Use this fish tank volume calculator to calculate the true water volume, total weight and equipment sizing for rectangular, cube, bow front, corner, cylinder and hexagonal tanks. Accounts for substrate and decor displacement — the real number most calculators miss.
- Six Tank Shapes
- US & Imperial Gallons
- Litres
- Actual Water Volume
- Total Weight
- Filter & Heater Sizing
Cube aquariums are ideal for small coral, shrimp and betta setups where footprint matters more than length.
Bow front volume is approximately 85% of a rectangular tank with the same length, max width and height.
Corner tanks are a quarter cylinder shape. Volume = π × r² × h ÷ 4.
Cylinder tanks are common for goldfish bowls, tall column aquariums and shrimp setups. Volume = π × r² × h.
Hexagonal tanks are measured across the flats (flat side to flat side). Volume = (√3 ÷ 2) × a² × h.
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Equipment sizing guide
Why Aquarium Volume Is Not What You Think
The number stamped on a fish tank box is the nominal volume — the raw geometric capacity of the glass. It is almost never the water volume you actually have. Once you add substrate, rock, driftwood and equipment, you lose 10–25% of the advertised capacity. Every dosing calculation, fish stocking estimate and filter sizing should be based on the actual water volume, not the box number.
Substrate displacement
A 2-inch gravel bed in a 55-gallon tank displaces about 6 gallons of water. A 3-inch sand bed in a planted tank displaces closer to 9 gallons. Depth matters.
Rock and driftwood
Heavy aquascapes and coral rock can displace 10–20% of total water. Large centrepiece rock formations in reef tanks often displace more water than the substrate.
Water level
Tanks are rarely filled to the rim. Most sit 1–2 inches below the top to prevent overflow, and a rim or lid trim reduces usable volume further. Real water level is typically 1 inch below nominal height.
Filter and sump volume
Canister filters, sumps and refugiums hold 5–20% additional water but this is separate from the display. Total system volume is display water plus filter water — the number that matters for dosing.
Using the box number to dose medication or fertiliser over-doses the tank by 10–25%. Livestock medications have narrow safety margins — over-dosing is a common cause of fish loss. Always calculate against actual water volume.
Standard Aquarium Sizes Reference
The standard tank sizes available in North America and Europe, with nominal volume and typical dimensions. Use this table when shopping for a new tank or checking whether a particular footprint will fit your stand.
| Volume | Dimensions (L × W × H) | Type | Best for |
|---|---|---|---|
| Nano tanks | |||
| 2.5 gal | 12 × 6 × 8 in | Rectangular | Betta, shrimp |
| 5.5 gal | 16 × 8 × 10 in | Rectangular | Betta, small community |
| 10 gal | 20 × 10 × 12 in | Rectangular | Nano community, quarantine |
| Small | |||
| 20 gal High | 24 × 12 × 16 in | Rectangular | Small community, betta sorority |
| 20 gal Long | 30 × 12 × 12 in | Rectangular | Community, planted |
| 29 gal | 30 × 12 × 18 in | Rectangular | Community, planted |
| Medium | |||
| 40 gal Breeder | 36 × 18 × 16 in | Rectangular | Reef, breeding, low profile |
| 55 gal | 48 × 13 × 21 in | Rectangular | Most popular community size |
| 65 gal | 36 × 18 × 24 in | Rectangular | Tall planted, reef |
| Large | |||
| 75 gal | 48 × 18 × 21 in | Rectangular | Wide community, reef |
| 90 gal | 48 × 18 × 24 in | Rectangular | Tall reef, cichlids |
| 125 gal | 72 × 18 × 21 in | Rectangular | Large community, cichlids |
| 180 gal | 72 × 24 × 25 in | Rectangular | Large reef, cichlids |
| Specialty shapes | |||
| 32 gal | 24 × 24 × 24 in | Cube | Reef cube, planted cube |
| 36 gal | 30 × 15 × 21 in | Bow front | Community display |
| 46 gal | 36 × 16 × 21 in | Bow front | Community display |
| 54 gal | 24 × 24 × 24 in | Corner | Small footprint corner |
| 92 gal | 48 × 24 × 24 in | Corner | Large corner feature |
Aquarium Volume Formulas
Every shape uses the same base principle: volume = base area × height. Only the base area formula changes.
Rectangular
Length × width × height.
V = L × W × H
Cube
Side cubed.
V = S³
Bow front
85% of a rectangular tank with the same L, max W and H.
V = L × W × H × 0.85
Corner (quarter cylinder)
π r² h ÷ 4.
V = π × r² × H ÷ 4
Cylinder
π r² h.
V = π × r² × H
Hexagon
(√3 ÷ 2) × a² × h where a is across flats.
