Almost every decision in fishkeeping starts with one number: how much water is in the tank. Heater wattage, filter flow, medication doses, salt mixes and stocking levels are all calculated per gallon or per liter. Get the volume wrong by 20% and every one of those decisions is off by 20% too. Fortunately, the maths is simple once you know which measurements to take.
The basic formula for rectangular tanks
A rectangular tank is a box, so its volume is length × width × height. The only trick is converting the result into the unit you want:
- Inches → US gallons: divide cubic inches by
231. One US gallon is defined as exactly 231 cubic inches. - Centimeters → liters: divide cubic centimeters by
1,000. - Feet → US gallons: multiply cubic feet by
7.48.
Example: a standard 29-gallon tank measures 30 × 12 × 18 inches. That is 6,480 cubic inches, and 6,480 ÷ 231 = 28.05 US gallons. In metric, 76.2 × 30.48 × 45.72 cm ≈ 106,190 cm³, or 106.2 liters.
Unit conversions you will need
| From | To | Multiply by |
|---|---|---|
| US gallons | Liters | 3.785 |
| Liters | US gallons | 0.2642 |
| Imperial (UK) gallons | Liters | 4.546 |
| US gallons | Imperial gallons | 0.8327 |
| Cubic feet | US gallons | 7.481 |
| Cubic feet | Liters | 28.32 |
Tank sizes sold in the US, such as "20 long" or "75 gallon", are always US gallons. Product labels from the UK and Europe may use imperial gallons or liters, so check before scaling a dose.
How to measure a tank correctly
Measure with a rigid tape measure along the outside of the glass: length along the front, width from front to back, height from the bottom of the glass to the top of the rim (not including the stand). These outside dimensions are what manufacturers use, which is why calculators that only take outside dimensions overstate the water you actually have. Three things reduce it:
- Glass thickness. A 55-gallon tank usually has quarter-inch glass. Subtracting two walls from the length and width and the bottom pane from the height costs about 3.4 gallons.
- The water line. Nobody fills to the brim. An inch below the rim on a 48 × 13 inch tank is another 2.6 gallons.
- Substrate, rock and decor. A 2-inch gravel bed displaces about 5 gallons in a 55; a big rockscape can take several more.
Real water volume of a 55-gallon tank
| Stage | US gallons | Liters |
|---|---|---|
| Outside dimensions (48 × 13 × 21 in) | 56.7 | 215 |
| Inside the 1/4-inch glass | 53.3 | 202 |
| Filled 1 inch below the rim | 50.8 | 192 |
| With 2 inches of gravel | 45.6 | 173 |
| With rock and decor (about 5% displaced) | 43.3 | 164 |
So a "55" holds about 46 gallons once set up, and less still with a heavy rockscape. That is the number to use for medication and fertilizer. The FishGallons volume calculator does this automatically when you open Glass, water level & substrate.
Bow-front tanks
A bow-front tank has a curved front pane. Seen from above, the footprint is a rectangle (length × depth at the ends) plus a circular segment: the curve. Measure four things: the straight length, the depth at the ends, the depth at the center of the bow, and the height.
The extra depth at the center is the segment's height, s. With the length L as the chord, the arc's radius is r = (L²/4 + s²) ÷ 2s and its angle is θ = 2 × asin(L ÷ 2r). The segment area is r²/2 × (θ − sin θ). Add it to the rectangle and multiply by the height. For a 36-inch bow-front with 12-inch ends, a 16.5-inch center and 20 inches of height, that gives 46.9 gallons. A quick approximation for shallow bows is two-thirds of length × bow depth for the curved part. See the bow-front calculator.
Cylinder tanks
Round column tanks use π × radius² × height. Measure the diameter through the center and halve it. If you can only measure around the tank, divide the circumference by π (3.1416) to get the diameter. An 18-inch diameter tank that is 36 inches tall holds π × 9² × 36 = 9,161 cubic inches, or 39.7 gallons. Half-cylinder tanks hold half as much. Try the cylinder calculator.
Hexagon tanks
A regular hexagon with side a has an area of 2.598 × a². Measure a single panel from corner to corner; if that is awkward, opposite corners are exactly two side lengths apart. A hex tank with 12-inch panels and 24 inches of height holds 2.598 × 144 × 24 ÷ 231 = 38.9 gallons. Use the hexagon calculator.
Odd shapes: the bucket method
Corner pentagons, L-shaped tanks and custom builds are easier to measure than to calculate. Fill the tank with a bucket of known volume and count — a 2-gallon bucket filled 23 times is 46 gallons. Do it when you first set the tank up (with substrate and rock in place) and you have the exact working volume for life. A water meter on a hose is faster still.
Why the number matters
Most dosing mistakes come from using the box size instead of the real volume. Treating a 46-gallon tank as 55 is a 20% overdose of medication; the reverse can leave a heater or filter undersized. Once you know your real volume, carry it through: size your heater and filter, check your stocking, and write the number on a label inside the stand.