Aquarium Volume Calculator
Water volume applies a 10% reduction for gravel, rocks, and decor displacement.
US Gallons20.0
Liters75.5
Actual Water (gal)18.0
Cubic Inches4,608
Knowing a tank size in gallons drives stocking limits, heater wattage, filter turnover, and dosing of water treatments. This calculator multiplies the inside length, width, and height of a rectangular aquarium to get its cubic-inch volume, then divides by 231 cubic inches per US gallon. It also converts to liters and trims about 10% for the gravel, rock, and equipment that displace water below the rim.
Formula
gallons = (L x W x H) / 231; water = gallons x 0.90
- L
- Inside length, inches
- W
- Inside width / front-to-back depth, inches
- H
- Inside height, inches
- 231
- Cubic inches in one US gallon
How it works
- Measure the inside length, width (front-to-back depth), and height of the tank in inches and enter all three.
- The volume in cubic inches is divided by 231 to give US gallons, then multiplied by 3.7854 for liters.
- A 10% displacement allowance is subtracted to estimate the actual water the tank holds once substrate and decor are in place.
Worked example
A 24 x 12 x 16 inch tank, a common 20-gallon long footprint.
- Volume = 24 x 12 x 16 = 4,608 cubic inches.
- Gallons = 4,608 / 231 = 19.95 US gallons.
- Water = 19.95 x 0.90 = about 18 gallons after displacement.
About 19.9 gallons (75.5 liters), roughly 18 gallons of actual water.
Frequently asked questions
- Should I use inside or outside dimensions?
- Use the inside measurements of the glass for the most accurate water volume. Outside dimensions include the glass thickness and trim, which slightly overstate capacity, especially on small tanks.
- Why is the actual water volume lower than the gallon figure?
- Substrate, rocks, driftwood, and equipment take up space, and tanks are rarely filled to the very top. The 10% displacement reduction gives a realistic estimate of how much water you actually add and treat.
- How do I convert US gallons to imperial gallons?
- This tool reports US gallons (231 cubic inches each). One US gallon equals about 0.833 imperial gallons, so multiply the US figure by 0.833 if you need UK imperial gallons instead.
- Does this work for cylindrical or bow-front tanks?
- No. The formula assumes a rectangular box (length x width x height). Round, bow-front, or hexagonal tanks need a different volume formula based on their cross-sectional shape.