Find the right air conditioner size in BTU, tons or kW using the six factors that actually change the load: room size, climate, sunlight, insulation, ceiling height and occupancy.
- Room Size
- Climate Zone
- Sun Exposure
- Insulation
- Ceiling Height
- Occupancy
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Closest standard AC size
Why Room Size Matters for AC Sizing
Room size is the biggest single driver of AC capacity — but it is not the only one, and it is not a one-size-fits-all number. A 300 sq ft bedroom and a 300 sq ft kitchen need completely different BTU ratings. A 300 sq ft room in Phoenix needs more than the same room in Seattle. Here is what actually changes the requirement.
Floor area sets the baseline
Larger floor plan means more air to cool and more surface area gaining heat. Area × climate factor gives the starting BTU.
Base = Area × BTU/ft²
Height changes the volume
A room with 10 ft ceilings has 25% more air than the same floor with 8 ft ceilings. Volume, not floor area, is what the AC must cool.
Ceiling factor = Height ÷ 8
People add load
Each person above the first two adds about 600 BTU/hr of sensible and latent heat. A home office needs more than an empty bedroom.
+600 BTU per person
Sun drives peak load
A west-facing room with large unshaded windows can gain 10–25% more heat than a north-facing room of the same size.
Sun factor ±10–25%
Room-by-Room BTU Requirements (Moderate Climate)
The most-searched BTU sizing chart for moderate climates (mid-Atlantic US, Central Europe, coastal Australia). Adjust up or down using the climate factor in the calculator above.
| Room size | Moderate | Warm | Hot | Typical unit |
|---|---|---|---|---|
| 100 sq ft (10×10) | 4,000 BTU | 5,000 BTU | 6,000 BTU | Window / portable |
| 144 sq ft (12×12) | 5,000 BTU | 6,000 BTU | 8,000 BTU | Window / mini-split |
| 200 sq ft (14×14) | 6,000 BTU | 8,000 BTU | 10,000 BTU | Window / mini-split |
| 300 sq ft (17×17) | 9,000 BTU | 12,000 BTU | 14,000 BTU | Mini-split / window |
| 400 sq ft (20×20) | 12,000 BTU | 14,000 BTU | 18,000 BTU | Mini-split / central |
| 500 sq ft (22×22) | 14,000 BTU | 18,000 BTU | 21,000 BTU | Mini-split / central |
| 700 sq ft (open plan) | 18,000 BTU | 24,000 BTU | 30,000 BTU | Central / multi-zone |
| 1,000 sq ft (small home) | 24,000 BTU | 30,000 BTU | 36,000 BTU | Central AC (2–3 ton) |
| 1,500 sq ft (medium home) | 36,000 BTU | 42,000 BTU | 48,000 BTU | Central AC (3–4 ton) |
| 2,000 sq ft (large home) | 48,000 BTU | 54,000 BTU | 60,000 BTU | Central AC (4–5 ton) |
The common “20 BTU per square foot” figure assumes a moderate climate, average insulation, 8 ft ceilings and two occupants. In a hot climate with poor insulation and heavy occupancy, the real load can be double that. That is why this calculator uses six inputs instead of one.
BTU Per Square Foot Reference
BTU per square foot varies from about 20 in cool climates to 45 in very hot climates. This table gives the exact factor used by the calculator, plus real-world notes on when to adjust it up or down.
| Climate zone | BTU per sq ft | Example regions | Notes |
|---|---|---|---|
| Cool / mild | 20 | UK, Ireland, Pacific Northwest, Northern Europe | Short cooling season; oversized units short-cycle more |
| Moderate | 25 | Mid-Atlantic US, Central Europe, Victoria AU | Baseline for real-world loads |
| Warm | 30 | Southern US, Mediterranean, coastal Australia | Long cooling season; humidity control matters |
| Hot | 35 | Texas, Arizona, Middle East, Northern India | Peak load driven by outdoor design temperature |
| Very hot | 45 | Gulf states, Sahara, Central Australia | Design for 45°C+ / 113°F+ outdoor temperatures |
Climate and Sunlight: The Two Biggest Adjustments
Two identical rooms in Phoenix and Seattle do not need the same AC. Climate drives the baseline BTU per square foot, and sunlight drives the peak-hour adjustment on top of it. These are the two factors most often skipped by online calculators, and they matter more than any other single input.
