How to Use the Aquarium Tank Size Calculator
This free tool calculates the exact water volume of your fish tank in both US gallons and litres, updating live as you type. Volume is the number every other decision hangs off — bioload capacity, dechlorinator dosing, medication dosing, and heater or filter sizing all start here.
- Choose your tank shape — rectangular, bowfront, or hexagonal.
- Select your unit — inches or centimetres.
- Enter length, width and height — results appear instantly as you type.
- Set your fill level — most tanks run at 80–90% of height.
How the Calculation Works
For a rectangular tank, volume is straight geometry: length × width × height. In centimetres that gives cubic centimetres, which divide by 1,000 for litres; in inches it gives cubic inches, which divide by 231 for US gallons. The fill-level percentage then scales the height term, because water you don’t add is volume you don’t have.
For a bowfront tank, the curved front adds a roughly elliptical segment to the footprint. The calculator approximates this by adding two-thirds of the bow depth × length × height to the rectangular base volume — the standard approximation used across the hobby, accurate to within a few percent for typical bow curves.
For a hexagonal tank, the footprint is a regular hexagon. Its area is (3√3 ÷ 2) × side² — about 2.598 × side² — multiplied by height and the fill factor.
All results convert both ways: litres alongside US gallons, centimetres alongside inches, so you never have to do unit maths yourself.
How to Interpret Your Results
The number you get is the geometric volume of the dimensions you entered. Real-world water volume is always a little lower: glass thickness steals from every side, and substrate, rock, driftwood and equipment all displace water. Subtract roughly 10–15% for a planted tank with substrate, or about 5% for a bare-bottom setup, to get a realistic working volume.
Use that working volume — not the nominal “sticker” size — for dosing water conditioner and medication, and as the input to stocking decisions. When you move on to stocking, remember volume is only one of three factors that matter: bioload capacity (how much waste your filter and plants can process), filtration performance (turnover and biological media), and swimming terrain (footprint and water-column space for the species you keep). Our Fish Stocking Calculator weighs all three rather than relying on outdated length-per-volume folklore.
Worked Examples — Real Tank Dimensions
Sometimes the easiest way to understand a calculator is to walk through real numbers. Here are four common aquarium configurations with the dimensions you would enter and the output you should expect. If your results don’t match these within a fraction of a gallon, double-check that you’ve selected the right tank shape and the right unit (inches vs centimetres).
Example 1: Standard 10-Gallon Rectangle
Choose Rectangular, set the unit to inches, and enter 20 × 10 × 12 with a 90% fill level. The calculator returns approximately 7.5 US gallons (29 litres). The nominal “10-gallon” label on the tank is the geometric maximum at 100% fill — once you account for the 10% air gap at the top, the real water volume is closer to 7.5 gallons. This is the figure you should use for dosing dechlorinator and medication, and it’s the figure our stocking calculator uses to work out how many fish the tank can support.
Example 2: Standard 20-Gallon “Long”
Choose Rectangular, set the unit to inches, and enter 30 × 12 × 12 with a 90% fill level. The calculator returns approximately 16.8 US gallons (64 litres). Note again that the “20-gallon” label refers to the geometric maximum; real working volume is about 16–17 gallons. The 30-inch length gives this tank a large footprint, which is why 20-gallon longs are considered the best beginner tank — more swimming space and more surface area for oxygen exchange than a 20-gallon tall of the same volume.
Example 3: Bowfront 36-Gallon
Choose Bowfront, set the unit to inches, and enter length 30, width 12, bow depth 6, height 16 with a 90% fill level. The calculator returns approximately 29.4 US gallons (111 litres). The bow adds roughly 4–5 gallons over a flat-fronted tank of the same rectangular footprint — the curved front creates extra volume that the calculator accounts for using the two-thirds bow-depth approximation described above.
