📐 Free Tool

Aquarium & Fish Tank Size Calculator

This free aquarium size calculator (fish tank size calculator) turns your tank's inside dimensions into total capacity, filled volume and usable water in US gallons, UK gallons and litres — a complete fish tank size calculation in one click, with a species planner underneath.

US & UK Gallons Litres Rectangular, Cube & Cylinder Partial Fill Species Planner

💧 Fish tank size calculator: enter your inside dimensions

Measure inside the glass and the aquarium size calculator returns total capacity, filled volume and usable water in gallons (US & UK) and litres.

Leave blank to use full tank height
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Enter inside dimensions

Results will show US gallons, UK gallons, litres and water weight.

Volume formulas & conversions used by this tank size calculator

Rectangular / cube: length × width × depth  |  Vertical cylinder: π × radius² × depth  |  Horizontal cylinder: circular segment area × length

Conversions (per NIST SP 811): 1 US gallon = 3.785411784 L  |  1 UK gallon = 4.54609 L  |  Measure inside the glass, not the outer frame.

Water weight excludes the tank, stand, substrate, rocks and equipment. Consult a qualified professional if floor or stand loading is a concern.

🐠 Fish tank size planner by species

Compare physical volume with a conservative species-based space estimate. Planning aid only — not a stocking guarantee.

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Choose a species

Results show minimum and recommended tank size with care notes.

Step by step

How to calculate the size of a fish tank in four steps

The calculator does the arithmetic; the accuracy comes from what you measure. Follow these four steps and the fish tank size calculation will match the water your fish actually live in.

Fish tank size calculator principle diagram: a tank cross-section showing inside length, inside width and water height measured inside the glass, with substrate and rock displacing water, beside the four-step calculation — length × width × water height, convert to litres, subtract displacement, convert to US and UK gallons — worked for a 20 gallon tank that yields 56.5 litres or 14.9 US gallons of usable water.
How the calculation works. Left: the three inside measurements and what displaces water. Right: the same four steps the calculator runs, worked for a standard "20 gallon high" tank — which ends up holding about 15 US gallons of usable water. Open full-size diagram.

  1. 1. Measure inside the glass

    Run the tape from inside surface to inside surface for length and width. Glass panels, silicone seams and trim take up 5–15% of the outside footprint, which is why a tank sold as "20 gallons" never holds 20 gallons of water. For a cube tank enter only one side; for a cylinder enter the inside diameter.

  2. 2. Use the water height, not the tank height

    Measure from the inside base to the line you actually fill to — usually 1–2 inches (2–5 cm) below the rim to leave room for a lid, filter outflow and surface agitation. The "Actual water depth" field takes this number; leave it blank and the calculator assumes a brim-full tank.

  3. 3. Deduct substrate and decor displacement

    Gravel, sand, rock, driftwood and internal equipment all push water out. Choose 0% for a bare quarantine tank, 10% for a typical community aquarium, or 15–20% for a heavily planted or hardscape layout. If you are still choosing a substrate depth, the aquarium substrate calculator sizes that layer from the same footprint.

  4. 4. Read the result in the unit your region uses

    The calculator prints US gallons, UK gallons and litres side by side. Use US gallons for American tank labels and most online stocking rules, UK gallons for tanks sold in the United Kingdom, and litres for Europe, Australia and most of Asia — and for dosing medication or conditioner, which is almost always labelled per litre.

By hand

Fish tank size calculation formulas and a worked example

Prefer to calculate size of fish tank volume yourself? These are the exact formulas and conversion factors the calculator uses.

Rectangular and cube fish tanks

Volume = inside length × inside width × water height. In inches, multiply the cubic inches by 0.004329 for US gallons or 0.016387 for litres. In centimetres, divide cubic centimetres by 1000 for litres, then divide litres by 3.785 for US gallons or 4.546 for UK gallons.

Cylinder fish tanks

Vertical cylinder: π × (inside diameter ÷ 2)² × water height. Horizontal cylinder: the area of the circular segment below the water line × length — the calculator handles the segment formula for you, since it is the one part of a fish tank size calculation that is genuinely awkward by hand.

Worked example: the size of a standard "20 gallon high" fish tank

A 20 High is sold as 24 × 12 × 16 inches, measured outside. Inside the glass it is closer to 23.6 × 11.6 × 16 in, and most people fill it to about 14 in of water.

StepCalculationResult
Brim-full capacity23.6 × 11.6 × 16 in = 4,380 cu in × 0.01638771.8 L · 19.0 US gal
At the water line23.6 × 11.6 × 14 in = 3,833 cu in × 0.01638762.8 L · 16.6 US gal
After 10% displacement62.8 L × 0.9056.5 L · 14.9 US gal · 12.4 UK gal
Water weight56.5 L × 0.998 kg/L56.4 kg · 124 lb (water only)

So the tank on the label as "20 gallons" gives your fish about 15 US gallons of water. That 25% gap is normal, and it is the number you should use for stocking, heater sizing and medication doses.

Reading the result

Fish tank size in US gallons, UK gallons and litres

Three units, one volume. Which figure you quote depends on where you shop and what you are dosing.

US gallons vs UK gallons

The tool shows both US gallons (3.785 L each) and UK Imperial gallons (4.546 L each). UK tank labels and many UK fish-keeping guides quote Imperial gallons, so a "40 gallon" tank in the United States is only about 33 UK gallons — a difference big enough to change a stocking plan.