How Sun Exposure Changes the Load
| Exposure | Adjustment | Typical scenario | Effect on sizing |
|---|---|---|---|
| Heavily shaded | −10% | North-facing room under mature trees | Smaller AC acceptable |
| Average | No change | Mixed or mild sun through the day | Use base BTU |
| Very sunny | +10% | Large east or south-facing windows | Step up one size |
| West-facing, unshaded | +20 to +25% | Afternoon sun on glass, no blinds | Step up one or two sizes |
Why West-Facing Rooms Are the Worst Case
West-facing glass receives peak solar gain in the late afternoon, exactly when outdoor air temperatures are at their highest. This overlap creates the worst-case cooling load of the day — around 4–6 PM in most climates. If your AC calculation is done at noon and sized only for that hour, the room will be too warm by evening. This is why the calculator applies a distinct sun multiplier on top of the base load.
BTU capacity is defined by AHRI Standard 210/240 (unitary air conditioners) and ANSI/ASHRAE Standard 55 (thermal comfort). One ton of cooling = 12,000 BTU/hr = 3.517 kW. Manual J (ACCA) is the industry standard for residential load calculation.
AC BTU Calculation Formulas
Every step this calculator performs uses one of the formulas below. You can reproduce the answer on paper by following the same sequence.
Base load
Floor area in sq ft times the climate BTU factor.
Base = Area × BTU/ft²
Sun-adjusted
Base BTU times the sun exposure multiplier.
After sun = Base × Sun
Insulation-adjusted
After-sun BTU times the insulation multiplier.
After ins = Sun × Insulation
Ceiling height
Multiply by ceiling height divided by 8 to get volume factor.
Ceiling factor = H ÷ 8
Occupancy load
600 BTU per person above the first two.
Occupancy = (People − 2) × 600
Kitchen add-on
Add 4,000 BTU if the room contains a kitchen.
+4,000 BTU if kitchen
Tonnage
Divide BTU by 12,000 to get US cooling tons.
Tons = BTU ÷ 12,000
Kilowatts
Divide BTU by 3,412.142 to get kW of cooling.
kW = BTU ÷ 3,412.142
BTU = ((Area × Climate) × Sun × Insulation × (Ceiling ÷ 8)) + ((People − 2) × 600) + Kitchen — then round to the nearest 100 and match to the nearest standard size.
How the AC BTU Calculator Works
Six inputs, applied in order. The base calculation starts with floor area and climate, then each adjustment scales the number up or down. Nothing is hidden — you can follow the arithmetic in the results panel.
Area × climate
Floor area in square feet or metres, multiplied by the climate BTU factor. This gives the base load.
Sun adjustment
Shade reduces load by 10%; full sun increases it by 10%. West-facing rooms can add up to 25%.
Insulation adjustment
Excellent insulation reduces load by 15%; poor insulation adds 15–30% depending on severity.
Ceiling height
Height divided by 8 gives the volume multiplier. A 10 ft ceiling adds 25%, a 12 ft ceiling adds 50%.
Occupancy load
Add 600 BTU/hr per person above the first two regular occupants. Kids count as half.
Kitchen add-on
If the room is a kitchen, add 4,000 BTU/hr for the cooking appliance and refrigerator load.
After all six adjustments, the calculator rounds the total to the nearest 100 BTU and matches it to the closest standard AC size (window, portable, mini-split or central). It also shows the next size up — because AC units are sold in fixed steps, and the right choice is often the next step rather than the exact calculated value.
AC Capacity Conversions
Air conditioner capacity is written three different ways depending on the market and the manufacturer. US manufacturers use BTU/hr and tons, European and Asian manufacturers use kW, and some legacy units use horsepower (HP). Here is how they relate.