Example 4: Hexagonal 5-Gallon
Choose Hexagonal, set the unit to inches, and enter side 8, height 12 with a 90% fill level. The calculator returns approximately 3.5 US gallons (13 litres). Hexagonal tanks have a surprisingly small footprint relative to their nominal size — a 5-gallon hex is a poor choice for most fish because the small surface area limits oxygen exchange and the tall, narrow shape provides little horizontal swimming space. They work for a single betta or a small shrimp colony, but not much else.
Common Aquarium Sizes — Quick Reference Table
If you’re shopping for a tank and want to know the dimensions and real water volume before you buy, this table lists the most commonly available aquarium sizes in the US market. All volumes assume a 90% fill level; subtract another 10–15% if you run a deep substrate bed.
| Nominal Size | Shape | L × W × H (in) | Real Volume (gal) | Real Volume (L) |
|---|---|---|---|---|
| 5 gallon | Rectangular | 16 × 8 × 10 | 3.7 | 14 |
| 10 gallon | Rectangular | 20 × 10 × 12 | 7.5 | 29 |
| 15 gallon | Rectangular | 24 × 12 × 12 | 11.2 | 42 |
| 20 gallon (high) | Rectangular | 24 × 12 × 16 | 14.9 | 56 |
| 20 gallon (long) | Rectangular | 30 × 12 × 12 | 16.8 | 64 |
| 29 gallon | Rectangular | 30 × 12 × 18 | 21.6 | 82 |
| 40 gallon (breeder) | Rectangular | 36 × 18 × 16 | 36.0 | 136 |
| 40 gallon (long) | Rectangular | 48 × 12 × 16 | 32.0 | 121 |
| 55 gallon | Rectangular | 48 × 13 × 21 | 45.7 | 173 |
| 75 gallon | Rectangular | 48 × 18 × 21 | 63.2 | 239 |
| 36 gallon | Bowfront | 30 × 12 × 16 (bow 6) | 29.4 | 111 |
| 46 gallon | Bowfront | 36 × 12 × 20 (bow 8) | 38.7 | 147 |
Notice how much real volume differs from the nominal size. A “10-gallon” tank holds about 7.5 gallons of actual water; a “55-gallon” holds about 46. The difference is the air gap at the top (10%) and the glass thickness (a few percent). This is why experienced aquarists always work with the calculated volume rather than the sticker on the tank — dosing errors compound quickly when you’re treating a sick fish or measuring out fertiliser.
Conversion Reference — Gallons, Litres, and Cubic Units
If you need to convert between units manually, here are the exact conversion factors the calculator uses internally. Bookmark this section if you’re working with a mix of metric and imperial measurements — for example, comparing a US tank plan to a European aquarium product.
| 1 US gallon | = 3.785 litres = 231 cubic inches = 0.003785 cubic metres |
| 1 litre | = 0.264 US gallons = 61.02 cubic inches = 1,000 cubic centimetres |
| 1 cubic inch | = 0.00433 US gallons = 0.0164 litres |
| 1 cubic foot | = 7.48 US gallons = 28.32 litres |
| 1 UK imperial gallon | = 1.201 US gallons = 4.546 litres (larger than a US gallon) |
The calculator always reports both US gallons and litres side by side, so you never have to do this maths yourself. If you’re comparing against a UK source that quotes imperial gallons, use the litre figure as the common ground — litres are the same everywhere.
Why Tank Volume Matters for Every Other Decision
Volume isn’t just a number — it’s the foundation of nearly every other aquarium calculation. Once you know your true water volume, every subsequent decision becomes more accurate:
- Dechlorinator dosing: Most water conditioners dose 1 ml per 10 gallons. Using the nominal tank size rather than the real water volume leads to under-dosing during water changes, leaving residual chlorine that damages your fish’s gills and kills the beneficial bacteria in your filter.
- Medication dosing: Fish medications are dosed by volume, and overdosing can be toxic — especially to invertebrates and scale-less fish. Always use the calculated working volume, not the sticker size, when treating ich, velvet, or bacterial infections.
- Fertiliser dosing: Planted-tank fertilisers (Estimative Index, low-tech regimes) are dosed by volume. A 10% error in your volume estimate translates to a 10% error in nutrient levels, which can swing a balanced tank into algae or nutrient deficiency.