V = (√3 ÷ 2) × a² × H
US gallons
Divide cubic inches by 231.
gal = in³ ÷ 231
Litres
Multiply US gallons by 3.785.
L = US gal × 3.785
Filter and Heater Sizing
Once you know the actual water volume, equipment sizing is a direct calculation. These are the accepted hobby ranges used by serious aquarists.
| Equipment | Rule | Example (55 gal display) | Notes |
|---|---|---|---|
| Filter turnover | 4–10× actual volume per hour | 240–550 GPH flow rate | Reef and cichlids favour the higher end |
| Heater wattage | 3–5 W per US gallon | 175–275 W | Use 5 W/gal for tanks under 40 gal |
| Water changes | 10–25% weekly | 6–14 gal per week | Heavier stocking and small tanks need more |
| Lighting (planted) | 2–4 W per US gal LED | 110–220 W LED equivalent | Depends on plant species and CO&sub2; |
| Lighting (reef) | 3–6 W per US gal LED | 165–330 W LED equivalent | Higher end for SPS corals |
| Substrate (planted) | 2–3 lb per US gal | 110–165 lb | More for root feeders and dirted tanks |
Fish Stocking Guide
The classic "one inch of adult fish per gallon" rule is a starting point, not a law. It works for slim-bodied community species in medium tanks with good filtration but breaks down for large messy fish, cichlids and reef setups.
Rule 1 — Slim community fish
1 inch of adult fish per US gallon. Tetras, rasboras, danios, corydoras, small livebearers.
Rule 2 — Goldfish and large fish
1 inch per 3 gallons. Goldfish, cichlids, plecos, large catfish. They produce far more waste per inch.
Rule 3 — Reef and saltwater
1 inch per 5 gallons for small reef fish. Aggressive territory and bioload limits apply far more than length.
Rule 4 — Surface area matters
Tall narrow tanks hold less usable swimming space than long shallow tanks of the same volume. Gas exchange happens at the surface — footprint beats height for stocking.
Total Weight and Floor Loading
Water weighs 8.34 lb per US gallon (1 kg per litre). Add the tank, substrate, rock, filter and stand, and a "55-gallon" setup easily weighs 600 lb. That matters for upper-floor installations.
| Tank size | Water weight | With tank + substrate + decor | Floor loading |
|---|---|---|---|
| 10 gal | 83 lb | 110–130 lb | Safe anywhere |
| 20 gal | 167 lb | 220–260 lb | Safe anywhere |
| 29 gal | 242 lb | 310–360 lb | Safe on ground floors, check upstairs |
| 40 gal | 334 lb | 450–520 lb | Ground floor or load-bearing upstairs |
| 55 gal | 459 lb | 600–700 lb | Load-bearing wall or ground floor |
| 75 gal | 626 lb | 830–950 lb | Concrete floor or engineered wood |
| 90 gal | 751 lb | 1,000–1,150 lb | Concrete floor recommended |
| 125 gal | 1,043 lb | 1,400–1,600 lb | Concrete floor, structural check |
| 180 gal | 1,502 lb | 2,000–2,300 lb | Concrete slab, professional advice |
Standard residential floors are designed for 40 lb per square foot (live load). A 55-gallon tank on a 48 × 13 inch footprint is 459 lb over 4.3 sq ft — about 107 lb per square foot. Always place tanks across joists, not parallel to them, and against a load-bearing wall. Above 75 gallons, get a structural opinion.
The US liquid gallon is defined as 231 cubic inches by NIST Handbook 44. The Imperial gallon is 277.42 cubic inches (4.546 litres). Volume conversions follow ISO 80000-3. Standard aquarium sizes are conventions set by manufacturers rather than a formal standard, though the AGA (Aquarium Association) footprint sizes are widely referenced. Water density is 8.34 lb per US gallon at 20°C / 68°F.
On a standard rectangular tank, the box volume is nominal and your real water volume is about 85–90% of that after substrate, decor and water level. Water weight is 8.34 lb per US gallon or 1 kg per litre. Filter flow should be 4–10× actual volume per hour. Heater wattage should be 3–5 watts per US gallon. A simple 48 × 12.5 × 21 inch tank (nominal 55 gal) holds about 47 gallons of actual water.
People Also Ask
How do I calculate the volume of my aquarium?
Measure the tank in inches (length × width × height), multiply the three together to get cubic inches, then divide by 231 to get US gallons. Divide by 277.42 for Imperial gallons. Multiply US gallons by 3.785 for litres. For non-rectangular tanks the shape formula changes — use the calculator above with the correct shape selected.
Why is my aquarium's actual volume lower than the box says?
Three reasons: substrate (gravel or sand bed) displaces 5–15% of the volume, decor and rock displace another 5–15%, and the water line sits 1–2 inches below the rim. Together these reduce actual water volume by 10–25% compared to the nominal rating. A 55-gallon tank typically holds 45–48 gallons of real water.