| BTU/hr | Tons | kW (cooling) | HP (approx.) | Typical room |
|---|---|---|---|---|
| 5,000 | 0.42 | 1.47 | 0.5 | Small bedroom (100–150 sq ft) |
| 6,000 | 0.50 | 1.76 | 0.6 | Small bedroom (150–250 sq ft) |
| 8,000 | 0.67 | 2.34 | 0.8 | Bedroom / small office |
| 9,000 | 0.75 | 2.64 | 0.9 | Medium bedroom |
| 10,000 | 0.83 | 2.93 | 1.0 | Living room (350–450 sq ft) |
| 12,000 | 1.00 | 3.52 | 1.2 | Large living room (450–550 sq ft) |
| 14,000 | 1.17 | 4.10 | 1.4 | Open-plan living |
| 18,000 | 1.50 | 5.28 | 1.8 | Whole floor / small apartment |
| 24,000 | 2.00 | 7.03 | 2.4 | Small house |
| 30,000 | 2.50 | 8.79 | 3.0 | Medium house |
| 36,000 | 3.00 | 10.55 | 3.6 | Large house (1,500 sq ft) |
| 48,000 | 4.00 | 14.07 | 4.8 | Very large house (2,000 sq ft) |
| 60,000 | 5.00 | 17.58 | 6.0 | Whole-home central (2,500 sq ft) |
How to Read an AC Nameplate
Every AC unit has a nameplate or data sticker. Here is what the numbers mean:
| Field on nameplate | What it tells you | Example value |
|---|---|---|
| Cooling capacity | The BTU/hr or kW rating in cooling mode | 12,000 BTU/hr / 3.52 kW |
| Tonnage | BTU divided by 12,000 | 1 ton |
| SEER / SEER2 | Seasonal efficiency ratio (US) | 14–22 SEER2 |
| EER / EER2 | Energy efficiency ratio at peak conditions | 11–14 EER2 |
| HSPF / HSPF2 | Heating efficiency (heat pumps only) | 8–10 HSPF2 |
| Refrigerant type | R-410A (older) or R-32 / R-454B (newer) | R-32 |
| Power input | Electrical draw at rated conditions | 1,100 W |
Common AC Sizes
Standard unit sizes do not come in every possible BTU rating. They jump in fixed steps. This is what the market actually sells, sorted by unit type.
| BTU size | Type | Where it fits | Best for |
|---|---|---|---|
| Window / portable units | |||
| 5,000 BTU | Window / portable | Small bedroom, dorm room | Single room, mild climate |
| 6,000 BTU | Window / portable | Bedroom, home office | Single room, moderate climate |
| 8,000 BTU | Window / portable | Medium bedroom, small living room | Single room, warm climate |
| 10,000 BTU | Window / portable | Living room, small studio | Large single room |
| 12,000 BTU | Window / portable | Large living room, studio | Open space, hot climate |
| Mini-split / ductless systems | |||
| 9,000 BTU | Mini-split | Bedroom, small office | Single zone, quiet operation |
| 12,000 BTU | Mini-split | Living room, master bedroom | Single zone, standard room |
| 18,000 BTU | Mini-split | Open-plan living, large room | Single large zone |
| 24,000 BTU | Mini-split | Studio, small apartment | Open layout or whole floor |
| 36,000 BTU | Multi-zone mini-split | 2–3 rooms | Whole home, multiple zones |
| Central air conditioning | |||
| 24,000 BTU (2 ton) | Central AC | 1,000–1,200 sq ft home | Small house |
| 36,000 BTU (3 ton) | Central AC | 1,400–1,600 sq ft home | Medium house |
| 48,000 BTU (4 ton) | Central AC | 1,800–2,200 sq ft home | Large house |
| 60,000 BTU (5 ton) | Central AC | 2,300–2,800 sq ft home | Very large house |
Room Type BTU Guide
Standard sizing by room function, assuming a moderate climate, 8 ft ceiling and average insulation.
| Room type | Typical size | Base BTU | Adjustments to consider |
|---|---|---|---|
| Bedroom (master) | 200–300 sq ft | 6,000–9,000 | Add 600 BTU if two adults + baby |
| Bedroom (guest / kids) | 120–180 sq ft | 5,000–6,000 | Reduce if unused most days |
| Living room | 300–500 sq ft | 10,000–14,000 | West-facing glass: add 20% |
| Kitchen (open plan) | 200–400 sq ft | 8,000–14,000 | Always add 4,000 BTU for kitchen |
| Home office | 100–200 sq ft | 5,000–8,000 | Add 600 BTU per extra person; add 500 for equipment |
| Dining room | 150–250 sq ft | 6,000–9,000 | Add 600 BTU per extra diner during meals |
| Whole apartment / house | 800–2,000 sq ft | 24,000–48,000 | Consider multi-zone or central system |
Manual J vs Rule of Thumb
Two sizing methods are used in practice: a full Manual J load calculation (the ACCA industry standard) and rule-of-thumb estimates (20–25 BTU per square foot). This calculator sits between the two — more accurate than a one-line rule, far faster than Manual J.
| Method | Accuracy | Time required | Best used for |
|---|---|---|---|
| Rule of thumb | ±40–50% | 1 minute | First estimate, ballpark pricing |
| Multi-factor calculator (this tool) | ±10–15% | 2 minutes | Choosing unit size before quote |
| Manual J (ACCA) | ±5% | 1–4 hours | Final contractor sizing, code compliance |
For a residential replacement, most contractors will perform a Manual J before quoting. For choosing a window unit, portable AC or a first mini-split estimate, this calculator gives you a defensible size that is close enough to avoid both oversizing and undersizing.
