ROUND() function does. If you write =ROUND(A2*0.45359237,1) and then convert back, you have introduced a real rounding error. If you only change the cell's display format, the stored value stays exact and the round-trip is clean. Know which one you are doing.
How many decimals each job actually needs
There is no single correct precision. The right number of decimals is set by what happens to the number next. The table below is the decision rule most converter pages never give.
| Use case | Round final answer to | Why |
|---|---|---|
| Body weight, personal tracking | 0.1 kg | Bathroom scales vary by ±0.2 kg anyway |
| Fitness ratios (W/kg, g/kg protein) | 0.01 kg | The ratio amplifies the rounding error |
| Clinical dosing (mg/kg) | 0.001 kg, or work in grams | Narrow-therapeutic-index drugs tolerate <1% error |
| Air freight chargeable weight | 0.5 kg | Carriers bill in half-kilo increments |
| Customs declaration | Form's printed precision | Declaring more precision than the form allows creates mismatches |
| Cooking, ingredient yield | 5 g (0.005 kg) | Below the resolution of a kitchen scale |
| Engineering, lab work | 4 significant figures | Matches typical instrument tolerance |
The pattern: round to the precision of the next operation, not the precision of the input. A weight that feeds a ratio needs more decimals than a weight that feeds a display.
Where the ÷2.2 shortcut is fine — and where it quietly fails
Dividing pounds by 2.2 is the most-taught shortcut in weight conversion, and it is wrong in a predictable direction. Because 2.2 is smaller than the true divisor of 2.20462262, dividing by it produces a kilogram figure that is 0.21% too high. The reverse shortcut — multiplying kilograms by 2.2 — produces a pound figure that is 0.21% too low.
On a 150 lb person, that is 143 g. On a 200 lb person, 191 g. On a 44 lb child, 42 g. Whether those numbers matter depends entirely on what happens next.
| Context | ÷2.2 acceptable? | Reason |
|---|---|---|
| Gym conversation, rough body weight | Yes | Nobody cares about 150 g on a 68 kg human |
| Tracking a weight-loss trend | Yes | Consistent bias cancels out across measurements |
| Protein or calorie targets | Usually | Targets are ranges, not thresholds |
| Paediatric drug dosing | No | mg/kg error compounds by dose and by day |
| Customs / carrier weight brackets | No | A few grams can shift a shipment into a higher tariff band |
| Contrast-media dosing | No | Under-dosing wastes a scan; over-dosing risks nephropathy |
| Food label net weight | No | Tolerances are legally defined and audited |
| Alloy or chemical formulation | No | Stoichiometry needs the exact molar mass ratio |
The freight case is the one that catches people by surprise. Many carriers set a threshold at 30 kg, above which a parcel reclassifies from parcel to freight with materially different fees. A shipment of 66.1 lb converts to 29.98 kg exactly but 30.05 kg under the ÷2.2 shortcut. The shortcut moves the shipment across a billing boundary. This is not a hypothetical: it is the reason logistics software uses the full factor.
Stone and ounces: the two composite units that break simple converters
Most online converters accept a single number. That is a problem for two large groups of users: anyone in the UK or Ireland describing body weight in stone, and anyone converting a birth weight or a small parcel recorded in pounds and ounces.
Stone
One stone is 14 pounds, and therefore exactly 6.35029318 kg. UK body weight is conventionally written as “11 stone 4” or “11 st 4 lb,” meaning 11 × 14 + 4 = 158 lb. The stone is always singular after a number — “11 stone,” not “11 stones” — when used as a unit of body weight.
Ounces
One avoirdupois ounce is exactly 28.349523125 g, and 16 ounces make a pound. A birth weight of “7 lb 8 oz” is 7.5 lb, which is 3.40 kg. The average full-term newborn is about 7 lb, or 3.2 kg, which is why both units appear on the same hospital record.
| Input | First step | Exact kg | Rounded |
|---|---|---|---|
| 8 st 0 lb | 8 × 14 = 112 lb | 50.8023454 | 50.80 kg |
| 10 st 7 lb | (10 × 14) + 7 = 147 lb | 66.6780784 | 66.68 kg |
| 12 st 13 lb | (12 × 14) + 13 = 181 lb | 82.1002190 | 82.10 kg |
| 5 lb 8 oz | 8 ÷ 16 = 0.5 → 5.5 lb | 2.4947580 | 2.49 kg |
| 6 lb 13 oz | 13 ÷ 16 = 0.8125 → 6.8125 lb | 3.0900955 | 3.09 kg |
| 9 lb 2 oz | 2 ÷ 16 = 0.125 → 9.125 lb | 4.1390304 | 4.14 kg |
Ounces must be converted to a decimal fraction of a pound before applying the factor. Converting ounces to kilograms separately and adding them introduces a second rounding step and a second error.
