Cubic Feet to Gallons Calculator
Type into either field, the other converts instantly, and the Imperial and metric figures below update with them.
Type into the cubic-feet or the US-gallons field and the other updates instantly, with Imperial gallons, quarts and liters recalculating alongside them. Built for the volumes people most often need in gallons rather than cubic feet: water tanks, aquariums, pools and any other container whose capacity matters more as a liquid figure than a solid one.
The 7.48 Gallons-Per-Cubic-Foot Factor
Cubic feet to US gallons
US Gallons = Cubic Feet × 7.4805
A US gallon is defined as 231 cubic inches; 1,728 ÷ 231 = 7.4805.
That factor is exact, not a rounded approximation, it falls straight out of the legal definitions of the cubic foot (1,728 in³) and the US gallon (231 in³). Multiply any cubic-feet figure by 7.4805 and you have US gallons with no further correction needed. This is the same 7.48 figure already used in this site's homepage conversion chart, so a result checked here will always agree with one checked there.
US Gallons vs. Imperial Gallons
The word "gallon" means two different sizes depending on where you're standing. A US gallon holds 231 cubic inches; an Imperial gallon, the one used in the UK, Canada and a handful of other countries, is roughly 20% larger, at about 277.4 cubic inches. That gap means a single cubic foot holds 7.4805 US gallons but only about 6.2288 Imperial gallons, and mixing the two up on a tank spec or a recipe scaled from a UK source produces a result that's off by a fifth. This calculator shows both figures side by side specifically so you don't have to remember which one applies, read whichever column matches the gallon type your supplier, spec sheet or recipe actually means.
Quarts and liters ride along with the same underlying cubic-feet value: a US quart is a quarter of a US gallon (29.9221 per cubic foot), and a liter is the exact metric unit tied to the cubic meter (28.3168 per cubic foot). If you need the metric side of this conversion in isolation, no gallons involved, the cubic meters calculator handles that more directly.
Liquid Gallons vs. Dry Gallons
There's a third, less commonly encountered gallon worth flagging: the US dry gallon, used historically for measuring dry goods like grain and now largely obsolete outside a few agricultural contexts. It's smaller than the liquid gallon this calculator uses, at about 268.8 cubic inches instead of 231. Unless you're dealing with a historical recipe, an old agricultural reference, or a specification that explicitly says "dry gallon," you can safely assume any gallon figure you encounter is the liquid gallon this page converts, it's the standard for water, fuel, paint and virtually every other liquid measured in gallons today.
Worked Example: Sizing an Aquarium
A tank measuring 2 ft × 1 ft × 1.5 ft
- Volume: 2 × 1 × 1.5 = 3 ft³
- US gallons: 3 × 7.4805 = 22.44 gal
- Liters: 3 × 28.3168 = 84.95 L
This is a 22-gallon aquarium, close to the common "22 long" size class.
Aquarium retailers round to a standard size name rather than quoting the exact calculated figure, so a real tank labelled "20 gallon" or "29 gallon" may not match this arithmetic exactly, treat the calculation as a way to check a tank's true capacity, not to guess its marketing name.
The same three steps size a rain barrel, a stock tank, or any other liquid container from its dimensions. If the container is cylindrical rather than rectangular, a drum or a round tank, the cylinder volume calculator gets you to cubic feet first using radius and height, and this page's conversion picks up from there.
Sizing a Rainwater Collection Barrel
Rainwater harvesting is one of the more common real-world reasons people end up needing a precise gallon figure rather than an estimate. A standard residential roof can shed a surprising volume during a single storm, roughly 0.6 gallons per square foot of roof area per inch of rainfall, which means even a modest roof easily overwhelms a single 55-gallon barrel in a heavy downpour. Working backward from a barrel's physical dimensions using this calculator, rather than trusting a manufacturer's rounded marketing figure, is worth doing before deciding how many barrels or what total tank size a collection system actually needs for your roof area and local rainfall.
The same logic applies to any liquid storage decision where the container's true capacity matters more than its advertised size, livestock water troughs, fuel tanks and cisterns are all cases where the gap between a nominal marketing figure and the calculated true volume is worth checking before you're relying on it.
Water Weight, Not Just Water Volume
Once you know a container's gallon capacity, its weight when full follows directly: water weighs about 8.34 lb per US gallon, so a filled 300-gallon tank carries roughly 2,500 lb, a figure worth checking against a deck, floor or vehicle's load rating before installing or transporting a tank that size. This is the same density-based calculation covered in more general terms on the weight-from-volume calculator, which lists water's density in cubic feet (62.4 lb/ft³) alongside several other materials rather than in gallons specifically, multiplying this page's cubic-feet figure by that density gets you to the same answer as multiplying gallons by 8.34, just starting from a different unit.
This calculation matters most for elevated or mobile installations, a rooftop tank, a trailer-mounted tank, an upper-floor aquarium, where the supporting structure has a hard weight limit that a "how many gallons fit" question alone doesn't answer.
A Quick Sanity Check
Since one cubic foot holds nearly seven and a half gallons, the gallon figure for any real container should always be noticeably larger than its cubic-feet figure, if you ever calculate a gallon result smaller than the cubic-feet number you started from, a decimal or a unit got mixed up somewhere upstream and it's worth re-checking the original measurement before trusting the result.
Fuel and Chemical Tanks Follow the Same Math
Everything on this page applies equally to fuel tanks, chemical totes and any other liquid container, with one added caution: never assume a tank's full geometric capacity is its safe fill level. Most tanks are rated for a working fill of somewhat less than their brim-full geometric volume specifically to leave expansion room for temperature swings and to avoid overflow during filling, check the tank's own rated capacity alongside this calculator's geometric figure rather than treating the two as interchangeable.
Frequently Asked Questions
How many gallons fit in one cubic foot of water?
About 7.48 US gallons, the exact figure is 7.480519, since a US gallon is defined as 231 cubic inches and a cubic foot holds 1,728 of them. That figure holds only for water; a liquid with a different density still occupies the same 7.48 gallons per cubic foot, since gallons and cubic feet are both units of volume, not weight.
Why is an Imperial gallon a different size than a US gallon?
They come from two separate historical definitions that were never unified, the Imperial gallon (used in the UK, Canada and a few other countries) is based on 10 pounds of water at a specific temperature, while the US gallon comes from an older English wine-trade measure. The Imperial gallon ends up about 20% larger.
How big a tank do I need for 100 gallons of water?
About 13.4 cubic feet, divide 100 by 7.4805 to convert gallons back to cubic feet, then size the tank's length, width and height (or diameter and height for a cylinder) to reach that volume. A cylindrical tank 2 feet across would need to stand about 4.3 feet tall to hold the same 100 gallons.
Does this calculator work for swimming pools?
Yes for a simple rectangular pool of uniform depth, multiply length by width by average depth in feet, then use this page to read the result in gallons. Pools with a sloped or varying floor need the volume calculated in sections first, the same divide-and-sum approach used for irregular room shapes.