What Happens If an AC Is Too Small?
An undersized unit does not cool the space, runs continuously and drives up the electricity bill. It is the less common mistake — but the one that shows up fastest on the thermostat.
Runs continuously
The compressor never rests. It runs at 100% from morning to night, wearing out faster and consuming far more electricity than a correctly sized unit.
Never reaches setpoint
On the hottest afternoons the room stays 3–5°F above the thermostat setting. The AC is fighting a load it cannot beat.
Higher energy bills
An undersized unit uses more kWh over a cooling season than a properly sized one, because it runs longer and harder to reach the same target.
Uneven temperature
Some corners stay warm while others are cool. The unit is throwing capacity at one spot and can never spread cool air evenly through the space.
What Happens If an AC Is Too Large?
This is the far more common mistake. It is harder to notice because the room does get cold — but the side effects are expensive and uncomfortable.
Short-cycling
The unit cools the room in minutes, shuts off, then triggers back on five minutes later. Start-stop-start-stop all day. Compressor starts are the main cause of premature failure.
Poor humidity control
The room drops to temperature before the coil has time to pull moisture out of the air. The thermostat says 72°F but the room feels clammy.
Wasted money upfront
You paid a premium for capacity you cannot use. A 24,000 BTU unit typically costs 30–40% more than an 18,000 BTU unit.
Shorter equipment life
Frequent compressor starts are the leading cause of premature AC failure. Oversized units often die 3–5 years before their rated lifespan.
SEER, EER and Efficiency Ratings Explained
Capacity and efficiency are two different things. Two ACs with the same BTU rating can have wildly different running costs. The efficiency numbers on the nameplate tell you how.
| Rating | What it measures | Typical range | What to look for |
|---|---|---|---|
| SEER / SEER2 | Seasonal efficiency over a whole cooling season | 14–22 (new units) | Higher is better. 15+ for moderate, 18+ for hot climates. |
| EER / EER2 | Efficiency at peak (95°F) conditions | 11–14 | More important in hot climates than SEER alone. |
| HSPF / HSPF2 | Heating efficiency (heat pumps only) | 8–10 | Only relevant if the unit also heats. |
For BTU sizing, SEER does not change the required capacity. A 1-ton unit at SEER 14 and a 1-ton unit at SEER 20 both deliver 12,000 BTU/hr. The difference is how much electricity each one uses to do it.
Signs Your AC Is the Wrong Size
Quick self-check list. If two or more of these apply, the unit is probably mismatched to the room.
| Symptom | Points to | Fix |
|---|---|---|
| Runs constantly on hot days, never shuts off | Undersized | Step up one size, or add a second unit |
| Shuts on and off every few minutes | Oversized | Step down one size |
| Room feels cold but clammy / sticky | Oversized (short cycling) | Smaller unit, or run in dehumidify mode |
| Some rooms cool, others stay warm | Undersized or wrong distribution | Add a second zone or upgrade air handler |
| Electric bill spikes on mild days | Oversized | Smaller unit, run shorter cooling cycles |
| Compressor died early (under 10 yrs) | Oversized short cycling | Replace with correctly sized unit |
People Also Ask
How many BTU do I need for a 500 sq ft room?
A 500 sq ft room needs roughly 10,000–12,000 BTU in a moderate climate, 12,500–15,000 BTU in a warm climate, and 15,000–18,000 BTU in a hot climate. The exact figure depends on sun exposure, insulation, ceiling height and how many people use the room.
What size AC for a 12×12 room?
A 12×12 room is 144 sq ft and typically needs a 5,000–6,000 BTU window or portable unit in a moderate climate. In a very sunny or poorly insulated room, 8,000 BTU gives better comfort. For a mini-split, 9,000 BTU is the standard minimum size.
Is a higher BTU AC always better?
No. This is the most expensive misconception in air conditioning. An oversized unit cools the room too quickly, shuts off before it can remove humidity, then short-cycles. You end up with a cold, clammy room and a higher energy bill.
How do I convert BTU to tons and kW?
1 ton = 12,000 BTU/hr. Divide BTU by 12,000 to get tons. 1 kW = 3,412 BTU/hr. Multiply kW by 3,412 to get BTU, or divide BTU by 3,412 to get kW. For a quick estimate, 12,000 BTU = 1 ton = 3.52 kW.
Why does my room feel humid even with the AC on?
Almost always an oversized AC. The unit cools the air below the thermostat setpoint in a few minutes, then shuts off. The coil never stays cold long enough to condense moisture out of the air. A correctly sized unit runs longer cycles, giving the coil time to dehumidify properly.
