Convert Imperial Gallon to US Gallon
Convert imperial gallons to us gallons instantly. 1 imperial gallon = 1.2009499255 us gallon — use the live calculator, the exact formula, a conversion table and worked examples. Also check the US Gallon to Imperial Gallon converter for the reverse conversion.
Units explained
Imperial Gallon
The imperial gallon is defined as exactly 4.54609 liters.
Set by the British Weights and Measures Act of 1824 as the volume of 10 lb of water.
Used for fuel and beverages in the UK and Commonwealth.
UK, 1824.
US Gallon
The US liquid gallon is defined as exactly 231 cubic inches (3.785411784 liters).
Based on the English wine gallon of 231 cubic inches, retained by the US after 1824.
The primary US unit for fuel, milk and large beverage containers.
English wine gallon; US since 1824.
Imperial Gallon to US Gallon conversion formula
The relationship between imperial gallons and us gallons:
To convert imperial gallons to us gallons, multiply the value in imperial gallons by 1.2009499255. To reverse, multiply us gallons by 0.8326741846.
How to use this converter
Type a value into the calculator. The result in us gallons updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the US Gallon to Imperial Gallon converter for the reverse direction.
Step-by-step: convert imperial gallons to us gallons
- Write down the value in imperial gallons (gal).
- Multiply that value by the factor 1.2009499255.
- The product is the equivalent value in us gallons (gal).
- To reverse, multiply the us gallon value by 0.8326741846.
Worked examples
Example 1 — Convert 1 gal to gal:
1 × 1.2009499255 = 1.2009499255 gal
Example 2 — Convert 100 gal to gal:
100 × 1.2009499255 = 120.0949925505 gal
Real-world example — Postcard and small-object dimensions
A postcard is about 5 imperial gallons wide. Converting to us gallons is essential for international postal addressing forms that ask for dimensions in different units across countries.
5 gal × 1.2009499255 = 6.0047496275 gal
Real-world example — Ruler-scale measurements
A 30-imperial gallon school ruler converts cleanly to us gallons — useful when buying a desk accessory from a retailer whose product specs use a different unit.
30 gal × 1.2009499255 = 36.0284977651 gal
Real-world example — Hardware-scale dimensions
A 10-imperial gallon fastener or component is about as long as a thumbnail. Mechanics and DIY enthusiasts convert between imperial gallons and us gallons daily when mixing metric and imperial tools.
10 gal × 1.2009499255 = 12.009499255 gal
Imperial Gallon to US Gallon conversion table
Standard reference values for converting imperial gallons to us gallons:
| Imperial Gallon [gal] | US Gallon [gal] |
|---|---|
| 0.01 | 0.0120094993 |
| 0.1 | 0.1200949926 |
| 1 | 1.2009499255 |
| 2 | 2.401899851 |
| 3 | 3.6028497765 |
| 4 | 4.803799702 |
| 5 | 6.0047496275 |
| 10 | 12.009499255 |
| 20 | 24.0189985101 |
| 30 | 36.0284977651 |
| 40 | 48.0379970202 |
| 50 | 60.0474962752 |
| 100 | 120.0949925505 |
| 500 | 600.4749627524 |
| 1000 | 1200.9499255049 |
Frequently asked questions
How many us gallons is 1 imperial gallon?
How do I convert imperial gallons to us gallons?
How do I convert us gallons back to imperial gallons?
How many us gallons is 100 imperial gallons?
Popular volume unit conversions
Convert Imperial Gallon to other volume units
Show all Imperial Gallon conversions
Metric / SI (15 units)
US Customary (Liquid) (15 units)
US Customary (Dry) (5 units)
Imperial (UK) (13 units)
Cubic (length-derived) (4 units)
Cooking / Culinary (5 units)
Sources & references
Conversion factor (1 gal = 1.2009499255 gal) verified against the following authoritative sources:
- BIPM — The International System of Units (SI Brochure 9th ed.)
Official BIPM publication defining the seven SI base units (including the meter) and the rules for their use. The global authority on units of measurement.
- NIST — Guide to the SI
US National Institute of Standards and Technology reference covering the SI base and derived units with definitions and usage rules for US technical practice.
- NIST Special Publication 811 — Guide for the Use of the International System of Units
Detailed NIST guide covering exact conversion factors between SI and US customary units along with formatting and rounding conventions.
- NIST — Refinement of values for the yard and pound (Federal Register 1959)
The treaty (signed by US
- International Hydrographic Organization — Resolution on the Nautical Mile
International authority that standardised the nautical mile at exactly 1852 m in 1929 — the value adopted worldwide for sea and air navigation.