Enter your fish tank's dimensions to get the exact volume in gallons and liters — the one figure that every stocking, dosing, and filtration decision depends on.
Volume alone doesn't keep fish alive — it's the input for four decisions that do.
How many fish your tank can hold depends on volume, surface area, and species — not just a flat "inch per gallon" rule.
Use the stocking calculatorFilters are rated by how many times they turn over your tank's volume per hour — get that wrong and waste builds up fast.
How the estimates workTreatments and dechlorinators are dosed per gallon or liter. An inaccurate volume means an inaccurate — and sometimes risky — dose.
Read the methodologyA weekly 20-30% water change is calculated straight from your real volume, not the number printed on the tank's box.
Plan around your stockingIt's exact for rectangular and cylindrical tanks, which cover most common sizes. For bow-front, hexagon, or corner tanks, use the rectangular option with your tank's widest measurements as a close estimate — actual volume will run 5-10% lower because of the curved or angled panels.
Measure the inside dimensions if you can reach them — glass thickness on larger tanks can shave off a few percent of volume. If you can only measure the outside, that's fine for planning purposes; just expect the real number to be slightly lower.
Manufacturer sizes are rounded and measured to the rim, but you never fill a tank completely to the top. Substrate, decorations, and equipment also displace water. Expect your real, usable volume to run 10-20% below the labeled size.
Use whichever matches your tape measure or spec sheet — the calculator converts either way. US tank manufacturers almost always list inches; most tanks sold outside the US are labeled in centimeters.
A small mismatch is normal and usually comes from rounded dimensions, glass thickness, or a tank shape that isn't a true rectangle or cylinder (bow-front, hexagon). If the gap is large, double-check that height was measured from the inside bottom to the rim, not the outside of the stand.
This calculator gives the tank's true geometric capacity to the rim, while manufacturers often round down or quote the volume at a safe fill line. A 10-15% difference between the two is typical and doesn't mean either number is wrong.
Volume is the input every other decision depends on: how many fish you can safely keep, how much water to change weekly, and how strong a filter or heater you need. Try the stocking calculator next, or browse the fish guides.