Rectangular
Cube
Cylinder
Half Cylinder
Corner Cylinder
Hexagon
Bow Front
L-Shape🐠 You bought the tank. Now you’re staring at it wondering: «How many gallons is this thing, actually?»
Maybe you need to dose a treatment and the bottle says «per gallon.» Maybe you’re about to add fish and you don’t want to overcrowd them. Maybe you’re just trying to buy the right filter and every product page asks for your tank size in gallons — but your tank only has centimetre measurements on the box.
You’re in the right place.
Enter your dimensions in the calculator above and get your answer in seconds — in US gallons, UK gallons, and litres simultaneously. Everything below is the math, the charts, and the reference data behind that number.
Fish Tank Volume Calculator — Gallons, Litres & All Tank Shapes
Enter your dimensions above to instantly calculate your aquarium’s capacity. Below, you will find the exact math, water weight charts, and standard tank size dimensions.
How many gallons is my fish tank?
Measure the inside glass length × width × height in inches, then divide by 231 to find your US gallons. A standard 36×18×18 inch tank holds roughly 50 US gallons.
This is the single most-asked question in fishkeeping — and the answer is simpler than most people think. You don’t need to fill the tank with a measuring jug. You just need a tape measure and 30 seconds. Measure from inside glass edge to inside glass edge — not the outer frame — and plug those three numbers into the calculator above for an instant result in gallons and litres simultaneously.
11,664 ÷ 231 = 50.5 US Gallons
192,556 ÷ 1,000 = 192.6 Litres
⚠ Always measure the inside of the glass, not the outer frame. Glass thickness typically reduces each dimension by 0.5–1 cm per side.
How to calculate fish tank volume — The Formula
For rectangular and square tanks: multiply Length × Width × Height, then divide by 231 for US gallons or 1,000 for litres (when measuring in cm).
These two formulas cover the vast majority of home aquariums. If your tank is a standard rectangular box — which most are — this is all you need. For non-rectangular shapes like cylinders or bow-fronts, scroll down to the shapes section below.
÷ 231
= US Gallons
÷ 1,000
= Litres
÷ 277.4
= UK Gallons
Why 231? One US gallon equals exactly 231 cubic inches — that’s the conversion constant baked into every aquarium sizing guide ever written. Why 277.4 for UK gallons? Because a UK imperial gallon is larger: 4.546 litres vs the US gallon’s 3.785 litres. This difference matters enormously when dosing treatments — always confirm which gallon your product label refers to.
Fish tank volume formulas by shape
Yes — the shape of your aquarium directly impacts its volume. A bow-front and a rectangular tank with identical outer dimensions will hold different amounts of water. Our calculator handles all shapes automatically.
This is where most basic online calculators fail you. They assume every tank is a box. But if you have a cylinder, a corner bow-front, or an L-shaped tank and you use the rectangular formula, you will overdose treatments or understock your filtration — both dangerous outcomes.
| Tank Shape | Volume Formula | What to Measure | Notes |
|---|---|---|---|
| Rectangular / Square | L × W × H ÷ 231 | Length, Width, Height (inches) | Most common home aquarium shape |
| Cube | Side³ ÷ 231 | One side length (inches) | All sides equal — measure once |
| Cylinder | π × r² × H ÷ 231 | Diameter, Height (inches) | r = diameter ÷ 2 |
| Half Cylinder | (π × r² × H ÷ 2) ÷ 231 | Diameter, Height (inches) | Common for corner/wall-mount tanks |
| Bow-Front | Rectangular volume + curved segment volume | Length, Width, Full Width, Height | Holds ~10–15% more than rectangular equivalent |
| L-Shape | Calculate as two rectangles, add together | Measure each rectangular section separately | Split at the corner, calculate each half |
The bow-front formula is the trickiest — it requires calculating the volume of a circular segment added to the base rectangle. Our calculator does this automatically. If you’re doing it manually: the curved section typically adds 8–15% extra volume over a standard rectangular tank of the same outer dimensions.
Fish tank sizes chart — Standard dimensions & litres
Standard aquariums range from 5 to 300+ gallons. Use this reference chart to identify your tank size by its dimensions — or to plan a new purchase.
