How this instrument works
A rectangular aquarium's water volume is simply length times width times height, all measured in inches, which gives a figure in cubic inches. Dividing by 231 — the exact number of cubic inches in one US gallon — converts that box measurement into the gallons every stocking chart, filter rating and medication label is written against.
This brim-full figure is a starting point, not the final answer. Substrate, rockwork, driftwood and any internal equipment all displace water, and no tank is ever actually filled to its rim, so the true volume of water sitting in a running aquarium typically runs five to fifteen percent below the number this instrument returns. Treat it as the upper bound for dosing and stocking math, then adjust down for your specific setup.
Manufacturers often round a tank's advertised size to the nearest common figure — a '55-gallon' tank and a '50-gallon' tank can share nearly the same footprint once you measure the actual glass. Measuring your own tank in inches and running the numbers here gives a figure specific to your setup rather than a marketing label, which matters most when a medication or water conditioner's dosing instructions are written strictly per gallon.
- Enter Tank length, width and height in inches, measuring the inside of the glass rather than the outer frame if you can.
- Read Water volume (US gallons) — the brim-full capacity of a rectangular tank at those dimensions.
- Subtract roughly 5-15% for substrate, rockwork and decorations to estimate the water you'll actually be dosing or stocking against.
- Use the gallons figure to size a filter or heater, or to calculate per-gallon doses of conditioner or medication.
- This formula assumes straight, right-angled glass sides — bowfront, cylinder and hexagonal tanks need a different volume formula.
Worked example — a 36 x 18 x 18 inch tank
Enter 36 inches for length, 18 inches for width and 18 inches for height — the footprint of a common 75-gallon-class tank. Multiplying gives 36 x 18 x 18 = 11,664 cubic inches of internal volume.
Dividing by 231 cubic inches per gallon, Water volume reads 50.49 US gallons. That is the brim-full figure; once you subtract a couple of inches of gravel and some rockwork, the actual water a canister filter or heater needs to handle for this tank lands closer to 45-48 gallons.
Questions
How many gallons is a 36x18x18 aquarium?
It holds about 50.49 US gallons brim-full: (36 x 18 x 18) / 231 = 50.49. That 36-by-18-inch footprint with an 18-inch height is a common size sold as part of the '75-gallon-class' family of tanks, though actual water volume runs a bit lower once substrate and decor are in place.
Why doesn't this match the gallon size printed on my tank?
Retail tank sizes are rounded to a familiar marketing number, and printed labels usually describe the tank family rather than its exact interior volume. Glass thickness alone can shave several percent off the interior dimensions this instrument needs, so measuring your own tank's inside dimensions in inches will usually give a more accurate figure than the number on the box.
Should I measure the inside or outside of the glass?
Measure the inside, where the water actually sits. Using the outside dimensions of the tank overstates the true water volume, because it includes the thickness of the glass panels themselves — typically a quarter- to half-inch difference per side on larger tanks, enough to shift the gallon figure noticeably.
Does this number include substrate and decorations?
No — this is the brim-full volume of an empty rectangular box. Gravel, sand, rockwork, driftwood and any internal filter housing all displace water, so the true volume once your aquarium is fully set up typically runs five to fifteen percent below this figure. For dosing purposes, many aquarists simply round down.
Can I use this for a bowfront or cylinder tank?
Not directly. This formula only applies to rectangular tanks with flat, right-angled glass panels. A bowfront, hexagonal, or cylindrical tank has a curved or angled cross-section and needs a volume formula built around that shape, such as a cylinder's pi times radius squared times height.
Why does my aquarium's gallon figure matter for dosing?
Water conditioners, medications and fertilizers are almost always dosed per gallon of actual tank water, and both filters and heaters are rated by the gallon range they can handle. Under-dosing wastes product; over-dosing a medication meant for a smaller volume can harm fish, so a reasonably accurate gallon figure protects both your wallet and your livestock.