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    Volume Conversion

    21 Gallon (US) to Gallon (UK)

    Quick answer

    21 Gallon (US) (gal) equals

    17.486158 gal (UK)

    To convert 21 Gallon (US)s to Gallon (UK)s, multiply by 0.8326741846. Therefore, 21 Gallon (US) (gal) equals 17.486158 Gallon (UK) (gal (UK)).

    Formula: gal (UK) = gal × 0.8326741846. Calculation: 21 × 0.8326741846 = 17.486158 gal (UK).

    Convert Gallon (US) (gal) to Gallon (UK) (gal (UK)) online with our free unit converter. In volume, 1 gal = 0.832674 gal (UK). Explore step-by-step conversion steps, mathematical formulas, and quick conversion reference tables below.

    Worked example: converting Gallon (US) to Gallon (UK)

    1. Step 1 — start with the formula. gal (UK) = gal × 0.8326741846
    2. Step 2 — substitute your figure. 21 gal × 0.8326741846
    3. Step 3 — read the result. 21 gal = 17.486158 gal (UK)
    4. Step 4 — check it in reverse. gal = gal (UK) × 1.2009499255 returns 21 gal, which matches the figure you started with.

    Volume Converter

    40 units available

    Decimals:
    17.486158

    21 gal = 17.486158 gal (UK)

    Formula:

    gal (UK) = gal × 0.8326741846

    With your value: 21 gal = 17.48615788 gal (UK)

    How big is that?

    Everyday references on a logarithmic scale, all expressed in gallon (us) so you can sanity-check the result.

    Milk carton (1/7.9e+4 your value)0.000264 gal
    Kitchen bucket (1/7.9e+3 your value)0.00264 gal
    Car fuel tank (1/1.6e+3 your value)0.0132 gal
    Bathtub (1/4.0e+2 your value)0.0528 gal
    Hot tub (1/53 your value)0.396 gal
    Your value21.0 gal

    Other Gallon (US) to Gallon (UK) conversions

    15 gal to gal (UK)16 gal to gal (UK)18 gal to gal (UK)20 gal to gal (UK)24 gal to gal (UK)25 gal to gal (UK)26 gal to gal (UK)Full Gallon (US) to Gallon (UK) converter

    Gallon (US) to Gallon (UK) Conversion Table

    Gallon (US) (gal)Gallon (UK) (gal (UK))
    0.50.416337
    10.832674
    21.665348
    54.163371
    108.326742
    1512.490113
    2016.653484
    2520.816855
    5041.633709
    10083.267418
    250208.168546
    500416.337092
    1000832.674185

    Gallon (UK) to Gallon (US) Conversion Table

    Gallon (UK) (gal (UK))Gallon (US) (gal)
    0.50.600475
    11.20095
    22.4019
    56.00475
    1012.009499
    1518.014249
    2024.018999
    2530.023748
    5060.047496
    100120.094993
    250300.237481
    500600.474963
    10001,200.949926

    Where these values come from

    Every factor on this page is traced to the document that defines it. See the full provenance dataset or our methodology.

    Gallon (US) (gal)

    Volume

    Exact conversion

    The US liquid gallon is exactly 231 cubic inches = 3.785 411 784 L.

    Source: NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices

    National Institute of Standards and Technology (US) · Reviewed 2026-07-01

    Gallon (UK) (gal (UK))

    Volume

    Exact conversion

    The imperial gallon is exactly 4.546 09 L (Weights and Measures Act 1985).

    Source: NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices

    National Institute of Standards and Technology (US) · Reviewed 2026-07-01

    How to Convert Gallon (US) to Gallon (UK)

    To convert from Gallon (US) (gal) to Gallon (UK) (gal (UK)), multiply the value by 0.8326741846.

    Formula: gal (UK) = gal × 0.8326741846

    Reverse: gal = gal (UK) × 1.2009499255

    Step-by-Step Example

    1. Start with your value in Gallon (US). For example: 5 gal
    2. Multiply by the conversion factor: 5 x 0.8326741846
    3. The result is: 4.163371 gal (UK)

    Gallon (US) and Gallon (UK) Relationship

    Both Gallon (US) and Gallon (UK) belong to the US Customary / Imperial system, making this an intra-system conversion within volume. Since 1 Gallon (US) represents a fraction of a Gallon (UK) (1 Gallon (UK) ≈ 1.2009 Gallon (US)s), the Gallon (UK) is the larger measurement interval. Understanding the relationship between Gallon (US) and Gallon (UK) is essential for accurate calculations in physics, engineering, commerce, and daily applications.

    About Gallon (US)

    The US gallon equals approximately 3.785 liters and is the standard unit for fuel and large liquid volumes in the United States.

    Fun Fact: The US gallon is about 20% smaller than the imperial (UK) gallon.

    Common uses of Gallon (US): fuel (US), beverages, swimming pools.

    About Gallon (UK)

    The imperial gallon equals exactly 4.54609 litres, about 20% larger than the US gallon — a frequent source of fuel-economy confusion.

    Common uses of Gallon (UK): UK fuel figures, brewing, older recipes.

    Frequently Asked Questions

    What is 21 Gallon (US) in Gallon (UK)?
    21 Gallon (US) equals 17.486158 Gallon (UK).
    How many Gallon (UK)s are in 1 Gallon (US)?
    There are exactly 0.8326741846 Gallon (UK)s in 1 Gallon (US). This means the conversion factor from Gallon (US) to Gallon (UK) is 0.8326741846.
    What is the formula to convert Gallon (US) to Gallon (UK)?
    The formula to convert Gallon (US) to Gallon (UK) is: gal (UK) = gal × 0.8326741846. For example, to convert 10 Gallon (US)s to Gallon (UK)s, you would multiply 10 by 0.8326741846.
    What is a Gallon (US)?
    The US gallon equals approximately 3.785 liters and is the standard unit for fuel and large liquid volumes in the United States.
    What is a Gallon (UK)?
    The imperial gallon equals exactly 4.54609 litres, about 20% larger than the US gallon — a frequent source of fuel-economy confusion.
    What are the common uses of Gallon (US)?
    The Gallon (US) is commonly used in fuel (US), beverages, swimming pools.

    Related Volume Conversions

    Liter → Gallon (UK)Milliliter → Gallon (UK)Microliter → Gallon (UK)Kiloliter → Gallon (UK)Cubic Meter → Gallon (UK)Cubic Centimeter → Gallon (UK)Cubic Millimeter → Gallon (UK)Cubic Kilometer → Gallon (UK)Gallon (US) → Liter

    Cite this page

    UnitConvertLab. (2026). 21 Gallon (US) to Gallon (UK). UnitConvertLab. Retrieved from https://unitconvertlab.com/convert/21-gallon_us-to-gallon_uk