# Volume converter

> Convert gallons to litres, cups to mL and fl oz to mL exactly, with US and imperial gallons, pints and fluid ounces kept separate.

Interactive version: https://www.calcopenly.com/conversion/volume-converter
Subject: Unit converters

Each volume unit is a fixed number of cubic metres. The US units are exact because they are built on the 1959 inch: a US gallon is 231 in³, which is 3.785411784 L. The imperial gallon is a different, larger unit, fixed by the UK Weights and Measures Act 1985 at exactly 4.54609 L.

The default converts 1 US gallon to 3.785411784 litres. Recipes and drink sizes are the other common use: a US cup is 1/16 gallon, 236.5882365 mL, and a US tablespoon is 3 teaspoons, 14.787 mL.

Choose the US or imperial version of a unit on purpose. An imperial pint (568 mL) is 20% larger than a US pint (473 mL), while an imperial fluid ounce (28.41 mL) is smaller than a US one (29.57 mL).

## Inputs

- **Value**: A number in the “from” unit, or with its own unit
- **From** (options: cubic metre (m³), litre (L), millilitre (mL), centilitre (cL), decilitre (dL), cubic inch (in³), cubic foot (ft³), cubic yard (yd³), US gallon (gal), US quart (qt), US pint (pt), US cup (cup), US fluid ounce (fl oz), US tablespoon (tbsp), US teaspoon (tsp), imperial gallon (gal (UK)), imperial quart (qt (UK)), imperial pint (pt (UK)), imperial fluid ounce (fl oz (UK)), metric cup (cup (metric)), metric tablespoon (tbsp (metric)), metric teaspoon (tsp (metric)), oil barrel (bbl), acre-foot (ac·ft))
- **To** (options: cubic metre (m³), litre (L), millilitre (mL), centilitre (cL), decilitre (dL), cubic inch (in³), cubic foot (ft³), cubic yard (yd³), US gallon (gal), US quart (qt), US pint (pt), US cup (cup), US fluid ounce (fl oz), US tablespoon (tbsp), US teaspoon (tsp), imperial gallon (gal (UK)), imperial quart (qt (UK)), imperial pint (pt (UK)), imperial fluid ounce (fl oz (UK)), metric cup (cup (metric)), metric tablespoon (tbsp (metric)), metric teaspoon (tsp (metric)), oil barrel (bbl), acre-foot (ac·ft))

## Results

- Converted value — main result
- Conversion
- Conversion factor
- Exactness

## Formula

$$
\begin{gathered} x_{\text{to}} = x_{\text{from}} \times \dfrac{f_{\text{from}}}{f_{\text{to}}} \\ f = \text{one unit expressed in } \text{m³} \end{gathered}
$$

## Worked examples

### 1 US gallon to litres

- Value: 1 gal
- From: US gallon (gal)
- To: litre (L)
- **Converted value: 3.785411784 L**
- **Exactness: Exact**
- Checked against: US gallon = 231 in³ × (0.0254 m)³ = 3.785411784 L (NIST HB 44 App. C); Python 3.8 fractions from the legal definitions

### 1 US gallon in cubic inches

- Value: 1 gal
- From: US gallon (gal)
- To: cubic inch (in³)
- **Converted value: 231 in³**
- Checked against: NIST HB 44 App. C: 1 US gallon = 231 in³

### Imperial gallon in US gallons

- Value: 1 gal (UK)
- From: imperial gallon (gal (UK))
- To: US gallon (gal)
- **Converted value: 1.2009499255 gal**
- Checked against: 4.54609 L (UK Weights and Measures Act 1985) ÷ 3.785411784 L = 568261250/473176473 (Python 3.8 fractions from the legal definitions)

### 1 US tablespoon in teaspoons

- Value: 1 tbsp
- From: US tablespoon (tbsp)
- To: US teaspoon (tsp)
- **Converted value: 3 tsp**
- **Exactness: Exact**
- Checked against: NIST HB 44 App. C: 1 tablespoon = 3 teaspoons

### 1 US cup in millilitres

- Value: 1 cup
- From: US cup (cup)
- To: millilitre (mL)
- **Converted value: 236.5882365 mL**
- Checked against: 1/16 US gallon = 231/16 in³ = 236.5882365 mL (Python 3.8 fractions from the legal definitions)

## Questions

### How many litres are in a gallon?

A US gallon is exactly 3.785411784 L and an imperial (UK) gallon exactly 4.54609 L, 20% more. The US gallon is defined as 231 cubic inches; the imperial gallon was first defined in 1824 as the volume of 10 lb of water and is now fixed in litres by the UK Weights and Measures Act 1985.

### How many millilitres are in a cup?

A US customary cup is 236.5882365 mL, 1/16 of a US gallon. US nutrition labels use a rounded 240 mL cup (21 CFR 101.9), and the metric cup used in Australia and New Zealand is 250 mL. The gap between 236.6 and 250 mL is about 6% of a cup, enough to matter in baking.

### Is a UK pint bigger than a US pint?

Yes, by 20%. An imperial pint is 20 imperial fluid ounces, 568.26125 mL; a US liquid pint is 16 US fluid ounces, 473.176473 mL. The fluid ounces differ as well, 28.4130625 mL against 29.5735295625 mL, so a UK pint of beer is 1.2 US pints.

### How many millilitres are in a tablespoon?

A US tablespoon is 14.78676478125 mL, half a US fluid ounce and exactly 3 teaspoons. The metric tablespoon is 15 mL, the figure US nutrition labels also round to. Australia's tablespoon is 20 mL (4 teaspoons), so an Australian spoonful holds 35% more than a US one.

### How many litres are in a barrel of oil?

Exactly 158.987294928 L. An oil barrel is 42 US gallons, and 42 × 3.785411784 L gives that figure, about 0.159 m³. Oil production and prices use this barrel; a US beer barrel is 31 gallons, so check which barrel a source means.

### How accurate is the volume converter?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “1 US gallon to litres” is checked against US gallon = 231 in³ × (0.0254 m)³ = 3.785411784 L (NIST HB 44 App. C); Python 3.8 fractions from the legal definitions.

### Where does the method come from?

NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors; NIST Handbook 44, Appendix C — General tables of units of measurement.

## Sources

- [NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44, Appendix C — General tables of units of measurement](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)