Mass, weight, and the customs form that asks for both
Strictly speaking, the pound and the kilogram are units of mass, not weight. Mass is the quantity of matter; weight is the force that gravity exerts on that mass. A 68 kg astronaut has the same mass on the Moon but roughly one-sixth the weight.
In commerce and law, this distinction has practical consequences. Shipping documents separate net weight (the goods alone), gross weight (goods plus packaging) and tare weight (packaging alone). All three are masses. A customs declaration that lists “weight” in kilograms is asking for mass, and the exact pound-to-kilogram factor applies without any gravity correction.
Where the confusion becomes real is in engineering, where the pound-force (lbf) is a separate unit equal to 4.4482216153 newtons — the force exerted by standard gravity on one pound of mass. A spring scale calibrated in pounds-force reads pounds of mass correctly only at the gravity where it was calibrated. A balance scale, which compares masses directly, does not have this problem.
If the number goes on a label, a form, a recipe or a prescription, treat it as mass and use 0.45359237. If the number is an engineering force calculation, you are in pound-force territory and need 4.4482216153 N per lbf — a different conversion entirely.
Shipping: actual weight versus dimensional weight
This is the gap almost every lbs-to-kg page leaves open. Converting the weight on the box does not tell you what the carrier will charge you, because most carriers bill on the greater of actual weight and dimensional (volumetric) weight.
Dimensional weight is calculated from the package's volume divided by a density divisor. The divisor differs by unit system and by carrier:
| Carrier convention | Formula | Divisor |
|---|---|---|
| International, metric | L × W × H (cm) ÷ 5000 | 5,000 cm³/kg |
| International, imperial | L × W × H (in) ÷ 139 | 139 in³/lb |
| Some US domestic services | L × W × H (in) ÷ 166 | 166 in³/lb |
| Some EU domestic services | L × W × H (cm) ÷ 6000 | 6,000 cm³/kg |
The conversion problem is subtle. If you calculate dimensional weight in pounds and the carrier bills in kilograms, you must convert the result, not the input dimensions. And because 139 in³/lb and 5,000 cm³/kg are not exact reciprocals — 139 × 16.387064 ÷ 1000 ≈ 2.278, not 2.2046 — the two routes can produce slightly different chargeable weights. Carriers resolve this by specifying which unit system their tariff uses and requiring the shipper to use that system throughout.
A worked example: a box measuring 40 × 30 × 30 cm has a volume of 36,000 cm³. Divided by 5,000, that is 7.2 kg dimensional weight. If the actual mass is 2 kg, the carrier bills 7.2 kg. Converting the 2 kg actual weight to pounds is irrelevant to the charge — the dimensional figure governs.
When the carrier's threshold is expressed in kilograms and your scale reads pounds, convert the actual weight with the full factor before comparing it to the dimensional weight. Rounding either figure before the comparison can flip which one is larger, and therefore which one you are billed on.
Batch conversion in Excel and Google Sheets
Converting one value by hand is easy. Converting a column of 4,000 values correctly is where the rounding rule earns its keep. There are two valid approaches and one common mistake.
Approach 1 — multiply by the exact factor
In Excel or Google Sheets, with pounds in column A and kilograms in column B:
This is the most portable option: it works in every version of every spreadsheet, exports cleanly to CSV, and never depends on an add-in. Wrap it in ROUND() only if the next step genuinely needs a rounded value, and if so, round in the same cell that produces the final figure — not in an intermediate helper column.
Approach 2 — Excel's CONVERT function
Excel has a built-in unit converter, but the unit token is not “lb.” It is “lbm”, for pound-mass:
Using “lb” instead of “lbm” returns a #N/A error. Google Sheets does not support CONVERT at all, which is why the multiplication approach is safer if the sheet will be shared across platforms.
