🐠 Aquarium Water Volume Calculator

Enter your tank's inside dimensions to find its water volume in gallons and liters.

Length (in)

Width (in)

Height (in)

How the tank volume is worked out

The calculator treats the tank as a rectangular box: it multiplies the three inside measurements to get cubic inches, divides by 231 (the cubic inches in a US gallon) and converts to liters.

cubic inches = length × width × height
US gallons   = cubic inches / 231
liters       = gallons × 3.78541

The 36 × 18 × 12 in tank loaded by default is 7,776 cubic inches, or 33.7 gallons and 127.4 liters. That is the brim-full figure - the geometry of an empty box, not the water you will pour in.

Measuring, and what the number leaves out

Measure inside the glass. Panel thickness and trim cost half an inch an axis, which on this tank is over a gallon.
Use the water line, not the rim. Most tanks run 1 to 1.5 in short; filling this one to 10.5 in leaves 29.5 gallons.
Substrate displaces water. Two inches of gravel over a 36 × 18 in footprint is 1,296 cubic inches, about 5.6 gallons of space, two thirds of it solid grain.
Stocking. The one-inch-per-gallon line suits small community fish only. Adult size, bioload and filter turnover decide it, and one large fish is not ten small ones.

Fresh water weighs about 8.34 lb per gallon, so 33.7 gallons is roughly 281 lb before glass, stand, substrate and rock. Check what the floor and the stand carry before filling a tank this size.

Frequently asked questions

How many gallons is a 36 x 18 x 12 inch tank?

33.7 US gallons, or 127.4 liters, brim full. Fill it to 1.5 in below the rim and you have 29.5 gallons; add two inches of gravel and the real water volume lands nearer 26.

Does this work for a bowfront or hexagonal tank?

No - the formula is length × width × height, so it fits a rectangular box and overstates a bowfront. For a cylinder, work out π × radius² × height in inches and divide by 231 yourself.

Why does tank volume matter for dosing?

Medications and conditioners are dosed per gallon. A 25% change on 33.7 gallons is 8.4 gallons, but if your true volume is 26 that same jug is a 32% change and a third more dose than the label intends, so work from the filled figure.