An aquarium volume calculator tells you exactly how many gallons your tank holds, so you can dose water conditioner correctly and plan the right water change. Knowing exactly how many gallons your aquarium holds makes everything else easier: dosing a water conditioner, picking a filter, and working out how much water to swap each week. Tank labels are often rounded, so measuring is more reliable than trusting the sticker.
Enter your tank’s water dimensions below. The calculator shows the water volume, the amount to replace for your chosen water change size, and how much the water weighs.
Aquarium volume & water change calculator
Works for rectangular and cylinder tanks, in inches or centimeters.
- Water in your tank
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- Replace each time
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- Water weight
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Results are estimates. Water weight does not include the tank, stand, substrate or decor.
Next steps: dose your water conditioner for the amount you’re replacing, and choose a filter rated for your tank’s volume.
How the Aquarium Volume Calculator: works
For a rectangular tank, multiply length by width by water height. The result is a volume in cubic inches or cubic centimeters, which is then converted:
- Inches to US gallons:
length × width × height ÷ 231 - Centimeters to liters:
length × width × height ÷ 1,000 - Cylinder tanks: use
π × (diameter ÷ 2)² × heightin place of length × width × height.
One US gallon is about 3.785 liters, and a gallon of water weighs about 8.34 pounds. A liter of water weighs about 1 kilogram.
Use the height of the water, not the glass. Most tanks are filled an inch or so below the rim, and substrate, rocks and decor take up space, so your real water volume is lower than a to-the-rim calculation. The substrate option above lets you account for that.
Common tank sizes and their volume
Tanks are sold under rounded labels. This table shows typical outside dimensions and the volume you get when you calculate all the way to the rim.
| Tank label | Typical size (L × W × H, in) | Calculated to the rim |
|---|---|---|
| 10 gallon | 20 × 10 × 12 | about 10.4 gal |
| 20 gallon long | 30 × 12 × 12 | about 18.7 gal |
| 29 gallon | 30 × 12 × 18 | about 28.1 gal |
| 40 gallon breeder | 36 × 18 × 16 | about 44.9 gal |
| 55 gallon | 48 × 13 × 21 | about 56.7 gal |
| 75 gallon | 48 × 18 × 21 | about 78.5 gal |
Exact dimensions vary by manufacturer, and glass thickness and water level reduce the real volume. Treat these figures as a guide, and measure your own tank for accuracy.
Planning for a specific fish? Our Blue Acara care guide includes tank size recommendations for single fish and pairs, and our freshwater aquarium setup guide covers choosing a tank for beginners.
How much water should you change?
For many freshwater aquariums, a partial water change of roughly 20–30% each week is a practical starting point. The right amount depends on your stocking level, feeding, filtration and test results, so adjust it based on what your tests show.
- 10% suits lightly stocked tanks or a quick top-up between larger changes.
- 25–30% is a common weekly routine for many community tanks.
- 50% can be useful when nitrate has climbed, but avoid replacing all of the water at once.
Match the temperature of the replacement water to the tank and treat it with a conditioner that handles chlorine and, where present, chloramine. Follow the product label for the volume of water you’re treating. For more on water quality and general aquarium management, the Merck Veterinary Manual is a trusted reference. To test before you change, see our guide to aquarium water quality, and for the cleaning routine itself, read how to clean a fish tank.
Frequently asked questions
How do I calculate how many gallons my aquarium holds?
Measure the length, width and water height in inches, multiply them together, then divide by 231. For example, a tank that is 36 × 18 × 16 inches holds about 44.9 gallons when filled to the rim.
Should I measure to the top of the tank or the water line?
Measure to the water line. That gives the volume of water you actually have, which is the number that matters for dosing and water changes.
Does substrate reduce the water volume?
Yes. Substrate, rocks, driftwood and decorations displace water, so your tank holds less than the raw calculation suggests. Use the substrate and decor option in the calculator to subtract an allowance.
How do I convert gallons to liters?
Multiply US gallons by 3.785 to get liters. To go the other way, divide liters by 3.785.
How much does an aquarium weigh when it’s full?
Water alone weighs about 8.34 pounds per US gallon, so a 40-gallon tank holds roughly 334 pounds of water. Add the weight of the glass, stand, substrate and decor, and make sure your stand and floor can support the total.