If you already own a tank and can’t find the paperwork, match your inside measurements to the chart below. These are industry-standard dimensions used by the major manufacturers. Minor variations of ±0.5 inch are normal between brands.
| Tank Size | Dimensions L×W×H (inches) | Dimensions L×W×H (cm) | US Gallons | UK Gallons | Litres |
|---|---|---|---|---|---|
| 5 Gallon | 16 × 8 × 10 | 41 × 20 × 25 | 5 | 4.2 | 19 |
| 10 Gallon | 20 × 10 × 12 | 51 × 25 × 30 | 10 | 8.3 | 38 |
| 20 Gallon Long Popular | 30 × 12 × 12 | 76 × 30 × 30 | 20 | 16.6 | 76 |
| 20 Gallon High | 24 × 12 × 16 | 61 × 30 × 41 | 20 | 16.6 | 76 |
| 29 Gallon | 30 × 12 × 18 | 76 × 30 × 46 | 29 | 24.1 | 110 |
| 40 Gallon Breeder Popular | 36 × 18 × 16 | 91 × 46 × 41 | 40 | 33.3 | 151 |
| 55 Gallon Popular | 48 × 13 × 21 | 122 × 33 × 53 | 55 | 45.8 | 208 |
| 75 Gallon | 48 × 18 × 21 | 122 × 46 × 53 | 75 | 62.5 | 284 |
| 90 Gallon | 48 × 18 × 24 | 122 × 46 × 61 | 90 | 74.9 | 341 |
| 125 Gallon | 72 × 18 × 21 | 183 × 46 × 53 | 125 | 104.1 | 473 |
| 150 Gallon | 72 × 18 × 28 | 183 × 46 × 71 | 150 | 124.9 | 568 |
| 180 Gallon | 72 × 24 × 25 | 183 × 61 × 64 | 180 | 149.9 | 681 |
| 300 Gallon | 96 × 24 × 31 | 244 × 61 × 79 | 300 | 249.8 | 1,136 |
📌 The 55-gallon tall aquarium dimensions (48×13×21 in) and 75-gallon tank length (48 inches) are the two most-searched size lookups — both are in the chart above. The 20-gallon tank size comes in two variants: Long (30×12×12) offers more swimming space; High (24×12×16) suits taller fish and plants.
US Gallons vs UK Gallons vs Litres — which one is your tank?
1 US Gallon = 3.785 Litres. 1 UK Gallon = 4.546 Litres. They are not the same — and confusing them when dosing treatments can harm or kill your fish.
This is the most dangerous unit confusion in fishkeeping. A bottle of water conditioner that says «1 cap per 10 gallons» is almost certainly referring to US gallons — because most aquarium products are manufactured or labelled to the US standard, even when sold in the UK. If you dose for UK gallons, you will underdose by nearly 20%.
| Litres | US Gallons | UK Gallons |
|---|---|---|
| 20 L | 5.3 US gal | 4.4 UK gal |
| 38 L | 10 US gal | 8.4 UK gal |
| 76 L | 20 US gal | 16.7 UK gal |
| 114 L | 30 US gal | 25.1 UK gal |
| 151 L | 40 US gal | 33.2 UK gal |
| 208 L | 55 US gal | 45.8 UK gal |
| 284 L | 75 US gal | 62.5 UK gal |
| 379 L | 100 US gal | 83.4 UK gal |
How much does a full fish tank weigh?
Fresh water weighs exactly 8.34 lbs per US gallon, or 1 kg per litre. A full 55-gallon tank weighs approximately 459 lbs of water alone — before glass, gravel, rocks, or stand.
