Article — Gallons to Cubic Feet Converter
Gallons to cubic feet: the exact factor is 7.480519
1 cubic foot equals 7.480519 US liquid gallons exactly, and 1 US gallon equals 0.133681 cubic feet. The conversion comes straight from the cubic-inch definitions: a cubic foot is 12³ = 1728 in³ and a US gallon is fixed at 231 in³ by congressional statute, so the ratio 1728 ÷ 231 = 7.480519 is a legal exact value rather than a measurement. Imperial UK gallons are larger: 1 ft³ = 6.22884 imperial gallons.
The calculator above does both directions. The article below explains where the constants come from, which gallon system to use, and how to apply the conversion to common tank, pool, and HVAC volume problems.
What is gallons to cubic feet conversion?
Gallons and cubic feet are both units of volume in the US customary system. Gallons are usually used for liquid capacity — fuel, water, paint, beverages. Cubic feet are used for solid volumes and dimensional spaces — tank interiors, refrigerator capacity, room air volume. Converting between them is a matter of one exact constant.
The exact constant exists because Congress fixed both units geometrically. The yard was tied to the meter in 1959 at 0.9144 m exactly; that fixed the foot at 0.3048 m. The US liquid gallon was fixed at 231 cubic inches by the 1707 Queen Anne wine gallon statute, retained by the US Congress in 1836. Those two definitions give the exact ratio.
The gallons to cubic feet formula
Two formulas, exact constants, no rounding:
ft³ = gallons / 7.480519 (gallons → ft³)gallons = ft³ × 7.480519 (ft³ → gallons)ratio 1728 in³ / 231 in³Worked examples: 50 US gallons ÷ 7.480519 = 6.684 ft³. 10 ft³ × 7.480519 = 74.81 gallons. The factor 7.480519 is sometimes rounded to 7.48 in informal use, which introduces an error of 0.07%. Use the full constant for permits, engineering drawings, and trade documentation.
US vs imperial gallons: which one?
The word "gallon" means different volumes in different countries. The US liquid gallon is 231 in³ = 3.78541 L. The imperial gallon used in the UK, Ireland, Canada (historically), and some Caribbean countries is 4.54609 L — about 20% larger. The conversion factor changes accordingly: 1 ft³ = 7.48052 US gallons but 1 ft³ = 6.22884 imperial gallons.
The US "dry gallon" exists as a separate unit equal to 268.8025 in³, used historically for grain and produce, but it is rarely seen today. The 231 in³ figure for the US liquid gallon came from Queen Anne's 1707 wine gallon statute. Britain adopted the larger 4.54609 L imperial gallon in 1824, but the US kept the older wine gallon. Two centuries later, the historical accident still confuses fuel and beer measurements worldwide.
If your source is American (US plumbing codes, US car manuals, US recipes), use the US conversion. If it is British, Canadian, or older Australian (pre-1970 metrification), check for imperial gallons. This calculator handles US gallons by default; flip the system if you are working from UK sources.
Tank volume in gallons and cubic feet
Tank manufacturers quote capacity in gallons; physical dimensions come in feet. Converting between the two lets you check whether a tank fits the available space and plan supporting structures.
- 50 gal water heater: 6.68 ft³ — typical residential storage
- 275 gal home heating oil tank: 36.77 ft³ — standard basement tank
- 500 gal propane tank: 66.84 ft³ — 9.5 ft long × 3 ft diameter cylinder
- 1,000 gal septic tank: 133.68 ft³ — minimum US residential code
- 10,000 gal fuel storage: 1,336.81 ft³ — small commercial
- 1,000,000 gal water tower: 133,681 ft³ — municipal scale
For a rectangular tank, multiply length × width × height in feet to get cubic feet, then multiply by 7.480519 to get gallons. A 4 ft × 3 ft × 2 ft tank: 24 ft³ × 7.48 = 179.5 gallons. For a cylindrical tank, use π × r² × h, then convert.
Pool and aquarium volume
Swimming pool chemistry (chlorine, pH buffer, algaecide dosing) is calculated per gallon. Pool dimensions are quoted in feet. A 20 ft × 40 ft rectangular pool averaging 5 ft deep holds 4000 ft³ = 29,920 gallons. Dose calculations multiply that volume by the chemical's per-gallon spec.
For round above-ground pools: gallons = π × r² × depth × 7.48. A 24 ft diameter, 4 ft deep pool: 3.14 × 144 × 4 × 7.48 = 13,529 gallons. Most chemical labels list dose rate per 10,000 gallons; divide the pool volume by 10,000 to find the multiplier.
Aquarium volumes go the other way more often. A 55 gallon long tank measures 48 in × 13 in × 21 in = 7.58 ft³ = 56.7 gallons (close, since glass walls take up some volume). Aquarium pump and filter ratings are gallons-per-hour; converting tank ft³ to gallons gives the turnover target.
HVAC and plumbing system volume
Closed-loop HVAC systems (chilled water, hot water, condenser water) need accurate volume estimates for expansion-tank sizing, glycol dosing, and pump selection. System volume in gallons is calculated from pipe length, pipe diameter, and equipment capacities — often combined across cubic-foot tank specs and gallon-per-foot pipe charts.
A 4 in schedule 40 pipe holds 0.661 gallons per linear foot. A 200 ft pipe run holds 132.2 gallons = 17.68 ft³. Expansion tank sizing typically uses 2% of total system volume as the expansion allowance, which is calculated in gallons but stocked in tanks rated by ft³.
Mental math shortcuts
Quick approximation: 1 ft³ ≈ 7.5 gallons. A 100 ft³ tank holds about 750 gallons. The error from 7.5 vs 7.480519 is 0.26% — irrelevant for capacity estimates, too much for trade.
Reverse: 1 gallon ≈ 0.134 ft³. 1000 gallons fits in about 134 ft³. For really rough mental math, 1000 gallons ≈ 130 ft³ works to within 3%.
Cubic feet (ft³) is volume; square feet (ft²) is area. A 100 ft² room with 8 ft ceilings has 800 ft³ of air volume, but only 100 ft² of floor space. HVAC airflow specs are in cubic feet per minute (CFM); flooring sales are in square feet. Mixing them up is a common source of pricing and sizing errors in renovation projects.
Common mistakes
Using imperial gallons in a US context. A 20% error propagates through every downstream calculation. US fuel pumps, water bills, and pool chemicals all use US gallons. Double-check non-US sources.
Rounding 7.480519 to 7.5 in trade documents. The 0.26% error sounds small but multiplies up. A 100,000 gallon municipal tank specified at 7.5 instead of 7.480519 reports 260 fewer ft³ than reality.
Forgetting wall thickness. A tank "holding 100 gallons" usually means usable interior volume. The external footprint in cubic feet is larger because of wall thickness. For space planning, ask the manufacturer for exterior dimensions, not capacity.