Litres — the international standard

Litres are the working unit in Europe, Australia and most of Asia, and almost every water conditioner, fertiliser and medication is dosed per litre. The litre figure shown is usable water after your chosen displacement, so it is the one to use for dosing.

Usable water, not box size

Labels print gross volume from outside dimensions. Actual usable water is always less. Because this tool works from inside measurements, the real water depth and a displacement allowance, it returns a conservative estimate of what your fish actually swim in — the figure filtration and stocking rules assume.

Common fish tank size benchmarks

Standard sizes: 10 US gal (37.9 L / 8.3 UK gal) · 20 US gal (75.7 L) · 29 US gal (109.8 L) · 55 US gal (208.2 L) · 75 US gal (283.9 L). Run your own tank through the calculator to see how far a lower fill line and hardscape pull the usable figure below the nominal size. For typical dimensions, filled weight and stocking at specific sizes, see our guides to the 50 gallon fish tank, 55 gallon fish tank and 100 gallon fish tank.

Species Calculators

Fish tank size calculators by species

Once you know the usable volume, the next question is whether it suits the fish. Each species calculator gives a minimum and a recommended tank size for the number you plan to keep — not a generic inch-per-gallon rule.

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Betta tank size

5–30 gal

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Goldfish tank size

20–40 gal first fish

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Guppy tank size

10 gal minimum

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Angelfish tank size

29 gal per pair

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Molly tank size

20 gal minimum

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Swordtail tank size

20 gal minimum

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Oscar tank size

75 gal per fish

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Discus tank size

55 gal group of 4

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Koi tank size

250 gal per koi

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Zebra danio tank size

10 gal for 6
View All Species Calculators →

Fish tank size calculator vs aquarium tank size calculator: is there a difference?

No. A fish tank size calculator, an aquarium tank size calculator and an aquarium volume calculator all answer the same question: how much water does this glass box actually hold? People reach for different words — some search "fish tank gallon calculator", others "calculate size of fish tank" or "fish tank size calculation in litres" — but the arithmetic is identical, and this page handles all of them.

What does change the answer is how you measure. Manufacturers name tanks by nominal outside size, while the number you need for stocking, dosing and heater sizing is the usable water volume — measured inside the glass, minus what substrate, rock and equipment displace. That is the figure this calculator returns, and it is the reason the result is usually 15–25% below the label.

So whichever phrase brought you here, the method is the same three moves: measure inside the glass, enter length, width and water height, then apply a displacement allowance. The result is the volume your fish, filter and test kit actually live with.

FAQ

Fish tank size calculator — frequently asked questions

Answers about US gallons, UK gallons, litres, displacement and how to interpret your result.

Measure the inside dimensions of your tank — length, width and water height — in inches or centimetres. Select the tank shape and your unit, then click Calculate. The fish tank size calculator shows total capacity, filled volume and estimated usable water in US gallons, UK gallons and litres instantly.
Multiply inside length × width × water height. In inches, multiply the result by 0.004329 for US gallons or 0.016387 for litres. In centimetres, divide by 1000 for litres. Then deduct about 10% for substrate and decor. A 23.6 × 11.6 × 14 inch fill works out to 62.8 litres filled and about 56.5 litres (14.9 US gallons) usable — see the worked example above.
One US gallon equals approximately 3.785 litres. One UK Imperial gallon equals approximately 4.546 litres — about 20% larger. If you are in the United Kingdom, compare the UK gallon figure with tank labels sold in UK pet shops. A tank described as "40 gallons" in the US holds about 33 UK gallons of water.
Tank labels typically print the gross volume using outside dimensions and a full fill height. This calculator uses your actual inside measurements and deducts a displacement allowance for substrate and decor, giving a realistic estimate of the water your fish actually swim in — usually 15–25% less than the label.
A bare tank with no substrate: 0%. A typical tank with sand or gravel and a few decorations: 10%. A heavily planted or hardscape setup with driftwood, rocks and an internal filter: 15–20%. The default 10% is a safe starting point for most freshwater community tanks.
The calculator converts measurements to litres using exact factors: 1 cubic inch = 0.016387 litres, 1 cubic centimetre = 0.001 litres. The litre result shown is usable water after deducting your chosen displacement percentage. Litres are the standard unit in Europe, Australia and most of Asia.
Yes. A fish tank size calculator, an aquarium tank size calculator and an aquarium volume calculator all perform the same fish tank size calculation: inside length × width × water height, converted to gallons and litres. Only the wording differs.
As a starting point, yes. The species planner on this page connects gallons to species minimums for betta, guppy, goldfish, discus, angelfish and more. Volume alone is not sufficient — also consider adult size, group needs, filtration capacity and water-change frequency.
Vertical and horizontal cylinders are supported directly — choose the shape and enter the inside diameter. Partial fills in a horizontal cylinder use the circular-segment formula. For a bow-front tank, enter the average of the front-to-back depth at the centre and at the ends as the width for a close estimate.
Yes. Every result includes US gallons, UK gallons and litres at the same time, so you do not need a separate converter. If you measure in centimetres the calculator works in metric throughout — length × width × height in centimetres divided by 1000 gives litres directly.
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Open Tool → 🧭 Aquarium Planner

Build a complete equipment, cycling and stocking checklist before buying animals.

Open Tool → 🪨 Aquarium Substrate Calculator

Work out how much gravel, sand or aquarium soil your tank footprint needs, in litres and pounds.

Open Tool → 📚 Water Parameters Guide

Expert guide to pH, ammonia, nitrite, nitrate, temperature and hardness for freshwater tanks.

Read Guide →