Article — Cubic Feet to Gallons Converter
Cubic Feet to Gallons: Pool, Tank, and Water-Bill Math
One cubic foot equals 7.48052 US gallons exactly. The factor is built into the unit definitions: a US gallon is 231 cubic inches, a cubic foot is 1,728 cubic inches (12³), and 1,728 ÷ 231 = 7.480519481. To convert cubic feet to gallons, multiply by 7.48052. The reverse: gallons × 0.13368 = cubic feet. For imperial gallons (UK, Canada), the factor is 6.22884.
This conversion shows up on every US water bill (most utilities meter in cubic feet), every pool fill estimate, every aquarium spec sheet, and most HVAC design documents. The math is fixed; the only variable is whether you are using US gallons or imperial gallons, which differ by about 20%.
The exact cubic feet to gallons factor
1 ft³ × 7.48052 = 7.48052 US gallons. For most household and trade work, 7.48 is precise enough — the rounding error is 0.007% (one gallon per 14,000). For very large volumes or for billing-quality math, use 7.48052. The full extended value is 7.4805194805…, a repeating decimal because 1,728 ÷ 231 has a non-terminating expansion.
The reverse direction, gallons to cubic feet, multiplies by 0.133681 (or divides by 7.48052). So 100 US gallons takes up 13.37 ft³. That is roughly the size of a low-profile chest freezer or a large kitchen pantry. A 55-gallon drum takes 7.35 ft³; a typical aquarium called a "10-gallon tank" holds 1.34 ft³ of water.
The US gallon was set at exactly 231 cubic inches by the British Wine Gallon Act of 1706 during Queen Anne's reign. The number had no scientific basis — it matched a standard wine cask of the era. The US kept that number after independence, while the UK redefined its imperial gallon in 1824 to a more "scientific" 10 pounds of water at 62 °F (about 277.42 in³). That single change made the imperial gallon 20% larger than the US gallon.
Cubic feet to gallons for pools
For a rectangular pool: length × width × average depth (all in feet) gives volume in cubic feet. Multiply by 7.48 for US gallons. A 16 × 32 ft pool that averages 5 ft deep holds 16 × 32 × 5 = 2,560 ft³ = 19,148 gal. An above-ground pool 15 ft across × 4 ft deep is (π × 7.5²) × 4 = 707 ft³ = 5,287 gal.
The trick on most in-ground pools is the average depth. A pool that runs 3 ft at the shallow end and 8 ft at the deep end averages (3 + 8) / 2 = 5.5 ft — but only if the slope is uniform. Pools with a flat-bottom shallow zone and a sloped deep end (the "lazy L" profile) need the depth weighted by area, not just averaged. For a quick estimate, use the simple average and add 5%.
Cubic feet to gallons for aquariums
Aquarium tanks are sold by their rated gallon capacity, which is essentially the dry internal volume converted to gallons. A 55-gallon tank measures roughly 48 × 13 × 21 inches = 13,104 cubic inches = 7.58 ft³ = 56.7 US gal (the slight extra over 55 is glass wall thickness on the listed exterior dimensions). When filled, gravel, plants, equipment, and a 1-2 inch water-level setback bring the actual water volume to about 50 gallons.
For stocking calculations, fish-keepers commonly use the rated capacity minus 10-15%. So a "75-gallon community tank" hosts fish loadings based on 65 gallons. Heater sizing uses the rated capacity (watts per gallon is a rough guide: 3-5 W/gal). The cubic-foot number matters for floor-load calculations: water weighs 62.4 lb/ft³, so a 75-gallon tank carries about 625 lb of water, plus 80 lb of tank glass and decor.
CCF, ft³, and your water bill
Most US water utilities meter in cubic feet and bill in CCF — hundred cubic feet. 1 CCF = 100 ft³ = 748.052 US gallons. So a typical residential bill showing 8 CCF means 800 ft³ or 5,984 gallons used during the billing cycle. A 4-person household in the US averages 9-15 CCF per month, depending on irrigation and climate.
Some West Coast utilities bill in HCF (same thing as CCF — hundred cubic feet, just written differently) or in 1,000-gallon increments. The conversion is the same: 1 HCF = 1 CCF = 748 gal. If your bill quotes "thousand gallons," divide your CCF reading by 1.336 to get the equivalent number of kilogallons (or simply multiply CCF by 0.748 to get thousands of gallons).
