How this instrument works
Twelve inches of length, width and depth enclose one cubic foot, which since 1959 means exactly 0.3048 metres on each edge and therefore 28.316846592 litres. US liquid gallons have stranger ancestry. They descend from Queen Anne's wine gallon, fixed by English statute in 1707 at 231 cubic inches — and even that figure was rounded, since vessels seven inches across and six inches deep actually hold 230.907 of them.
Britain abandoned Anne's vessel in 1824 for an imperial gallon built on ten pounds of water, but young America kept her number, and 231 cubic inches remains its legal definition today. Since 12 × 12 × 12 = 1728, dividing 1728 by 231 counts how many gallons fit inside one cubic foot. Cancel their shared factor of three and what remains is 576⁄77 — an honest fraction, whose decimal repeats forever in blocks of six digits: 7.480519 480519 480519…
That ratio is why American water bills read as they do. Residential meters register volume in cubic feet, and utilities across much of this country charge by CCF — one hundred cubic feet, or 748.05 gallons — then print gallons on your statement, because customers picture gallons. Pool builders, well drillers, septic installers, aquarium keepers and rainwater harvesters run this multiplication daily: measure your tank in feet, multiply by 7.480519, quote it in gallons.
- Type your volume into the Cubic feet (ft3) field; it opens at 10 for reference.
- Sizing rectangular tanks or pools? Multiply length × width × average depth in feet first, then enter that product.
- Read your answer on the US gallons (gal) line, which recomputes as you type.
- Working from meter readings, subtract one from another and enter their difference to price that usage.
- Going backwards, divide any gallon figure by 7.48051948052 to recover cubic feet.
Worked example — when your meter moves 10 cubic feet
You read your household water meter before bed at 4,118 cubic feet and again at breakfast at 4,128. Enter 10 into the Cubic feet (ft3) field and the US gallons (gal) line returns 74.8051948052 — roughly 1.5 bathtubs gone overnight, with no tap open and nobody awake.
Nine gallons an hour is no weeping faucet — a steady drip loses perhaps five gallons a day, while a toilet flapper that stops sealing can lose several hundred. Billed by CCF, those ten cubic feet amount to a tenth of one unit: pennies as line items go, yet repeated every night until someone fits a two-dollar rubber part.
Questions
Is the 7.48051948052 factor exact?
Its underlying ratio is exact without qualification; this printed decimal is exact to twelve significant figures. US gallons are defined as 231 cubic inches while one foot cubed spans 1728, so their true relationship is 576⁄77, whose expansion 7.480519480519… never terminates. Any finite decimal has to stop somewhere, and stopping at 7.48051948052 leaves error near one part per trillion — far below what any meter, tank or pool could ever resolve.
Does this work for imperial gallons used in Britain or Canada?
No. Imperial gallons run 4.54609 litres, about 20 percent larger than US gallons at 3.785411784 litres, so one cubic foot holds only 6.22884 of them. Britain redefined its measure in 1824 around ten pounds of water and never returned to Queen Anne's 231 cubic inches, while America kept it. Quoting 7.4805 gallons per cubic foot to readers who understand them as imperial inflates your stated volume by roughly one fifth.
Why does my water bill show cubic feet rather than gallons?
Because your meter counts cubic feet. Most American residential registers do exactly that, or else work in CCF — one hundred of them, equal to 748.05 gallons. Charging by CCF keeps figures small: households drawing 6,000 gallons per month appear as 8 CCF. NIST Handbook 44 sets accuracy and marking rules those devices must meet, and either unit is legal for trade provided its register is labelled honestly.
What about the US dry gallon?
It is its own unit, nearly extinct outside grain tables. Dry gallons hold an eighth of each Winchester bushel — 268.8025 cubic inches — which puts only 6.42851 of them inside one cubic foot. Berries and shellfish were once sold this way, but modern trade prefers dry quarts, bushels or plain litres. Unless your document says dry explicitly, an American gallon means the 231-inch³ liquid measure this sheet applies.
Is rounding to 7.48 close enough for swimming pools?
For sizing pumps or ordering chlorine, easily. Using 7.48 understates volume by seven thousandths of one percent — under two gallons across 25,000 gallons of pool water, and your average-depth estimate is wrong by far more than that. Where fuller precision earns its keep is reconciling meter readings, tracking irrigation allocations, or any quantity that gets multiplied by some rate and later audited line by line.
How do I turn gallons back into cubic feet?
Divide by 7.48051948052, or multiply by its reciprocal 77⁄576 = 0.133680555… Your 50-gallon water heater therefore occupies 6.684 cubic feet, and an acre-foot of 325,851 gallons is 43,560 cubic feet — precisely those 43,560 square feet in an acre, flooded one foot deep. That reciprocal is worth memorising as 'roughly an eighth', since 0.1337 sits close to 0.125.