Gallons to Liters Calculator

Background.

A gallons-to-liters calculator earns its keep the moment you buy an aquarium, a fish tank filter, or a pool pump. US pet stores sell tanks by the gallon - a '40-gallon breeder' is a standard size - but the filter's flow rate, the water-conditioner dosage chart, and the heater wattage guide that came in the box are all specified in liters per hour or liters of tank volume, because the equipment is manufactured to a metric standard sold worldwide. Getting the conversion wrong means dosing the wrong amount of dechlorinator or undersizing a filter for the actual water volume, both of which matter to the animals living in the water.

This calculator converts gallons to liters using the exact US liquid gallon (3.785411784 liters, fixed by the 1959 international yard-and-pound agreement and published in NIST Special Publication 811) and, as a second mode, the UK imperial gallon (exactly 4.54609 liters, fixed by the 1985 UK Weights and Measures Act). A 40-gallon US aquarium is 151.41647136 liters; if the same number were read as imperial gallons instead, it would be 181.8436 liters - a 20 percent difference large enough to under-dose a water treatment or under-power a filter. Both results are always shown side by side so you never have to guess which gallon a spec sheet meant.

What is gallons to liters calculator?

A gallon is a US customary and imperial unit of volume; a liter is the metric unit of volume equal to one cubic decimeter (1000 cubic centimeters). Two gallons are in everyday use and they are not the same size: the US liquid gallon is defined as exactly 231 cubic inches (3.785411784 liters), while the UK imperial gallon is defined as exactly 4.54609 liters - about 20 percent larger. Pet-supply, home-improvement, and fuel retail in the United States use the US gallon; the UK, Ireland, and parts of the Commonwealth use the imperial gallon. This calculator converts a gallon value into liters under both definitions simultaneously, so a US-gallon-derived result and an imperial-gallon-derived result are always available together, and the select field simply chooses which one is treated as primary.

How to use this calculator.

1. 01Enter the volume in gallons - for example, an aquarium size, a pool-fill target, or a drum or tank capacity.
2. 02Choose the gallon type: US gallon (used in the United States) or imperial gallon (used in the UK and parts of the Commonwealth).
3. 03Read the converted value in liters, shown to full precision.
4. 04Check the liters-from-US and liters-from-imperial fields side by side if your equipment documentation does not specify which gallon it means.

The formula.

The conversion is purely multiplicative because volume has no offset - zero gallons is zero liters in every system. The calculator multiplies the gallon value by the exact size of one gallon in liters. For the US gallon, that factor is 3.785411784, derived from the historical 231-cubic-inch US gallon combined with the exact 2.54 cm inch fixed by the 1959 international yard-and-pound agreement (231 × 2.54³ cm³ = 3785.411784 cm³ = 3.785411784 L). For the imperial gallon, the factor is 4.54609, fixed directly by the UK Weights and Measures Act 1985. In the worked example, 40 gallons multiplied by 3.785411784 gives 151.41647136 liters if the tank is US-gallon sized; the same 40 gallons multiplied by 4.54609 gives 181.8436 liters if it is imperial-gallon sized - a difference of more than 30 liters on the same nominal tank size. All arithmetic runs through Decimal.js at full precision so the result matches the defined factor exactly rather than drifting the way ordinary floating-point multiplication can.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Institute of Standards and Technology. NIST Special Publication 811, 2008 Edition - Guide for the Use of the International System of Units (SI), Appendix B. Source for the exact US gallon = 3.785411784 L conversion factor.
2. [2]United Kingdom. Weights and Measures Act 1985, Schedule 1. Statutory definition of the imperial gallon as exactly 4.54609 liters.