This is the question people forget to ask until the tank is already full and the floor is creaking. A standard residential floor is rated for 40–50 lbs per square foot. A large aquarium can easily exceed this — especially when you factor in the glass, cabinet, substrate, and decorations. Always calculate total weight before choosing a location.
| Tank Size (US Gal) | Water Weight (lbs) | Water Weight (kg) | Approx. Total Weight* |
|---|---|---|---|
| 10 Gallon | 83 lbs | 38 kg | ~111 lbs / 50 kg |
| 20 Gallon | 167 lbs | 76 kg | ~225 lbs / 102 kg |
| 29 Gallon | 242 lbs | 110 kg | ~330 lbs / 150 kg |
| 40 Gallon | 334 lbs | 151 kg | ~458 lbs / 208 kg |
| 55 Gallon | 459 lbs | 208 kg | ~625 lbs / 284 kg |
| 75 Gallon | 626 lbs | 284 kg | ~850 lbs / 386 kg |
| 125 Gallon | 1,043 lbs | 473 kg | ~1,400 lbs / 635 kg |
*Approx. total weight includes water + glass tank + gravel/substrate (estimated at 1.5 lbs per gallon). Decorations, sump, and stand add additional weight.
How many fish can I keep? — Tank stocking rules
The baseline rule: 1 inch of adult fish per 1 US gallon of water, or 1 cm of fish per litre. But this rule has serious limitations — body shape, waste output, and filtration capacity all matter more than length alone.
The «inch per gallon» rule was designed for small, slim-bodied community fish like tetras and danios. It completely breaks down for deep-bodied fish like goldfish and angelfish, or messy fish like cichlids. Use it as a starting floor — not a ceiling.
🐠 Betta Fish
Minimum: 5 US gallons (19L). Bettas need horizontal swimming space, not height. A 5-gallon long tank beats a 5-gallon tall every time. One betta per tank — always.
🐟 Goldfish
Minimum: 20 gallons for the first fish, +10 gallons per additional fish. Goldfish produce enormous waste. The inch-per-gallon rule does not apply. They need heavy filtration — 8–10× tank volume per hour.
🐡 Angelfish
Minimum: 30 gallons for a pair. Angelfish are tall-bodied — they need tanks at least 18 inches deep. A best fish for 30 gallon aquarium list should include a single pair maximum.
🦈 Community Fish (75 Gal)
A 75-gallon tank suits medium schooling fish beautifully. Best fish for 75 gallon aquarium include large tetras, rainbowfish, medium cichlids, and peaceful bottom dwellers like corydoras.
| Tank Size | Slim-Bodied Fish (inch/gal rule) | Goldfish | Recommended Filtration |
|---|---|---|---|
| 10 Gallon | Up to 10 inches total | Not recommended | 80–100 GPH filter |
| 20 Gallon | Up to 20 inches total | 1 fancy goldfish | 160–200 GPH filter |
| 29 Gallon | Up to 29 inches total | 1–2 fancy goldfish | 230–290 GPH filter |
| 55 Gallon | Up to 55 inches total | 3–4 fancy goldfish | 440–550 GPH filter |
| 75 Gallon | Up to 75 inches total | 5–6 fancy goldfish | 600–750 GPH filter |
| 125 Gallon | Up to 125 inches total | 8–10 fancy goldfish | 1,000–1,250 GPH filter |
GPH = Gallons Per Hour. Always oversize your filter — a filter rated for your exact tank volume is already underpowered for a fully stocked aquarium.
Frequently Asked Questions
Direct answers to the most-searched aquarium sizing questions — written for fish owners, not textbooks.
How do I measure a fish tank correctly?
Always measure from inside glass edge to inside glass edge — not the outer frame or trim. The outer dimensions include the glass thickness (typically 6–12mm per side on larger tanks), which will give you a falsely large volume calculation.
Measure three dimensions:
- Length — front panel, left edge to right edge (inside)
- Width — side panel, front glass to back glass (inside)
- Height — bottom to your actual waterline, not the top of the tank (most tanks are filled 1–2 inches below the rim)
Use a flexible tape measure or a rigid ruler. For tanks already filled with water, measure to the waterline — not the tank rim — for an accurate volume of water currently in the tank.
Does the shape of my aquarium affect the volume?
Yes — significantly. Two tanks with identical outer dimensions but different shapes will hold different volumes of water. A bow-front tank holds approximately 10–15% more water than a rectangular tank of the same length and height, because the curved front panel pushes outward.
