The exact equivalent
A US gallon is exactly 3.785411784 litres. The imperial gallon is 4.54609 litres, and confusing them is a 20% error.
1 gal = 3.78541178 l
1 l = 0.26417205 gal
How to convert it by hand
Just multiply by the conversion factor. To go from US gallons to litres, multiply by 3.785412; to come back, divide by that same number or multiply by 0.264172.
For example: 25 gal × 3.785412 = 94.6353 l.
A tip that saves mistakes: check the result goes the way you expect. If litres are a smaller unit than US gallons, the number has to come out bigger. If it comes out smaller, you divided when you should have multiplied.
Conversion table
| gal | l |
|---|---|
| 1 | 3.7854 |
| 2 | 7.5708 |
| 5 | 18.9271 |
| 10 | 37.8541 |
| 20 | 75.7082 |
| 50 | 189.2706 |
| 100 | 378.5412 |
Reference points to picture it
| Reference | Equals |
|---|---|
| 1 US gallon | 3.79 l |
| A 15-gallon tank | 56.8 l |
| A US gallon of milk | 3.79 l |
| 1 imperial gallon | 4.55 l |
The trick for doing it in your head
Multiply by 4 and take off 6%. For 10 gallons: 40 minus 2.2 gives 37.8 litres, which is the exact answer.
It is fine for getting by without a phone, but not for anything that depends on precision: in a recipe, on a building site or in a dose, use the exact conversion.