How many gallons is a 48 × 12.5 × 21 tank?
48 × 12.5 × 21 = 12,600 cubic inches. Divided by 231, that is 54.5 US gallons — the standard 55-gallon tank. Actual water volume after substrate and decor is typically 45–48 gallons.
How much does aquarium water weigh?
Freshwater weighs 8.34 lb per US gallon (1 kg per litre) at 20°C. Saltwater is slightly heavier at about 8.55 lb per US gallon (1.025 kg per litre). A 55-gallon tank of fresh water weighs 459 lb just for the water, before the tank, substrate and stand.
What size filter do I need for a 55-gallon tank?
Aim for a filter that turns the tank over 4–10 times per hour. For a 55-gallon tank with 47 gallons of actual water, that is a 190–470 GPH flow rate. For community fish, 250–350 GPH is plenty. For cichlids, goldfish or reef, aim for the higher end at 400–500 GPH.
What wattage heater for a 55-gallon tank?
Use the 3–5 watts per US gallon rule. For 55 gallons, that is 165–275 W. A single 200 W or 250 W heater is standard. For tanks above 55 gallons, use two smaller heaters instead of one large one — if one fails on, it will not cook the tank.
How many fish can I put in my aquarium?
The classic rule is 1 inch of adult fish per US gallon for slim-bodied community fish, and 1 inch per 3 gallons for large or messy fish like goldfish and cichlids. Reef tanks: 1 inch per 5 gallons. These are starting points — filtration, water change schedule and adult size of the species matter more than the raw inches.
How do I measure a bow front or corner tank?
For a bow front, measure the straight back length, the maximum width (front to back at the widest point) and height. Volume is approximately 85% of a rectangular tank with those same dimensions. For a corner tank, measure the radius (distance from the corner to the outer curve) and height. Volume is a quarter cylinder: π × r² × h ÷ 4.
Does aquarium volume include the filter and sump?
The display tank volume and the total system volume are different numbers. Canister filters hold 1–3 gallons, and sumps hold 10–40 gallons on larger systems. For dosing medications and fertilisers, use the total system volume (display + filter + sump). For stocking and equipment sizing, use the display volume.
How much substrate do I need for a planted tank?
Planted tanks want 2–3 inches of substrate depth, which works out to roughly 2–3 lb of substrate per US gallon of tank volume. A 55-gallon planted tank needs 110–165 lb of substrate. Root-feeding plants like Amazon swords and cryptocorynes want the deeper end of that range.
How do I convert gallons to litres for my aquarium?
Multiply US gallons by 3.785 to get litres. Multiply Imperial gallons by 4.546. A 55 US gallon tank is 208 litres. A 55 Imperial gallon tank (UK convention) is 250 litres. Always check which gallon convention your tank is rated in — the difference is 20%.
Can I put a 55-gallon tank on an upper floor?
Yes, if you place it correctly. Put it against a load-bearing wall and perpendicular to the floor joists (across them, not parallel). Spread the load with a stand that has a solid base rather than four legs. Above 75 gallons, get a structural opinion. Residential floors are designed for 40 lb per square foot live load — a 55-gallon setup is typically 100–140 lb per square foot on its footprint, which is fine when placed correctly.
What is the difference between US and Imperial gallons for aquariums?
A US gallon is 231 cubic inches (3.785 litres). An Imperial gallon is 277.42 cubic inches (4.546 litres). US tanks are rated in US gallons; UK tanks are sometimes rated in Imperial gallons — a 55 Imperial gallon tank is 66 US gallons. Always confirm which unit a tank is rated in before comparing sizes.
Does a bigger tank need less maintenance?
Yes, relatively. Larger water volumes dilute waste and buffer against parameter swings. A 55-gallon tank is much more forgiving than a 10-gallon tank for the same stocking density. The trade-off is that water changes require more volume and equipment cost is higher. Most aquarists find 40–75 gallons to be the sweet spot of stability versus manageable maintenance.
Related Calculators
References
- National Institute of Standards and Technology. NIST Handbook 44 — Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. Current edition.
- International Organization for Standardization. ISO 80000-3 — Quantities and units, Part 3: Space and time.
- American Society for Testing and Materials. ASTM D1193 — Standard Specification for Reagent Water (reference for water density and purity).
- American National Standards Institute / Aquarium industry practice. Standard glass aquarium footprint conventions as published by major manufacturers.
- Interlocking Concrete Pavement Institute (reference for volume displacement calculations by similar geometric methods).
Aquarium volume, weight and equipment sizing are estimates based on the geometry you enter. Actual capacity varies with glass thickness, tank construction and water level. Always confirm floor loading and equipment compatibility with the manufacturer before installing a large aquarium.