Free Aquarium Volume calculator
Measure your tank once and get its real capacity in gallons and litres — for rectangular, cylinder and bow-front shapes — plus the true water volume after substrate and decor, updated live, as you type.
On this page15 sections
The standard glass box. Volume = length × width × height ÷ 231.
Estimates only, from the dimensions you enter — not a substitute for professional fishkeeping or veterinary advice. Confirm your true water volume before dosing. How accurate is this?
Results are estimates. Consult a professional.
How the aquarium volume calculator works
An aquarium holds water by volume, so the only thing that matters is the three-dimensional space inside the glass. This calculator multiplies your tank's measurements to get cubic inches, divides by 231 — the number of cubic inches in one US gallon — and converts the same figure to litres. It then estimates the true water volume, which is always a little less than the full capacity once gravel, rocks and decorations take their share.
Gross capacity versus true water volume
Two numbers do the work here. Gross capacity is the geometry — the water the empty tank would hold filled to the brim. Net, or true, water volume is what is left once substrate, hardscape and the inch or two of freeboard below the rim are subtracted. The calculator shows both, because the net figure is the one that matters for dosing and day-to-day care.
Gross capacity vs. true water volume
Most volume calculators stop at the gross figure. That is the trap. Your "20-gallon" tank rarely holds 20 gallons of water — once gravel, rocks, driftwood and the unfilled top are in place, the real volume is closer to 16 or 17 gallons.
The gap is not small. Industry calculators and tank sellers note that actual water volume usually runs about 85–90% of the calculated figure for a planted community tank, and lower still for a reef or heavily aquascaped setup where rock can take a fifth of the space. This calculator makes that adjustment visible with a displacement allowance you can dial from 0 to 30%.
A worked example using the aquarium volume calculator
Priya has a standard 30 × 12 × 12 inch rectangular tank — the classic "20-gallon long" footprint. She wants to know how much water it really holds so she can dose a water conditioner correctly after a change.
Step 1 — Find the cubic inches
Multiply the three dimensions: 30 × 12 × 12 = 4,320 cubic inches.
Step 2 — Convert to gallons and litres
Divide by 231: 4,320 ÷ 231 = 18.7 US gallons. Multiply by 3.785411784 for litres: 18.7 × 3.785 ≈ 70.8 litres. That is the gross capacity, full to the brim.
Step 3 — Subtract the displacement
Priya keeps a 2-inch gravel bed and a few rocks, so she sets the displacement allowance to 10%. True water volume = 18.7 × 0.90 = 16.8 gallons (about 63.7 litres). That is the figure she doses to.
How to calculate volume for each tank shape
The conversion to gallons is identical for every tank — cubic inches divided by 231. Only the way you find those cubic inches changes with the shape. The calculator handles the three most common designs.
Rectangular tanks
The simplest case, and the most common. Multiply length by width by height, all in inches, then divide by 231. A 24 × 12 × 16 inch tank is 4,608 cubic inches, or about 19.9 gallons.
Cylinder tanks
Round column tanks use the circle area times height: π × (diameter ÷ 2)² × height ÷ 231. A 20-inch-diameter, 24-inch-tall cylinder works out to about 32.6 gallons. Measure the diameter across the inside of the tank, not the outer rim.
Bow-front tanks
A bow-front has a flat back and a curved front that bulges outward. The calculator treats it as a rectangular body plus a half-ellipse bow section running the length of the tank, so you enter the flat-panel length, width and height plus the bow depth — how far the curve bulges past the sides at its deepest point. The curved front typically adds 10–15% over the plain rectangular figure.
Bulk Reef Supply's water volume calculator covers rectangular, cylinder and bow-front shapes and likewise flags that real water volume is reduced by sand, rock, equipment and the inhabitants themselves.Common aquarium sizes and their water volume
If you want a ballpark before you measure, this table lists the gross and net volumes for common rectangular tank footprints. Net figures assume a 10% displacement allowance for substrate and decor — set yours higher for a reef or planted tank with heavy hardscape.
| Tank (L × W × H, in) | Gross (gal) | Gross (L) | Net @ 10% (gal) |
|---|---|---|---|
| 20 × 10 × 12 (10-gal) | 10.4 | 39.3 | 9.4 |
| 30 × 12 × 12 (20-long) | 18.7 | 70.8 | 16.8 |
| 24 × 12 × 16 (20-high) | 19.9 | 75.5 | 18.0 |
| 36 × 18 × 18 (40-breeder) | 50.5 | 191.1 | 45.4 |
| 48 × 13 × 21 (55-gal) | 56.7 | 214.7 | 51.1 |
| 48 × 18 × 21 (75-gal) | 78.5 | 297.3 | 70.7 |
Gross volume = length × width × height ÷ 231; net = gross × 0.90. Nominal names (10-gal, 55-gal) are rounded marketing labels; the calculated capacity is the precise geometry.
