How to calculate cubic feet for water
Water volume in cubic feet depends only on the shape and dimensions of the container. For rectangular tanks, multiply length × width × water height. For cylindrical tanks, use π × radius² × water height. Then convert to whatever unit your application uses — gallons for plumbing, liters for chemistry, pounds for load calculations.
One cubic foot of water always weighs ~62.43 lbs — useful for sizing supports.
Worked examples
Example 1: A 4 × 8 × 4 ft fish tank, full
4 × 8 × 4 = 128 ft³ = 957 US gallons = 7,991 lbs of water.
Example 2: A 6 ft diameter cylindrical tank, 5 ft of water
π × 3² × 5 ≈ 141.4 ft³ ≈ 1,058 US gallons.
Example 3: A 100 × 50 ft pond, 4 ft average depth
100 × 50 × 4 = 20,000 ft³ ≈ 149,610 US gallons.
Water unit reference
| Per 1 ft³ of water | Conversion factor | Worked example |
|---|---|---|
| US gallons | × 7.48052 | 7.48 gal |
| UK (imperial) gallons | × 6.22884 | 6.23 gal |
| Liters | × 28.3168 | 28.32 L |
| Quarts (US liquid) | × 29.9221 | 29.92 qt |
| Pounds (water) | × 62.43 | 62.43 lbs |
Tips and considerations
Use water level, not tank dimensions
Most tanks aren't filled to the brim. Measure the actual water height, not the tank wall height, for accurate volume.
Account for displacement
Decorations, gravel, plants, and equipment displace water. For a stocked aquarium, subtract about 10–15% from gross volume to get actual water volume.
Common mistakes to avoid
- Using cubic feet when gallons is expected. Water tanks, pools, and most plumbing specs are in gallons. 1 ft³ ≈ 7.48 US gallons.
- Forgetting that water weighs 62.43 lb/ft³. A 100-gallon fish tank weighs 835 lb of water alone, plus rocks, plants, and the tank itself — floor-load matters.
- Treating cubic meters and cubic feet as interchangeable. 1 m³ = 35.3 ft³. International tank specs are usually metric.
- Confusing capacity and usable volume. A “100-gallon” tank usually holds 90–95 gallons of usable water — the rest is unfillable outlets and freeboard.
Related concepts and calculators
Water volume connects to several practical calculators:
- Cubic feet to gallons — the standard US unit for water volume.
- Cubic feet to liters — the metric equivalent for international tanks and aquariums.
- Cubic feet to pounds — water weighs 62.4 lb/ft³, so floor and roof loads matter.
- Swimming pool calculator — handles pool shapes with chemical-dosing tables.
Frequently asked questions
Related calculators
Calculate cubic feet and gallons for rectangular, round, and oval pools.
OpenConvert cubic feet to US gallons. 1 ft³ ≈ 7.48 US gallons.
OpenConvert cubic feet to liters. 1 ft³ ≈ 28.3168 L.
OpenCalculate the cubic feet of a cylinder from radius or diameter and height. Mixed-unit support.
OpenCalculate the cubic feet of any rectangular box, room, or container using length, width, and height.
Open