Cubic Feet to Gallons: Where 7.48 Comes From

A cubic foot and a gallon both measure volume, so converting between them should be simple — and it is, once you know the one number that links them. A cubic foot holds about 7.48 US gallons, or 6.23 imperial gallons if you are working in older UK units. The cubic feet to gallons calculator does the arithmetic in both directions and shows litres alongside, but this guide explains where that 7.48 comes from, why the US and imperial answers differ, and the everyday places this conversion actually turns up — from a water bill to a fish tank to a header of concrete.

What “cubic feet to gallons” really converts

A cubic foot is the volume of a cube measuring one foot on every side. A gallon is a fixed unit of liquid capacity. Neither depends on what is inside — a cubic foot of water, air, or gravel is the same amount of space — so the conversion is a pure geometry-to-capacity swap, not a physics problem. That is why the same factor works whether you are sizing a rainwater butt or reading a utility meter.

The one wrinkle is that “gallon” is not a single unit. The US liquid gallon and the imperial (UK) gallon are different sizes, and picking the wrong one is the single most common mistake here. A US gallon is 3.785 litres; an imperial gallon is 4.546 litres, roughly 20% larger. Because each imperial gallon is bigger, a given cubic-feet figure converts to fewer imperial gallons than US gallons. Everything below states which one it means.

How the conversion works

Start with the exact definitions. The international foot is exactly 0.3048 metres, so a cubic foot is 0.3048³ = 0.028 316 846 592 cubic metres, which is 28.316 846 592 litres (a litre is one-thousandth of a cubic metre). A US liquid gallon is defined as 231 cubic inches, which works out to exactly 3.785 411 784 litres. Divide one by the other:

1 ft³ ÷ 1 US gal = 28.316846592 L ÷ 3.785411784 L = 7.480519 US gallons

There is a tidier way to see the same number that avoids litres entirely. A cubic foot is 12 × 12 × 12 = 1,728 cubic inches, and a US gallon is 231 cubic inches by definition. So the factor is simply 1,728 ÷ 231 = 7.480519. That is the whole derivation — no measured constants, just two definitions divided. It is exact; the only rounding is in how many decimals you show.

The imperial gallon is not tied to cubic inches. It is defined directly as 4.546 09 litres (UK Weights and Measures Act 1985), so its factor comes back through litres: 28.316 846 592 ÷ 4.546 09 = 6.228 84 imperial gallons per cubic foot. The converter pivots every unit through litres for exactly this reason: one intermediate unit keeps cubic inches, cubic metres, US gallons and imperial gallons all mutually consistent.

Going the other way, invert the factor: one US gallon is 1 ÷ 7.480 52 = 0.133 681 cubic feet, and one imperial gallon is 0.160 544 cubic feet.

Worked example

Two everyday cases show the conversion running in each direction. Type the numbers into the cubic feet to gallons calculator to follow along.

A rectangular tank. A storage tank measures 4 ft long, 2 ft wide and 1.5 ft deep. Its volume is 4 × 2 × 1.5 = 12 cubic feet. In US gallons that is 12 × 7.480 52 = 89.77 US gallons; in imperial gallons, 12 × 6.228 84 = 74.75 imperial gallons; and 12 × 28.3168 = 339.8 litres. If the tank were cylindrical or oval rather than a neat box, the tank volume calculator would give you the cubic feet first, and you would convert from there.

A water bill. Suppose your meter shows 12 CCF used this month. A CCF is one hundred cubic feet, so that is 1,200 cubic feet. Multiply: 1,200 × 7.480 52 = 8,977 US gallons. That single conversion turns an abstract meter reading into a number you can actually reason about — more on utility units next.

Reading a water bill: CCF, HCF and the gallon

This conversion earns its keep on the utility bill. Many US water utilities meter usage in hundreds of cubic feet, printed as CCF (the first C is the Roman numeral for 100) or equivalently HCF (hundred cubic feet). One CCF is 100 cubic feet, which is 748 US gallons (100 × 7.4805). So if your bill jumps from 8 CCF to 14 CCF, that is a rise of 6 CCF — about 4,488 gallons — and now you can judge whether a leak or a filled pool explains it.

Utilities often price water per CCF, so the conversion also lets you translate a rate. If water costs, say, US$4.20 per CCF, that is 4.20 ÷ 748 = about US$0.0056 per gallon — useful when you are comparing the running cost of a gallon-rated appliance. Be careful with one lookalike: some gas utilities also bill in CCF, but there a cubic foot measures a volume of gas, not a liquid gallon, so the 7.48 factor does not apply to your gas usage.

Factors that change the answer

US gallon versus imperial gallon

This is the big one. The US liquid gallon (3.785 411 784 L) and the imperial gallon (4.546 09 L) differ by about 20%, so the same cubic-feet value gives two different gallon answers. A 1,000 ft³ pool is 7,481 US gallons but only 6,229 imperial gallons — a gap of over 1,200 gallons. Match the gallon to the context: US gallons for anything in the United States, imperial for older British figures. When in doubt, convert to litres, which is unambiguous.

Liquid volume versus gas volume

The 7.48 factor answers “how many gallons of liquid fill this space?” It does not convert a cubic foot of natural gas into gallons of anything meaningful, because gas volume depends on temperature and pressure and is billed as energy, not liquid capacity. If your bill lists therms or CCF of gas, that is a different calculation entirely. Use cubic-feet-to-gallons for liquids, tanks and containers.

