A gallon label is incomplete unless you know which gallon it means. This page works through container and recipe conversions. For the definitions and background, see why US and UK gallon sizes differ.
Choose the unit before multiplying
A US liquid gallon is 3.785411784 liters. An imperial gallon is 4.54609 liters. A US dry gallon is a separate unit and should not be substituted in liquid conversions. The examples below use US liquid and imperial gallons.
Conversion reference: NIST unit-conversion tables.
Example 1: a five-gallon container
Five US liquid gallons hold 5 × 3.785411784 = 18.92705892 L, approximately 18.93 L. Five imperial gallons hold 5 × 4.54609 = 22.73045 L, approximately 22.73 L. The difference is approximately 3.80 L even though both labels say “five gallons.”
Example 2: convert a recipe
If a recipe explicitly requires half a US liquid gallon, use 1.892705892 L. Half an imperial gallon is 2.273045 L. Copying the number 0.5 without copying the unit changes the recipe by about 380 mL.
When the source only says “gallon,” check its stated measurement system or ask the author. Do not infer the unit solely from where you are cooking. For ingredient scaling, first settle the volume unit, then apply the serving multiplier using the recipe scaler.
Example 3: liters back to gallons
For a 20-liter volume, divide rather than multiply: 20 ÷ 3.785411784 gives approximately 5.28344 US liquid gallons; 20 ÷ 4.54609 gives approximately 4.39938 imperial gallons. Round for display only after completing the calculation.
A repeatable conversion worksheet
- Write the starting amount and the complete unit name.
- Convert to liters using the matching factor.
- If necessary, divide liters by the destination gallon factor.
- Check whether the result is a container capacity or a usable fill amount.
A nominal tank size is not a safe-fill instruction: headspace, temperature, manufacturer limits and the contents can matter. Volume also does not establish mass without density. Use the unit converter to check units and the volume calculator for geometric capacity.
Why not round every intermediate value?
Using 3.8 L for each gallon repeatedly can compound rounding differences across large batches. Keep the full factor in a worksheet and round the final amount to a precision appropriate to your measuring equipment. A displayed decimal is not a claim that a household measuring jug is that precise.