- Heater sizing: The standard rule is 3–5 watts per gallon. A 20-gallon tank needs a 60–100 watt heater. Using real volume rather than nominal size ensures your heater can actually maintain temperature in winter.
- Filtration turnover: A filter should turn over 4–10 times the tank volume per hour (a 20-gallon tank needs a filter rated 80–200 gph). Using real volume ensures you’re not under-filtering.
- Stocking density: The stocking calculator uses your real water volume as the input for bioload, surface-area, and terrain calculations. An accurate volume figure is what makes its stocking recommendations trustworthy.
The pattern is simple: every aquarium decision is a function of volume. Get the volume right once — using the calculator above — and every subsequent decision gets easier and more accurate. Get it wrong, and you’ll be fighting water-quality, dosing, and stocking problems for as long as you own the tank.
Frequently Asked Questions
Should I measure inside or outside the glass?
Outside is easier and is what most people do; the calculator’s result is then the geometric maximum. For a more precise figure, measure inside the glass or subtract roughly twice the glass thickness (commonly 6–12 mm / 0.25–0.5 in per pane) from each dimension.
Why is my real water volume lower than the calculator says?
Glass thickness, substrate, hardscape and equipment all displace water, and most tanks aren’t filled to the brim. Subtracting 10–15% from the geometric volume of a decorated, planted tank is a reliable working estimate.
Are these US gallons or UK (imperial) gallons?
US gallons (3.785 litres). UK imperial gallons are larger (4.546 litres), so if you’re comparing against an imperial figure, the litre value is the safest common ground.
What fill level should I use?
80–90% suits most setups: it leaves space for gas exchange, jumpy fish, and equipment. Open-top planted tanks often run lower; tightly covered tanks can run higher.
Does tank shape change how many fish I can keep?
Yes — significantly. Two tanks of identical volume can have very different footprints, and footprint (swimming terrain) often matters more than raw litres, especially for active or bottom-dwelling species. That’s exactly why our stocking tool considers terrain and bioload, not just volume.
How do I calculate the volume of a cylinder or corner tank?
The current calculator supports rectangular, bowfront, and hexagonal shapes — the three most common aquarium footprints. Cylindrical (pillar) tanks and corner (pentagonal) tanks use different geometry. For a cylinder, volume = π × radius² × height; for a corner pentagon, treat it as half a hexagon and use half the hex area formula. We’re working on adding these shapes to the calculator in a future update.
How accurate is the bowfront calculation?
The two-thirds bow-depth approximation is accurate to within 2–3% for typical bow curves (bow depth 4–8 inches on a 30–36 inch tank). For extreme bows (very deep curves on short tanks), the approximation underestimates volume slightly. For practical purposes — dosing, stocking, and equipment sizing — the accuracy is more than sufficient.
Can I use this calculator for a pond?
Yes — the rectangular and hexagonal calculations work for any water-holding vessel of those shapes, including ponds. For irregular pond shapes (kidney, freeform), measure the average length, width, and depth and treat the pond as rectangular for an approximate volume. Pond volume is particularly important for medication dosing and pump sizing.
How do I account for water displacement from substrate and rocks?
Substrate displaces roughly 1 litre per 1.5 kg of substrate (gravel/sand is denser than water). A 2-inch (5 cm) sand bed in a 20-gallon long displaces about 1.5 gallons. Large rocks and driftwood displace their full volume — a fist-sized piece of dragon stone displaces about 0.5 litres. For most planted tanks, a flat 10–15% reduction from the calculated geometric volume accounts for all displacement accurately.
What’s the difference between rated volume and real volume?
Rated (nominal) volume is the manufacturer’s label — a “20-gallon” tank. Real volume is the actual water the tank holds when filled to a normal level (80–90% of height). Real volume is always 10–20% lower than rated volume because of the air gap at the top and glass thickness. Always use real volume for dosing, stocking, and equipment sizing — this is what the calculator reports.