The water-weight shortcut

A neat cross-check for water specifically: one cubic foot of water weighs about 62.4 pounds, and a US gallon of water weighs about 8.34 pounds. Divide and you get back the same factor — 62.4 ÷ 8.34 ≈ 7.48 gallons. This only works for water, whose density happens to sit near 62.4 lb/ft³; oil, milk or fuel weigh differently, but they all still occupy the same 7.48 gallons per cubic foot, because volume conversion ignores what is inside.

How many decimals to keep

For a mental estimate, 7.48 is plenty — it is accurate to better than one part in ten thousand. For a large volume where a fraction of a percent matters (sizing a commercial tank, reconciling a big water account), use 7.480 52. The difference between the two only shows up past the thousand-gallon mark: at 10,000 ft³ the rounded factor is off by about 7 gallons.

Converting by hand, quickly

You will not always have the converter open, so a few shortcuts help.

• Cubic feet to US gallons: multiply by 7.48. For a rough head figure, “seven and a half times” gets you within a percent.
• US gallons to cubic feet: divide by 7.48, or multiply by 0.1337. A 40-gallon water heater holds 40 × 0.1337 ≈ 5.35 cubic feet.
• Cubic feet to imperial gallons: multiply by 6.23 — a little over six.
• CCF to gallons: multiply the CCF reading by 748 for US gallons. One hundred cubic feet, 748 gallons.
• Anchor values worth memorising: 1 ft³ ≈ 7.48 US gal; 1 CCF = 748 US gal; 1 US gal ≈ 0.134 ft³.

Common mistakes

Mixing up US and imperial gallons

Grabbing 7.48 when the figure is actually in imperial gallons (or vice versa) introduces a 20% error — enough to badly mis-size a tank or misread a bill. If a source does not say which gallon it means, treat US as the default for anything American and imperial for older UK material, and sanity-check against litres.

Confusing cubic feet with square feet

Square feet measure area; cubic feet measure volume. You cannot convert square feet to gallons without a depth. A pool with a 300 ft² surface tells you nothing about its capacity until you multiply by average depth to get cubic feet — then the 7.48 factor applies.

Rounding too early on a big volume

Using 7.5 instead of 7.48 is fine for a bucket, but on a 50,000 ft³ reservoir it overstates the capacity by roughly 1,000 gallons. Carry the full factor through the calculation and round only the final answer.

Treating gas cubic feet as liquid

A cubic foot of natural gas on your energy bill is not 7.48 gallons of fuel. Gas is metered as a compressible volume and billed by energy content; the liquid-gallon conversion simply does not apply.

When the converter is not enough

The cubic feet to gallons calculator assumes you already have a volume in cubic feet. Getting to that figure is often the harder half:

• Irregular or curved containers. For a cylindrical drum, oval stock tank or capsule, work out the cubic feet with the tank volume calculator or the cylinder volume calculator first, then convert.
• Small kitchen or recipe volumes. If you are scaling between cups and gallons rather than cubic feet, the cups to gallons converter handles US, imperial and metric cups.
• Any other unit pair. To move between millilitres, fluid ounces, litres and gallons without touching cubic feet, the volume converter covers the full set in one place.
• Dry bulk materials. Cubic feet of gravel, soil or concrete are sold by weight or by the cubic yard, not by the gallon — the concrete calculator and gravel driveway calculator take volume to the units those trades actually use.

Frequently asked questions

What is a CCF on my water bill, and how many gallons is it? CCF stands for one hundred cubic feet (the C is the Roman numeral 100). One CCF equals 100 cubic feet, which is 748 US gallons. Some utilities print the same unit as HCF (hundred cubic feet) — it is identical.

How much does a cubic foot of water weigh? About 62.4 pounds (28.3 kilograms) at ordinary room temperature. Since a US gallon of water weighs roughly 8.34 pounds, dividing 62.4 by 8.34 returns the familiar 7.48 gallons per cubic foot — a handy way to check the conversion.

How many cubic feet is a 40-gallon water heater? Multiply gallons by 0.133 681: a 40 US-gallon tank is about 5.35 cubic feet, and a 50-gallon tank about 6.68 cubic feet. That is the internal water volume, not the heater’s external footprint.

Why is the factor 7.48 rather than a round number? Because a cubic foot is 1,728 cubic inches and a US gallon is defined as 231 cubic inches, and 1,728 ÷ 231 = 7.4805… The two units were defined independently, so their ratio was never going to land on something tidy.

How do I convert gallons back to cubic feet? Divide US gallons by 7.4805, or multiply by 0.133 681. For imperial gallons, divide by 6.2288 or multiply by 0.160 544. A 500 US-gallon tank, for instance, is 500 × 0.133 681 ≈ 66.8 cubic feet.

Does water temperature change the conversion? No. The cubic-feet-to-gallons factor is a fixed ratio between two volume units and never changes. Temperature slightly alters water’s density (and so its weight per cubic foot), but the space it occupies still converts at 7.48 gallons per cubic foot.

How many gallons is a swimming pool measured in cubic feet? Multiply the pool’s cubic feet by 7.48 for US gallons. A pool of 2,250 ft³ (say 30 ft × 15 ft × 5 ft average depth) holds about 16,831 US gallons. Work out the cubic feet from length × width × average depth first.

Related calculators

Pair the cubic feet to gallons calculator with these tools for the rest of a volume workflow.

• Volume converter — move between litres, gallons, millilitres, fluid ounces and cubic centimetres in one step.
• Tank volume calculator — capacity of cylindrical, oval and capsule tanks, in cubic feet and gallons.
• Cylinder volume calculator — volume from radius and height for any round container.
• Cups to gallons converter — US, imperial and metric cups scaled up to gallons.