A running toilet wastes about 200 gallons per day, or 27 ft³ — roughly 0.27 CCF. Left for a full 30-day billing cycle, that is 8.1 CCF or 6,055 gallons added to your bill. At $5 per CCF (the US average), the running flapper costs you $40 a month until you fix it.
US vs. imperial gallons
The biggest source of cubic-feet-to-gallons errors is mixing US and imperial gallons. 1 ft³ converts to 7.48052 US gallons but only 6.22884 imperial gallons — a 20% difference. UK water bills, Canadian utility bills, and product specs sold into Commonwealth markets all use imperial. US specs use the US gallon. Marine and aviation fuel pricing tends to use imperial gallons even in US ports because of historical contract conventions.
A 55-US-gallon drum is not the same as a 55-imperial-gallon drum. The US version holds 7.35 ft³; the imperial would hold 8.83 ft³ (208 vs. 250 liters). When buying drums for import or export, always confirm which gallon. The same is true for fuel tanks on boats and aircraft — fueling stations in the UK will dispense in imperial gallons or in liters; in the US, US gallons.
Cubic feet vs. CFM in HVAC
HVAC engineers care about both. Cubic feet measures the air volume in a space; CFM (cubic feet per minute) measures the flow rate through ducting or a vent. A 12 × 12 × 8 ft room contains 1,152 ft³ of air. The standard 4 air-changes-per-hour rule for ventilation means (1,152 × 4) ÷ 60 = 76.8 CFM of fresh air must flow through it.
The gallons-per-minute analog for water plumbing follows the same logic: a 75-gallon water heater (10 ft³) drained at 5 GPM takes 15 minutes to empty. Conversion between flow rates: 1 CFM ≈ 7.48 GPM, so a 1 CFM air handler moves the equivalent of 7.48 gallons of "air-volume" per minute. That mental conversion is useful when comparing the air-handling and plumbing systems on the same building plan.
Cubic feet to gallons for non-rectangular tanks
For a vertical cylindrical tank (water heater, propane bottle, well pressure tank): V = π × r² × h, all in feet. A 30-inch-diameter water heater 5 ft tall has V = π × (1.25)² × 5 = 24.5 ft³ = 183 gal. The rated capacity will be smaller because the calculation assumes 100% fill; tanks reserve top space for thermal expansion.
For spherical tanks (large propane bullets, some industrial process tanks): V = (4/3) × π × r³. A 6-ft-diameter sphere = (4/3) × π × 3³ = 113 ft³ = 845 gal. For a horizontal cylinder partially filled, the math gets messier — you need the chord length, the height of the fill, and either an integral or a precomputed chart. Most propane suppliers publish dipstick charts for common tank sizes.
A residential propane tank rated "500 gallons" is the water-capacity rating. The actual propane fill is 80% of that (400 gal) — the top 20% is reserved for thermal expansion. Water heater "60-gallon" ratings are the actual hot-water capacity, but useful daily output is more like 80% because of the cold-water inlet. Always check whether a stated gallon figure is the geometric volume or the working capacity.
Common cubic-feet-to-gallons mistakes
The biggest one is using 7.48 for everything when the spec is imperial gallons — that uses the wrong factor (should be 6.23) and creates a 20% error. Second is forgetting that pool average depth is not just (shallow + deep) ÷ 2 unless the slope is uniform. Third is treating CFM as if it were a volume; it is a flow rate per minute. Fourth is filling a tank to its listed water-capacity when the working fill is 80-90% of that.
- 1 ft³ = 7.48052 US gallons (exact)
- 1 ft³ = 6.22884 imperial gallons
- 1 US gallon = 0.13368 ft³
- 1 ft³ of water = 62.43 lb at 4 °C, 62.37 lb at 60 °F
- 1 CCF = 100 ft³ = 748.05 US gallons
- 1 acre-foot = 43,560 ft³ = 325,851 US gallons
- 55-gal drum = 7.35 ft³ at rated capacity
- Average US family uses ~9-15 CCF/month
- 231 cubic inches = 1 US gallon (by 1706 definition)
- 1,728 cubic inches = 1 cubic foot (12³)