Does ceiling height really matter for AC sizing?
Yes. AC cools volume, not floor area. A room with 10 ft ceilings has 25% more air than the same floor plan with 8 ft ceilings. The calculator multiplies the base BTU by ceiling height ÷ 8. Cathedral ceilings are a common source of undersizing.
How much extra BTU do people add to a room?
The first two occupants are already accounted for in the base load. Each additional person adds about 600 BTU/hr of sensible and latent heat. A home office with four people needs 1,200 BTU more than the same room with two people.
Should I add BTU if the room has a kitchen?
Yes. A kitchen adds about 4,000 BTU/hr to the cooling load because of the stove, oven, dishwasher, refrigerator and general appliance heat. This is the AHAM and ENERGY STAR recommendation.
What is the difference between a mini-split and a window AC for the same room?
For the same BTU rating, a mini-split is quieter, more efficient (higher SEER), and does not block a window. A window unit is cheaper upfront and easier to install. The BTU requirement is the same for both. Choose the type based on budget, noise tolerance and installation constraints.
How accurate is a rule-of-thumb BTU calculation?
A simple "20 BTU per square foot" rule can be off by 40–50% in real rooms. It ignores climate, sun exposure, insulation, ceiling height and occupancy. A multi-factor calculator like this one gets within about 10–15% of a full Manual J load calculation.
Does a west-facing room really need more BTU?
Yes. West-facing glass receives peak solar gain in the late afternoon, exactly when outdoor air temperatures are highest. This is the worst-case cooling load of the day. A west-facing room can need 10–25% more BTU than a north-facing room with the same floor area.
What happens if I install an AC that is slightly oversized?
Slightly oversized — within 10% of the correct size — is usually fine, especially on inverter units that can modulate down. Significantly oversized (20%+ too large) causes short-cycling, poor humidity control, uneven cooling and premature compressor failure.
What is a Manual J calculation and do I need one?
Manual J is the ACCA standard method for calculating residential cooling loads room by room. It factors in wall construction, window area, orientation, insulation R-values, air infiltration and duct losses. Contractors use it for system replacement and new construction. You do not need a full Manual J to buy a window unit.
Does AC tonnage match the ton of refrigerant?
No. The "ton" in AC tonnage is a unit of cooling capacity, defined as 12,000 BTU/hr. It comes from the historical fact that 1 ton of ice melting in 24 hours absorbs about 12,000 BTU/hr of heat. It has nothing to do with how much refrigerant is in the system.
What is the right AC size for a bedroom?
Most bedrooms (120–250 sq ft) need 5,000–8,000 BTU in moderate climates. A 12×12 master bedroom with two adults: 6,000 BTU is typical. A larger master with west-facing windows might need 8,000–10,000 BTU. Quieter operation matters more in bedrooms — consider a mini-split if noise is a concern.
How do I size an AC for an open-plan living and kitchen area?
Treat the whole open-plan space as one room for BTU calculation, then add the 4,000 BTU kitchen allowance. A 700 sq ft open-plan space in a warm climate works out around 24,000–30,000 BTU. For this size, a ducted mini-split or central system is usually better than a single wall unit.
What is the smallest AC size available?
The smallest window units are 5,000 BTU. Some portable units are rated at 6,000–8,000 BTU nominal but deliver less than that in practice because of duct losses. For very small rooms (under 100 sq ft), a 5,000 BTU window unit is usually the correct answer.
Why does the calculator recommend the next size up?
Because AC units come in fixed size steps, not every number. If your calculation lands at 11,400 BTU, no unit exists at that exact rating. The nearest standard is 12,000 BTU (1 ton). Choosing the next size up covers hotter-than-average days and accounts for the real-world load being slightly higher than the calculation.
Related Calculators
References
- AHRI Standard 210/240 — Performance Rating of Unitary Air-Conditioning & Air-Source Heat Pump Equipment. Air-Conditioning, Heating and Refrigeration Institute.
- ANSI/ASHRAE Standard 55 — Thermal Environmental Conditions for Human Occupancy. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
- ACCA Manual J — Residential Load Calculation. Air Conditioning Contractors of America.
- ENERGY STAR — Room Air Conditioner Sizing Guidelines. U.S. Environmental Protection Agency.
- ASHRAE Handbook — Fundamentals, Chapter 18: Nonresidential Cooling and Heating Load Calculations.
Confirm the final unit size with a qualified HVAC contractor before purchase. This calculator provides an estimate, not a Manual J load calculation.