Formatting a column to zero decimal places does not round the values. The stored number remains 81.6466266 even when the cell displays 82. If that column is later exported, re-imported, or summed, the underlying precision reappears — and any totals computed from the displayed values will not match the totals computed from the stored values. Use ROUND() when you want a real rounded value; use cell formatting when you only want a cleaner display.
Price per pound to price per kilogram
Unit-price comparison is the most common reason people convert weight outside of body tracking, and it uses the reciprocal in a way that is easy to get backwards. If an item costs P dollars per pound, the price per kilogram is P × 2.20462262, because a kilogram contains 2.2046 pounds and therefore contains 2.2046 times as much product.
| Price per lb | Price per kg (exact factor) | Price per kg (using 2.2) |
|---|---|---|
| $1.00 | $2.2046 | $2.2000 |
| $2.50 | $5.5116 | $5.5000 |
| $3.50 | $7.7162 | $7.7000 |
| $5.99 | $13.2057 | $13.1780 |
| $12.00 | $26.4555 | $26.4000 |
The shortcut understates the kilogram price by about 0.21%, which is small in absolute terms but consistent in direction — it will always make the per-pound item look marginally cheaper than it is. For a grocery comparison across dozens of items, that bias is small enough to ignore. For contract pricing on bulk commodities, it is not.
Mental math, and exactly how far off it is
The most popular mental shortcut is “halve the pounds, then subtract 10%.” It works because 0.5 × 0.9 = 0.45, which is close to 0.45359237. The residual error is predictable: the result is 0.79% low, every time.
For 200 lb: 200 ÷ 2 = 100; 10% of 100 = 10; 100 − 10 = 90 kg. The exact answer is 90.72 kg, so the shortcut is 0.72 kg light.
| Pounds | Halve, minus 10% | Exact kg | Error |
|---|---|---|---|
| 100 lb | 45.0 | 45.359 | −0.36 kg |
| 150 lb | 67.5 | 68.039 | −0.54 kg |
| 200 lb | 90.0 | 90.718 | −0.72 kg |
| 250 lb | 112.5 | 113.398 | −0.90 kg |
| 300 lb | 135.0 | 136.078 | −1.08 kg |
Two corrections improve it. Using × 0.454 instead of the halve-and-subtract routine gives a result that is 0.09% high — about eight times more accurate than the 0.45 method. Or keep the halve-and-subtract result and add 0.8% back: for 200 lb, 90 × 1.008 = 90.72 kg, which is exact to two decimals.
Quick reference: pounds to kilograms
Values in the range people actually look up. The exact column is the product of the pound figure and 0.45359237; the rounded column is the same number cut to two decimals — the value most scales and forms display.
| Pounds | Kilograms (exact) | Kilograms (2 dp) | Typical context |
|---|---|---|---|
| 80 lb | 36.2873896 | 36.29 | Child, medium dog |
| 100 lb | 45.3592370 | 45.36 | Small adult, checked bag |
| 120 lb | 54.4310844 | 54.43 | Adult, light |
| 140 lb | 63.5029318 | 63.50 | Adult |
| 150 lb | 68.0388555 | 68.04 | Adult |
| 160 lb | 72.5747792 | 72.57 | Adult |
| 180 lb | 81.6466266 | 81.65 | Adult |
| 200 lb | 90.7184740 | 90.72 | Adult |
| 220 lb | 99.7903214 | 99.79 | Just under the 100 kg mark |
| 250 lb | 113.3980925 | 113.40 | Adult, heavy |
| 300 lb | 136.0777110 | 136.08 | Adult, heavy |
A useful landmark: 100 kg is 220.46 lb, not 220. The difference is 0.46 lb, or about 209 g — enough to matter when a 100 kg bodyweight class is on the line.
Kilograms to pounds
| Kilograms | Pounds (exact) | Pounds (2 dp) |
|---|---|---|
| 50 kg | 110.2311311 | 110.23 |
| 60 kg | 132.2773573 | 132.28 |
| 70 kg | 154.3235835 | 154.32 |
| 75 kg | 165.3466966 | 165.35 |
| 80 kg | 176.3698097 | 176.37 |
| 90 kg | 198.4160360 | 198.42 |
| 100 kg | 220.4622622 | 220.46 |
Questions people actually search for
Twelve answers, each self-contained. Tap to expand.
What is the exact factor to convert lbs to kg?
