How this instrument works
Two exact chains meet inside this factor. Since 1959 the international foot has measured 0.3048 metres precisely, so one cubic foot encloses 0.028316846592 cubic metres, or 28.316846592 litres. US liquid gallons hold 231 cubic inches, a count carried over from colonial wine measure that Congress never saw reason to amend, and that 25.4 mm inch turns it into 3.785411784 litres, equally precisely. Divide one litre figure by another and out drops 0.1336805…, exactly what 231 ÷ 1728 gives.
That decimal behaves oddly, and its denominator explains why. Reduce 231⁄1728 by their shared factor of three and 77⁄576 remains, where 576 breaks down into 2⁶ × 3². Those six twos let six decimal places settle — 0.133680 — while what 3² leaves behind keeps one digit cycling endlessly after them, so 5 repeats without limit. This page rounds at its twelfth place, which is why it closes on 556 rather than 555.
Both units earn their keep in different rooms of one building. Liquid is bought, stored and taxed by gallon; space is sold by cubic foot — freight cube, container capacity, racking, refrigerated volume, clearance under floors. Anyone quoting chemical shipments by density class, working out how many 275-gallon totes one mezzanine will take, or excavating pits for buried tanks begins in gallons and has to finish in cubic feet. Flow work runs this same errand: pumps are catalogued in gallons per minute while channels, weirs and ducts are rated in cubic feet per second.
- Enter your volume in the US gallons (gal) field; it opens at 100 gal so this factor is visible immediately.
- Read Cubic feet (ft3) below it, refreshed to six decimal places on every keystroke.
- Confirm your gallon is US liquid measure — imperial and dry gallons carry their own factors.
- Reversing? Divide your Cubic feet (ft3) figure by 0.133680555556 to land back on gallons.
Worked example — 100 gallons of diesel
One fabricator quotes an auxiliary fuel tank for his pickup bed. Typing 100 into US gallons (gal) returns 13.3680555556 in Cubic feet (ft3) — call it 13.37 on his drawing. Full-size boxes swallow roughly 62 cubic feet, so this tank claims about one fifth of that, and at diesel's 7.1 pounds per gallon its load runs near 710 pounds before any toolbox goes in.
That number cross-checks two ways. Diesel sits near 53 pounds per cubic foot, and 13.368 × 53 comes back to 709 pounds, matching what per-gallon density already said. Spread those 13.37 cubic feet over one standard 48 × 40 inch pallet, which covers 13.333 square feet of floor, and your pile stands 12.03 inches tall: one pallet footprint, one foot deep, is very nearly 100 gallons.
Questions
Is 0.133680555556 an exact factor?
Its underlying ratio is exact; only this printed decimal stops one notch short. US gallons are defined as 231 cubic inches and cubic feet contain 1728, so true value here is 77⁄576 — 0.133680 trailed by fives that never run out. Rounding at twelve places nudges that final digit up to 6. Push one million gallons through it and your surplus comes to roughly one hundredth of a millilitre, about a quarter of one drop, which no tank gauge, meter or laboratory balance will ever notice.
What if my gallons are imperial rather than US?
Then your factor shifts by about one fifth. Imperial gallons have stood at exactly 4.54609 litres since 1985, which works out to 0.1605437 cubic feet against 0.1336806 for US liquid gallons used here. Fuel, heating oil and marine capacities published in Britain, Ireland and parts of the Caribbean still lean on imperial measure, so any figure lifted from UK datasheets and run through this page lands roughly 20 per cent small. US dry gallons differ again, 0.1555570 cubic feet, and never apply to liquids.
How do I convert gallons per minute into cubic feet per second?
Convert volume first, then fix time. Multiplying gallons per minute by 0.133680555556 yields cubic feet per minute, and dividing by 60 carries you to cubic feet per second, so gpm ÷ 448.831 does both at once. One 100 gpm irrigation pump therefore shifts 13.37 cubic feet each minute, or 0.2228 cubic feet per second — precisely the form stream gauges, culvert tables and stormwater models expect.
Why are gas cylinders rated in cubic feet when they hold only a few gallons?
Because that rating counts gas after it escapes, not steel around it. One common 80 cubic foot aluminium cylinder encloses about 11 litres of internal volume — under three US gallons, or four tenths of one cubic foot — and reaches 80 only once its contents expand from some 3000 psi down to ordinary air pressure. Converting cylinder water capacity here gives physical space it occupies, which is what racks, manifests and lift calculations need, rather than that free gas figure stamped on its shoulder.
Does 13.37 cubic feet tell me how much truck space a 100-gallon tank needs?
No — that figure covers liquid, not parcel. Moulded tanks add wall thickness, ribbing, filler necks and fittings, and carriers charge on whatever box encloses all of it, so billable cube commonly runs a third to a half above your liquid volume. Use converted cubic feet for weight, dosing and hydraulics; measure outside dimensions for density class, doorway clearance and rack height. Mixing those two up is how pallets turn up that will not pass through your roll-up door.
Is there a quick way to do this in my head?
Divide by seven and one half. That treats 7.5 gallons as one cubic foot, and since 7.5 gallons is really 1.0026 cubic feet, this shortcut runs low by roughly one quarter of one per cent: 100 gallons lands on 13.33 instead of 13.37. For sizing pumps, ordering fill or sanity-checking spreadsheets, that is plenty. Bring back all twelve places when your figure gets multiplied by some rate and audited later — chemical dosing, custody transfer and metered billing repay those extra digits.