Why true water volume matters for dosing treatments
Get the volume wrong and every treatment is wrong with it. Water conditioners, medications, fertilisers and pH adjusters are all measured per gallon of water, so an over-stated volume leads to an over-dose that can stress or kill fish — and an under-stated one to an under-dose that does nothing.
A common dechlorinator instructs 1 mL per 10 US gallons. Dose our 16.8-gallon example tank by its 20-gallon label and you add 2.0 mL where about 1.7 mL is needed — roughly 19% too much. The same trap hits a reef tank dosed by its glass capacity when rock fills a fifth of it: you over-treat the real water by that margin. The fix is to dose to the net figure from this calculator. For a water change, dose only the new water you are adding, not the whole tank.
Per-gallon dosing is standard across aquarium products — API's Tap Water Conditioner, for example, directs 1 mL per 10 US gallons of water being treated, which depends on actual water volume.The 1 inch of fish per gallon guide, and its limits
You will hear that a tank holds "one inch of fish per gallon." It is a handy first sniff-test for a beginner, but it is a rough rule of thumb, not a stocking law — and leaning on it too hard is how tanks get overcrowded.
- It ignores adult size. A one-inch juvenile can mature into a 12-inch fish. Stock to the adult length, never the size at the shop.
- It ignores body shape. A slim neon tetra and a chunky goldfish of the same length produce wildly different waste and need different space.
- It ignores bioload. A fish twice as long can have several times the body mass and waste output, which the filter has to keep up with.
- It only loosely fits small fish. The guide is least misleading for small, slim, schooling species in tanks of roughly 15 gallons or more.
Use the net water volume here as your starting denominator, then research each species' adult size, temperament and filtration needs. For stocking plans and any fish-health question, ask an aquatic veterinarian or an experienced fishkeeper. You can also size feeding and care for other pets with our dog food calculator and cat calorie calculator.
Aquarium Science details why the inch-per-gallon rule breaks down — body shape, adult size and waste production matter more than length — and proposes surface area and filtration as better stocking guides.How much volume do gravel and rocks take up?
Substrate and hardscape are the main reason the tank holds less water than the glass suggests. The deeper the bed and the more rock you add, the larger the gap between gross and net volume.
| What's in the tank | Rough volume lost |
|---|---|
| Gravel, 1 inch deep | about 3–5% |
| Gravel, 2 inches deep | about 6–10% |
| Sand, 1 inch deep | about 4–6% |
| Rock / hardscape (planted) | 5–10% on top of substrate |
| Live rock (reef tank) | 10–20% or more |
Typical ranges; exact loss depends on tank footprint and how much hardscape you use. Add the freeboard you leave below the rim — usually an inch or two — for the full picture.
For most community tanks, a 10–15% displacement allowance is realistic. A bare tank with a thin substrate can sit at 5%. A reef or heavily aquascaped tank stacked with rock can lose 20–25% or more, so set the slider to match what you keep.
Bulk Reef Supply and other aquarium references put displacement from substrate, rock and decor at roughly 10–25% of calculated volume, with reef tanks at the high end.Aquarium volume definitions
How accurate is this aquarium volume calculator?
The volume math is exact. Length times width times height divided by 231 is the precise gallon capacity of a rectangular tank, the cylinder uses π and the bow-front adds a standard half-ellipse approximation for the curve. If your measurements are right, the gross figures are right to the decimal.
The net water volume is an estimate, and on purpose. How much your gravel, rock and decor displace depends on your specific setup, so the displacement allowance is a planning figure rather than a measured one — adjust it to match your tank. For the most accurate net volume, measure to the real water line rather than the rim, and treat the net result as a close working estimate. This tool helps you plan water changes, dosing and stocking; it is not a substitute for professional fishkeeping or veterinary advice. For any fish-health concern, consult an aquatic veterinarian.
Frequently asked questions about the free Aquarium Volume calculator
About this Aquarium Volume calculator
This aquarium volume calculator runs entirely in your browser. Nothing you type is sent anywhere — the gross capacity and the true water volume are worked out on your device the moment you change a dimension, so you get an instant, private estimate for any tank shape.
It is one of our pet calculators, alongside the rest of our free calculators. Results are planning estimates for water changes, dosing and stocking — not a substitute for professional fishkeeping or veterinary advice.