A cylindrical tank of the same height and diameter as a rectangular tank’s width holds considerably less water — because the circle’s area (π × r²) is smaller than the square’s area (side²). Always select your tank’s shape in the calculator above before entering dimensions to get an accurate result.
What is the difference between US and UK gallons for fish tanks?
A US gallon = 3.785 litres. A UK imperial gallon = 4.546 litres. That is a 20% difference — large enough to cause serious problems when dosing medications, dechlorinators, or fertilisers.
The vast majority of aquarium products — even those sold in the UK — use US gallons on their dosing instructions, because most brands originate in North America. When in doubt, convert your tank volume to litres first, then use the litre-based dosing rate if available. Our calculator outputs all three units simultaneously so you always have the right number for any product label.
How do I calculate litres in a fish tank?
If your tank dimensions are in centimetres: Length × Width × Height ÷ 1,000 = Litres. This works because 1,000 cubic centimetres (cm³) equals exactly 1 litre.
Example: A tank measuring 60cm × 30cm × 36cm = 64,800 cm³ ÷ 1,000 = 64.8 litres. If your dimensions are in inches, multiply the result in cubic inches by 0.01639 to get litres, or simply divide by 61.02. The calculator above handles all of this automatically — just select your preferred unit of measurement before entering dimensions.
What size heater do I need for my aquarium volume?
The standard rule is 5 watts per US gallon for tropical fish tanks in a room-temperature environment (68–72°F / 20–22°C). For tanks in cold rooms, garages, or basements, increase to 7–10 watts per gallon.
- 10 gallon tank → 50W heater
- 20 gallon tank → 100W heater
- 29 gallon tank → 150W heater
- 55 gallon tank → 250–300W heater
- 75 gallon tank → 300–400W heater
- 125 gallon tank → 500–600W heater (or two heaters split across the tank)
For tanks over 75 gallons, always use two heaters placed at opposite ends of the tank. This ensures even heat distribution and provides a backup if one unit fails — a single heater failure in a large tank can drop temperature dangerously before you notice.
How many fish can I put in a 30-litre tank?
Using the 1cm-per-litre rule, a 30-litre tank can house approximately 30cm of slim-bodied fish. In practice, that means a small school of 6–8 nano fish such as ember tetras, chili rasboras, or endlers livebearers.
A 30-litre tank is not suitable for goldfish, cichlids, or any fish that grows beyond 5cm in adult length. It is an excellent beginner tank for a single betta fish or a carefully chosen nano community. Always factor in your filtration capacity — a 30-litre tank needs a filter rated for at least 150–240 litres per hour to maintain water quality in a stocked setup.
How do I convert my tank dimensions from cm to gallons?
There are two ways to convert centimetres to gallons. Method 1 (via litres): Multiply L × W × H in cm, divide by 1,000 to get litres, then divide by 3.785 for US gallons or 4.546 for UK gallons.
= 64,800 cm³ ÷ 1,000 = 64.8 litres
÷ 3.785 = 17.1 US gallons
÷ 4.546 = 14.3 UK gallons
Method 2 (direct): Multiply L × W × H in cm, then multiply by 0.000264172 for US gallons. Both methods give the same result. Or — just use the calculator above and skip the arithmetic entirely.
Is a 400-litre tank small or large?
A 400-litre tank is considered a large home aquarium. It equates to approximately 105 US gallons or 88 UK gallons — placing it firmly in the «advanced hobbyist» category in terms of maintenance, equipment cost, and floor loading requirements.
At 400 litres, you are looking at a water weight alone of 400 kg (882 lbs). Add the glass, cabinet, substrate, and decorations and total weight will typically reach 550–650 kg. This tank requires a dedicated, reinforced stand and should be positioned on a ground floor or against a structural wall on upper floors. In terms of stocking, 400 litres opens the door to medium-large cichlids, large schools of community fish, or a serious planted aquascape.
🐠 Ready to size your tank perfectly?
Use the calculator at the top of this page to get your exact volume in US gallons, UK gallons, and litres — then use the charts above to choose the right filter, heater, and stocking level for a thriving aquarium.
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