Aquarium Calculator

Everything about an aquarium starts with its real water volume, and that is not the number on the box. A "20 gallon" tank holds about 20 gallons of glass, but once you subtract the inch below the rim and the gravel, rock and wood inside it, the actual water is closer to 17.

That matters because every other figure is derived from it. Heater wattage, filter flow, dosing, water changes and stocking all scale from water volume, so starting from the marketing number puts everything else out by ten percent or more.

Heater sizing is the part most guides oversimplify. The old "5 watts per gallon" rule ignores the thing that actually decides the load: how far the room drops below your target temperature. A tank in a warm room barely needs a heater; the same tank in a cold garage needs two.

1. Step 1 Water volume gallons = length × width × water_height ÷ 231 231 cubic inches per US gallon. Measure inside the glass and use water height, not tank height.
2. Step 2 Deduct for substrate and decor net = gross × 0.9 Ten percent is typical for a normally-decorated tank.
3. Step 3 Heater wattage watts = gallons × watts_per_gallon(temperature_gap) 2.5 W/gal for a 5 °F gap, 3.5 for 10 °F, 5 for 15 °F, 6.5 above that.
4. Step 4 Filter flow GPH = gallons × turnover, then ÷ 0.65 for the rated figure Filters are rated empty; media cuts real flow by about a third.
5. Step 5 Stocking rule of thumb inches_of_fish ≈ gallons A crude starting point that fails for heavy-waste species.

Example: a 30 × 12 × 16 in tank holds 24.9 gallons of glass, or 22.4 with decor. In a room that drops to 68 °F with a 78 °F target, that is a 10-degree gap, needing 78 W — so a 100 W heater. At 5× turnover it needs 112 GPH of real flow, meaning a filter rated around 172.

• Volume uses internal dimensions and water height rather than tank height. Measuring the outside of the trim overstates volume by several percent.
• The 10% displacement allowance covers a normal gravel bed and moderate decor. A heavily aquascaped tank with deep substrate and large hardscape can lose 20% or more.
• Heater wattage is based on the gap between the coldest room temperature and the target, which is what actually determines the load. Tanks in unheated garages, conservatories or basements need substantially more than the old flat watts-per-gallon rules suggest.
• Above a 15 °F gap, two smaller heaters are safer than one large one — a stuck thermostat cannot overheat the tank as fast, and a failure leaves partial heating rather than none.
• Filter ratings are manufacturer figures for an empty chamber. Real flow with media installed is typically 30 to 40 percent lower, which is why the rated figure shown is higher than the target turnover.
• The one-inch-per-gallon stocking rule is a crude heuristic included because people search for it. It is roughly usable for small slim community fish such as tetras and rasboras, and badly wrong for goldfish, plecos, cichlids and anything with a heavy bioload.
• Nothing here models the nitrogen cycle, water chemistry, species compatibility or minimum tank sizes for particular fish — all of which matter more than volume alone.
• Marine and reef tanks have different requirements for turnover, flow and equipment, and this calculator is aimed at freshwater.