Gallon Calculator

Convert gallons to liters, cups, quarts, pints, and cubic feet instantly.

Covers US, UK, and dry gallons plus a water gallons-to-weight mode.

Updated August 7, 2026

CreatorFrank Zhao

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Introduction / overview

The Gallon Calculator converts a single volume into every unit it understands at once. Type a value next to any unit — gallons, liters, cubic feet, cups, pints, fluid ounces, barrels — and every other field fills in automatically. It works in both directions, so you can start from whichever unit you happen to have.

It is aimed at everyday situations: adapting a recipe that calls for cups when your jug reads liters, working out how many gallons a water tank or fuel container holds from its cubic-foot volume, or checking a utility figure against familiar units. The second mode, Water gallons to weight, goes one step further and turns a volume of water into a weight in kilograms or pounds.

All unit factors are exact: 1 US gallon = 3.785411784 liters and 1 UK gallon = 4.54609 liters. Because every field is tied to the same underlying volume, entering one value always produces consistent results across all the others.

How to use / quick start

1Keep the default "Volume converter" mode, or switch to "Water gallons to weight converter" if you need the weight of water.
2Pick the unit that matches what you have — for example, set the Gallons field to US gal if that is the unit on your container.
3Enter any number into any field. The rest of the fields update instantly to show the same volume in other units.
4To convert the other way, simply edit a different field. The calculator re-derives everything from whichever value you typed last.

Example — convert 5 US gallons

Type 5 into the Gallons field (US gal).

\mathrm{V_L}

=

5 \times 3.785411784

=

18.92705892\ \mathrm{L}

The same volume shows up as 80 US cups, 20 US quarts, 640 US fluid ounces, and roughly 0.668 cubic feet. All fields describe the same amount of liquid, just in different units — which one you read depends on your measuring cup, jug, or container.

Real-world examples

How many gallons does a 100 cubic-foot tank hold?

You measured a rectangular water tank as 100 cu ft of capacity.

In the volume mode, open the first Cubic... field, switch its unit to cu ft, and type 100. The Gallons field fills in:

\mathrm{V_{US\,gal}}

=

\frac{100\ \mathrm{ft^3}}{0.1336806\ \mathrm{ft^3/US\,gal}}

\approx

748.05\ \mathrm{US\,gal}

That is the usable capacity before you account for walls or fill levels. If you only know the tank's length, width, and height in feet, work out the cubic-foot volume first with the cubic feet calculator, then bring that number back here.

How heavy is a full 20-liter aquarium?

The water mode converts a volume of water into weight.

Switch to Water gallons to weight converter. In the Volume (metric) field choose liters, type 20, and read the weight. With the default density of 998.34 kg/m³ (water near 20 °C):

\mathrm{m}

=

\mathrm{V} \times \rho

=

0.02 \times 998.34

=

19.9668\ \mathrm{kg}

Add the weight of the glass and substrate, and you have a realistic idea of what the stand must support. The tool also shows the same weight in pounds in the Imperial weight field. If you already know a liquid's weight and want to find how many gallons it takes up instead, the kg to gallons converter works in that direction.

Calculation method

Every volume field is converted from the same underlying volume, so the relationships are simple multiplications by exact factors. The two you will use most often:

\mathrm{V_L} = \mathrm{V_{US\,gal}} \times 3.785411784

\mathrm{V_{cups}} = \mathrm{V_{US\,gal}} \times 16

1\ \mathrm{US\,gal}

=

4\ \mathrm{qt}

=

8\ \mathrm{pt}

=

128\ \mathrm{fl\,oz}

=

231\ \mathrm{in^3}

For the water mode, weight follows directly from density and volume:

\mathrm{m_{kg}} = \mathrm{V_{m^3}} \times \rho_{\mathrm{kg/m^3}}

Which way can it solve?

Every field is a two-way input. Type a weight and the tool derives the matching volume from the density you have set, or clear the density field and enter both a volume and a weight — the density is then worked out from those two. There is no fixed "input" or "output" side.

Related concepts

US gallon vs UK gallon

They are different sizes. The UK (imperial) gallon is about 20% larger: 1 UK gallon ≈ 1.20095 US gallons, while 1 US gallon ≈ 0.8327 UK gallons. If a recipe, label, or vehicle spec does not say which gallon it uses, it is usually worth checking — the difference is large enough to matter in fuel, paint, and chemical mixing.

Liquid gallons vs dry gallons

The U.S. customary system keeps separate volume units for dry goods and liquids — bushels and dry gallons for dry commodities, liquid gallons for liquids — as the NIST volume units reference notes. A US dry gallon is exactly 268.8025 in³, which is about 16% larger than a liquid gallon. Use the dry gallon unit for dry goods such as grain; use liquid gallons for anything you pour.

Cups and spoons in the kitchen

This calculator treats a US customary cup as exactly 8 US fluid ounces (236.5882365 ml). The tablespoon and teaspoon options use the common 15 ml and 5 ml metric kitchen convention. Different countries round cups differently (237, 240, or 250 ml), so if you are scaling a recipe, stick to one convention throughout rather than mixing measuring cups and scales.

Water density depends on temperature

Water is densest around 4 °C (about 1000 kg/m³) and gets slightly lighter as it warms. The water mode uses a default density of 998.34 kg/m³, which corresponds to water near 20 °C. That is why "1 liter of water weighs 1 kg" is a good rule of thumb but not exact at every temperature. For a different temperature or a liquid other than fresh water, edit the density field before reading the weight.

FAQs

How do I convert US gallons to UK gallons?

Set the Gallons field unit to US gal, type your number, then switch the same field to UK gal — or read the liters and divide by 4.54609. As a quick check, 1 US gallon equals about 0.8327 UK gallons, and 1 UK gallon equals about 1.20095 US gallons.

How much does one gallon of water weigh?

One US gallon of water at the default density (998.34 kg/m³, near 20 °C) weighs about 3.78 kg, or about 8.33 lb. Switch to the water mode and enter 1 in the Volume (gallons) field to see both numbers. The weight changes slightly with water temperature and for saltwater.

What is a dry gallon used for?

The US dry gallon is a customary unit for dry commodities such as grain. It equals 268.8025 in³ — about 16% larger than the US liquid gallon. If your bill or product is measured in CCF (hundred cubic feet) rather than gallons, the CCF to gallons converter handles that specific unit for water and gas bills.

Limitations

A few practical boundaries to keep in mind

Dry and liquid units are different
Use the dry gallon for dry goods and the liquid gallon for liquids — mixing them changes the result by roughly 16%.
The "rain drops" unit is an estimate
It uses a fun 0.05 ml-per-drop approximation; real drops vary with the dropper and the liquid.
Water weight depends on density
The default of 998.34 kg/m³ suits fresh water near 20 °C; saltwater, warmer or colder water, and other liquids need a custom density.
Kitchen spoon and cup conventions vary
This calculator uses the US customary cup and the 15 ml / 5 ml metric tablespoon and teaspoon, which may differ from local conventions.
Volumes and weights must be non-negative
A zero density with a positive weight target cannot produce a volume, and the tool shows an error instead of a number.

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