gallons to cubic feet calculator
A gallons to cubic feet calculator multiplies US gallons by 0.133681 (or divides by 7.48), so 100 gallons is 13.368 ft³ and 500 gallons is 66.840 ft³.
How to convert gallons to cubic feet
This gallons to cubic feet calculator multiplies US gallons by 0.133681 to give cubic feet, and by 3.78541 to give liters. The formula is cubic feet = gallons × 0.133681, which is the same as gallons ÷ 7.48052. Enter 100 gallons and you get 13.368 cubic feet and 378.5 liters.
Cubic feet are shown to three decimal places and liters to one. To get cubic yards, divide the cubic feet by 27.
Where 0.133681 comes from
The US gallon has a fixed size: exactly 231 cubic inches. A cubic foot is a 12-inch cube, or 1,728 cubic inches. Divide one by the other and you get 231 ÷ 1,728 = 0.133681. Picture a one-foot cube box. It would take seven full gallon jugs, plus an eighth jug poured almost halfway, to fill it.
The two directions are:
- Gallons to cubic feet: × 0.133681
- Cubic feet to gallons: × 7.48052
Worked examples
- Calculator default: 100 gallons
- Cubic feet: 100 × 0.133681 = 13.368 ft³
- Liters: 100 × 3.78541 = 378.5 L
That is about the size of a compact rain barrel setup or a large water heater.
- A 500-gallon water storage tank
- Cubic feet: 500 × 0.133681 = 66.840 ft³
- Liters: 500 × 3.78541 = 1,892.7 L
- Cubic yards: 66.840 ÷ 27 = 2.48 yd³
- Water weight: 500 × 8.34 = about 4,170 lb
That weight is why tank pads matter. Spread over a 6 × 6 ft pad (36 sq ft), the full tank presses down at about 116 lb per square foot.
Gallons to cubic feet reference table
| US gallons | Cubic feet | Liters | Typical example |
|---|---|---|---|
| 1 | 0.134 | 3.8 | Paint can |
| 5 | 0.668 | 18.9 | Bucket |
| 40 | 5.347 | 151.4 | Water heater |
| 55 | 7.352 | 208.2 | Steel drum |
| 100 | 13.368 | 378.5 | Rain barrel bank |
| 275 | 36.762 | 1,041.0 | IBC tote |
| 500 | 66.840 | 1,892.7 | Storage tank |
| 1,000 | 133.681 | 3,785.4 | Septic or cistern |
| 1,500 | 200.522 | 5,678.1 | Larger septic tank |
| 20,000 | 2,673.620 | 75,708.2 | Backyard pool |
How to use the conversion on a job
- Checking a tank rating. Measure the inside of a rectangular tank in feet, multiply length × width × height, and compare with the rated gallons converted here. Rated capacity often includes the space up to the brim, which you will not fill.
- Excavation and backfill. A buried 1,000-gallon tank displaces about 134 ft³ of soil, roughly 5 yd³, before you count its walls and bedding.
- Pool chemistry and fill. Pools are dosed by gallons but sized in feet. The pool gallons calculator goes from dimensions straight to gallons.
- Concrete and liquid mixes. Water, admixtures, and sealers come in gallons. Convert to cubic feet when you need to compare against a mixer drum or container size.
Common mistakes
- Using 7.5 or 8 as a shortcut. 7.5 is fine for rough estimates. 8 overstates gallons by about 7%, which matters when dosing chemicals.
- Mixing up US and imperial gallons. Imperial gallons are 20% larger. UK pump and tank specs usually use them.
- Confusing dry and liquid measure. Some potting mixes and grains are sold by the dry gallon or quart. This calculator uses the US liquid gallon.
Going the other way? Use the cubic feet to gallons calculator. For a round tank by its dimensions, use the tank volume calculator. For metric, see the gallons to liters calculator.
Frequently asked questions
How many cubic feet are in a gallon?
One US gallon is 0.133681 cubic feet. It works the other way too: a cubic foot holds 7.48052 gallons. The gallon is defined as exactly 231 cubic inches, and a cubic foot is 1,728 cubic inches, so 231 ÷ 1,728 = 0.133681.
Why do tank specs mix gallons and cubic feet?
Liquid capacity is sold and dosed in gallons, but the space a tank takes up, and the soil or gravel around it, is figured in feet. A 275-gallon tote holds 36.762 ft³ of liquid, which lets you compare it with a measured opening or a pallet footprint.
Is this the US gallon or the UK imperial gallon?
The US gallon, 3.78541 liters. The imperial gallon used in the UK is 4.54609 liters, about 20% larger, and equals 0.160544 ft³. If a spec sheet is British, convert imperial gallons to liters first.
How much does a cubic foot of water weigh?
About 62.4 lb. A US gallon of water weighs about 8.34 lb, and 7.48 gallons × 8.34 lb is roughly 62.4 lb. So 100 gallons, or 13.4 ft³, of water weighs about 834 lb.
What's the reverse conversion?
Multiply cubic feet by 7.48052 to get gallons. A 4 × 2 × 2 ft trough is 16 ft³, which is 16 × 7.48 = 119.7 gallons. The cubic feet to gallons calculator does it for you.
Estimates only. Verify quantities with your supplier before purchasing.