Measure your tank's dimensions to find the gallon volume
The fastest way to find your aquarium's gallon capacity is to measure the length, width, and height of the tank in inches, then use a straightforward formula. Measure from the inside of the glass — not the outside frame — because that is the actual water space. Write down all three numbers.
Once you have the measurements, multiply length × width × height. Take that result and divide it by 231. That final number is your tank's capacity in gallons. For example, a tank that is 20 inches long, 10 inches wide, and 12 inches tall would be (20 × 10 × 12) ÷ 231 = 10.4 gallons.
This formula works for rectangular tanks, which are the most common shape. If your tank is a different shape — cube, bow-front, or hexagon — the math changes slightly, but the measurement step is the same.
Key Takeaways
- Measure the inside length, width, and height in inches, then divide (length × width × height) by 231 to get gallons.
- Subtract 10 to 15 percent from the calculated volume to account for gravel, decorations, and the space the filter takes up.
- If your tank came with paperwork or a label, the manufacturer's stated capacity is usually accurate and saves you the math.
- Cylindrical tanks and corner tanks need different formulas; measure the diameter and height for a cylinder, or look up the shape-specific calculation online.
Check the manufacturer's label or paperwork first
Most aquariums come with a label on the back or bottom that states the gallon capacity. Check the glass itself, the frame, or any paperwork that came with the tank — this is often the fastest answer and is usually correct.
If you bought the tank used or the label has worn off, the original box or receipt may list the size. A quick search for the brand and model number online will often pull up the specifications as well. This route saves you from doing the math yourself.
Account for gravel, decorations, and equipment when calculating usable space
The gallon number you calculate is the total volume of the tank when it is completely full of water. In reality, you will lose space to gravel, rocks, plants, decorations, and the filter intake. Subtract 10 to 15 percent from your calculated volume to get a more realistic picture of the actual water you are holding.
For example, if your tank calculates to 20 gallons, subtract 2 to 3 gallons for substrate and décor. That leaves you with roughly 17 to 18 gallons of actual water. This matters because fish stocking guidelines and medication dosing are based on the true water volume, not the tank's theoretical capacity.
Use the formula for different tank shapes
Rectangular tanks use the straightforward formula above. For a cylindrical tank (like a drum or column), measure the diameter across the top and the height, then use this formula: (diameter ÷ 2) squared × 3.14 × height ÷ 231.
A cube tank is just a rectangular tank where all three sides are the same length, so the formula stays the same. Bow-front tanks (curved glass in front) and hexagon tanks are harder to calculate by hand because the shape is irregular. For these, your best option is to find the manufacturer's stated capacity or search for a calculator designed for that specific shape.
Corner tanks and other custom shapes usually come with a label, so check there first before attempting to measure.
Convert from liters if your tank was sold outside the United States
Tanks sold in Canada, Europe, and other countries are often labeled in liters rather than gallons. To convert liters to US gallons, divide the liter amount by 3.785. A 75-liter tank, for example, is about 19.8 US gallons.
If you are working with an imperial gallon (used in the UK and Canada for some products), note that an imperial gallon is larger than a US gallon — divide by 4.546 instead. This matters less for small tanks but becomes significant for larger setups.
Double-check your math with an online calculator
If you are unsure about your measurements or the formula, several free online aquarium calculators will do the math for you. Enter your tank's length, width, and height, select the shape, and the calculator returns the gallon capacity when ready. This is a good way to verify your hand calculation.
Keep your measurements written down for future reference — you will need the gallon capacity when buying filters, heaters, medications, or when stocking fish. Knowing the exact volume makes it easier to maintain the tank properly.
Frequently Asked Questions
What if my tank is not a standard rectangle?
Measure the diameter and height for cylinders, or look up the manufacturer's capacity for bow-front and hexagon tanks. Custom shapes are difficult to calculate by hand, so checking the label or original paperwork is usually faster and more accurate than trying to measure an irregular shape.
Should I fill the tank all the way to the top?
No. Most tanks are filled to about 1 inch below the rim to leave room for the filter intake and to prevent water from splashing out. This means the actual usable water is slightly less than the calculated capacity, which is why subtracting 10 to 15 percent is a good practice.
Does the thickness of the glass affect the gallon count?
Yes, slightly. Thicker glass takes up more interior space, which is why measuring from the inside of the glass matters. For most home aquariums, this difference is small — usually less than 1 percent — but it is why inside measurements are more accurate than outside ones.
Can I use this gallon number for fish stocking guidelines?
Use the adjusted number after subtracting for gravel and décor. Fish stocking rules like "one inch of fish per gallon" are based on actual water volume, not the tank's theoretical capacity. A 20-gallon tank with 3 gallons of substrate and decorations holds about 17 gallons of water for fish.
What if I measured in centimeters instead of inches?
Divide the result by 3,785 instead of 231. The formula stays the same (length × width × height), but the divisor changes because centimeters and milliliters require a different conversion factor to reach US gallons.