Gallons in, liters out
One US gallon is 3.78541 liters. That’s the number doing all the work here. Type any gallon figure and the liter equivalent appears instantly, no clicking through dropdowns to find the unit you want.
So 10 US gallons is 37.85 liters. A typical 15 gallon car tank holds about 56.8 liters. And a single gallon of milk? Just under four liters, at 3.79.
US versus Imperial, because it matters
Here’s the trap. There are two gallons in common use, and they’re meaningfully different sizes.
- US gallon = 3.78541 liters. This is what you want for almost anything American: fuel prices, recipe volumes, paint cans, milk jugs.
- Imperial gallon = 4.54609 liters. Used in the UK and some Commonwealth countries. About 20% larger than the US gallon.
The converter has a toggle for both. If you grab the wrong one, a “5 gallon” conversion swings from 18.9 liters (US) to 22.7 liters (Imperial). That gap is enough to matter when you’re filling a tank or pricing fuel, so the default is US and the Imperial option sits right beside it.
Fuel economy: where the conversion gets useful
This is the big one. American cars advertise miles per gallon; the rest of the world thinks in liters per 100 km. To compare them honestly you have to convert the gallon part first.
Say a car gets 30 US MPG. There are 3.78541 liters in that gallon, so you’re already halfway to the metric figure. The conversion also explains why US and UK MPG numbers never line up. A UK gallon is bigger, so the same physical fuel efficiency produces a higher “MPG” on Imperial gallons. A car rated 40 mpg in Britain is not as efficient as a 40 mpg US car. The bigger gallon inflates the number.
Beyond fuel, water tanks are the other constant: a “55 gallon” drum (a US standard) is 208 liters, and pool and rainwater equipment sold in the States lists capacity in gallons that you’ll often need in liters for dosing or planning.
Using the tool
Enter your gallon value, read the liters. It updates as you type, no submit step. Flip to Imperial if your source is British. The table at the bottom has common amounts (1, 2, 5, 10, 20, 55 gallons) pre-calculated both ways for quick reference. Every calculation happens in your browser with the exact 3.78541 factor, so nothing uploads and there’s no rounding surprise baked in.
Worth memorizing: 1 gallon rounds to about 3.8 liters, and you can multiply gallons by roughly 3.785 in your head for a close estimate.
FAQ
How many liters are in a gallon?
A US gallon is 3.78541 liters. An Imperial (UK) gallon is 4.54609 liters. The tool uses both exact values, depending on which toggle you pick.
Why is the UK gallon bigger?
Britain redefined its gallon in 1824 around ten pounds of water, landing at 4.546 liters. The US stuck with an older 231 cubic inch wine gallon, which is smaller. They’ve been different ever since.
Can I convert MPG to L/100km with this?
This tool handles the gallon-to-liter half of that calculation. Convert your gallons to liters first, then divide by the distance in 100 km chunks. For a full fuel-economy figure you’ll also need the distance conversion.
Is 4 liters a gallon?
Roughly. A US gallon is 3.785 liters, so 4 liters is slightly more than one US gallon (about 1.057). People round the two together casually, but they aren’t identical.
Does this need internet?
Only to load the page. The actual conversion is plain browser math, so it keeps working offline and never sends your numbers anywhere.