0.45359237. One international avoirdupois pound equals exactly 0.45359237 kilograms, as defined by the 1959 International Yard and Pound Agreement. This is the factor used by NIST and every national metrology institute. The reverse factor is 1 ÷ 0.45359237 = 2.2046226218… lb per kg, a repeating decimal.
Is 1 lb exactly 0.45 kg?
No. 0.45 is a rounded approximation that is 0.79% low. One pound is 0.45359237 kg exactly. The 0.45 figure is useful for mental arithmetic but drifts by 3.6 g per pound — about 540 g on a 150 lb person. Use 0.454 if you want a mental multiplier that is only 0.09% high.
How do I convert 150 lbs to kg?
150 × 0.45359237 = 68.0388555 kg, which rounds to 68.04 kg at two decimals or 68.0 kg at one. The ÷2.2 shortcut gives 68.18 kg, which is 143 g too high. The libs to kgs calculator returns the exact value and the substituted arithmetic in one step.
How do I convert kg back to lbs?
Divide by 0.45359237, or multiply by 2.20462262 — the two are the same operation. For 70 kg: 70 ÷ 0.45359237 = 154.3235835 lb, or 154.32 lb at two decimals. Multiplying by 2.2 instead gives 154.00 lb, which is 0.32 lb (147 g) low.
Why do different converters give different answers?
Because they round different parts of the expression. A converter that multiplies by 0.4536 loses 1.1 g on a 150 lb input. A converter that divides by 2.2 loses 143 g on the same input. A converter that uses 0.45359237 loses nothing. The factor, not the arithmetic, is the variable. If two tools disagree, check which factor each one publishes.
How many decimals should I round to?
Match the precision to the next operation, not the input. Two decimals (0.01 kg) is enough for body weight and fitness ratios. Three decimals (0.001 kg) or grams is safer for clinical dosing. Four significant figures is the floor for lab work. Round once, at the end — never round the factor or an intermediate value. The full decision table is in §05 above.
What is the difference between lb and lbs?
Nothing. lb is the correct abbreviation for both singular and plural in scientific and legal writing; lbs is an informal plural. Both derive from the Latin libra, which is also why the unit symbol is lb rather than p. You may also see lbm (pound-mass) in engineering contexts to distinguish it from lbf (pound-force).
Is a pound a unit of mass or weight?
Legally and practically, the avoirdupois pound is a unit of mass, the same quantity the kilogram measures. The pound-force (lbf) is the corresponding force unit, equal to 4.4482216153 newtons. For labels, recipes, prescriptions, customs forms and shipping, you are converting mass and the 0.45359237 factor applies without any gravity correction.
How do I convert stone and pounds to kg?
Multiply the stone value by 14, add the remaining pounds, then multiply by 0.45359237. For 11 st 4 lb: (11 × 14) + 4 = 158 lb, and 158 × 0.45359237 = 71.67 kg. One stone alone is 6.35029318 kg exactly. The libs to kgs converter has a dedicated stone + lb mode that performs both steps.
How do I convert pounds and ounces to kg?
Divide the ounces by 16 to get a decimal fraction of a pound, add the whole pounds, then multiply by 0.45359237. For 7 lb 8 oz: 8 ÷ 16 = 0.5, so 7.5 lb, and 7.5 × 0.45359237 = 3.40 kg. Convert the ounces before applying the factor; converting them separately to kilograms and adding introduces a second rounding error.
Does the conversion matter for shipping?
Yes, in two ways. First, carriers bill on the greater of actual weight and dimensional weight (volume ÷ 139 for in³/lb, or ÷ 5000 for cm³/kg), so converting the actual weight does not change the charge if the dimensional figure governs. Second, many carriers set kilogram thresholds — 30 kg is a common parcel-to-freight boundary — and the ÷2.2 shortcut can push a shipment across one. Use the full factor before comparing.
What is the fastest accurate way to convert without a calculator?
Multiply by 0.454. It is one operation, and the result is only 0.09% high — about 14 times more accurate than the halve-and-subtract-10% trick. If you prefer the halving method, add 0.8% back to the result to correct the built-in shortfall: for 200 lb, 90 kg × 1.008 = 90.72 kg, which is exact to two decimals.
Convert any value without the arithmetic
The calculator applies the full 0.45359237 factor, accepts plain pounds, pounds and ounces, kilograms, or stone and pounds, and shows the substituted working so you can check it by hand.
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