Math

Convert Gallons to Liters (US & UK)

Q: What is the exact value of a US gallon in liters?

The US liquid gallon equals exactly **3.785411784 liters**. This is derived from the definition of the US gallon as 231 cubic inches, combined with the exact conversion 1 inch = 25.4 mm. NIST Handbook 44 (Appendix C) lists this as the official factor used in US commerce and legal metrology.

Q: What is the exact value of a UK (Imperial) gallon in liters?

The Imperial gallon equals exactly **4.54609 liters** by definition, as established in the UK Weights and Measures Act 1985 and confirmed by the international yard and pound agreement. This value is exact — not an approximation — and is the standard used in the UK, Canada (historically), and Commonwealth countries for certain applications.

Q: How much bigger is the UK gallon compared to the US gallon?

The Imperial gallon is **approximately 20.095% larger** than the US liquid gallon (4.54609 ÷ 3.785411784 ≈ 1.20095). This means 1 UK gallon = ~1.201 US gallons, or conversely 1 US gallon ≈ 0.8327 UK gallons. This gap is large enough to cause serious errors in fuel economy comparisons between US and UK vehicle specs.

Q: Does Canada use US gallons or Imperial gallons?

Canada officially switched to the **metric system (liters)** for fuel and trade in the 1970s–1980s. Before metrication, Canada used Imperial gallons. Today, Canadian gas stations pump liters exclusively. However, some older Canadian appliances, well-water systems, and rural water tanks may still reference Imperial gallons in documentation, so the distinction remains relevant.

Q: Why do US and UK fuel economy numbers look so different for the same car?

Because **UK MPG uses the larger Imperial gallon** while US MPG uses the smaller US gallon. A car rated at 40 US MPG ≈ 48 UK MPG (multiply by ~1.2009), even though both measure the same physical fuel efficiency. To compare fairly, always convert both figures to L/100km: `L/100km = 235.214 ÷ MPG_US` or `282.481 ÷ MPG_UK`.

Q: Is there a US dry gallon, and should I use it here?

Yes — the **US dry gallon ≈ 4.40488 liters** and is used for agricultural commodities like grains and berries (e.g., a US dry gallon of corn). It is legally distinct from the US liquid gallon and is NOT used for fuel, beverages, or most industrial liquids. This converter handles the US liquid gallon (3.785411784 L), which covers the overwhelming majority of everyday use cases.

Q: How do I convert gallons per minute (GPM) to liters per minute (LPM)?

Apply the same factor to the numeric value: **LPM = GPM × 3.785411784** (US) or **LPM = GPM × 4.54609** (UK). For example, a US pump rated at 10 GPM delivers 37.85 LPM. Flow rate conversions are linear — the time unit (minutes, hours) cancels out and does not affect the gallon-to-liter factor.

Q: What conversion factor should I use for quick mental math?

A reliable mental shortcut: **1 US gallon ≈ 3.785 L** (or roughly 'multiply by 3.8'). For Imperial gallons, use **1 UK gal ≈ 4.546 L** (or roughly 'multiply by 4.5'). The error with these shortcuts is under 0.5%, acceptable for everyday estimates. For billing, engineering, or legal compliance, always use the full NIST-certified factors.

Q: Do US and UK gallons differ for gases or only for liquids?

The gallon — in all its definitions — is a **unit of volume**, applied equally to liquids and gases when measured at a specified pressure and temperature. In practice, gas volumes in the US are more commonly expressed in cubic feet or cubic meters at standard conditions (14.696 psi, 60 °F per API standards), so gallons are rarely used for gases commercially. The liquid-to-liter conversion factors remain the same regardless of the substance.

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This converter handles the two legally distinct gallon definitions still in active use worldwide: the US liquid gallon (231 cubic inches = 3.785411784 L) and the Imperial (UK) gallon (277.419 cubic inches = 4.54609 L). The two differ by roughly 20%, which is large enough to cause costly real-world errors in fuel, trade, and recipe measurement. Use this tool whenever you need to move between gallons and liters—or between US and Imperial gallons—with exact, standard-body-certified values. The NIST defines both conversions precisely in NIST Handbook 44 Appendix C, so no approximations are used here.

Last reviewed: May 12, 2026 Verified by Hacé Cuentas Team Source: NIST Handbook 44 (2024 ed.) – Appendix C: General Tables of Units of Measurement, Wikipedia – Gallon (US liquid, US dry, and Imperial definitions), NIST Special Publication 811 (2008) – Guide for the Use of the International System of Units 100% private

When to use this calculator

Converting a US car's fuel efficiency (MPG) into L/100km for a European road trip budget
Calculating how many liters of paint or pesticide a US 5-gallon bucket contains for international shipping labels
Adapting a UK brewing recipe that lists Imperial gallons into liters for American homebrew equipment
Verifying fuel-pump readings at a Mexican or Canadian border station where pumps dispense liters but the tank capacity is listed in US gallons
Comparing water-heater or pool-pump capacities listed in UK gallons on British product pages with US-gallon specs
Re-labeling bulk food or beverage containers for export compliance when the country of origin uses a different gallon standard

Calculation Example

Typical Value
Result

Result: Result

How it works

3 min read

How It Is Calculated

Both conversions use the exact factors adopted by the international yard and pound agreement (1959) and codified by NIST:

US liquid gallon → liters:
  L = gal_US × 3.785411784

Imperial (UK) gallon → liters:
  L = gal_UK × 4.54609

Liters → US liquid gallon:
  gal_US = L ÷ 3.785411784

Liters → Imperial (UK) gallon:
  gal_UK = L ÷ 4.54609

US gallon → Imperial gallon:
  gal_UK = gal_US × (3.785411784 ÷ 4.54609) ≈ gal_US × 0.832674

Imperial gallon → US gallon:
  gal_US = gal_UK × (4.54609 ÷ 3.785411784) ≈ gal_UK × 1.200950

> Why are the factors exact? The liter is defined as exactly 1 cubic decimeter (1 dm³). The US gallon is defined as exactly 231 in³, and 1 inch = 25.4 mm exactly, giving the exact 3.785411784 L figure. The Imperial gallon was redefined in 1985 as exactly 4.54609 L by UK statute.

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Reference Table

Gallons (US)	Gallons (UK / Imperial)	Liters
0.25	0.208	0.946
0.5	0.416	1.893
1	0.833	3.785
2	1.665	7.571
5	4.163	18.927
10	8.327	37.854
15	12.490	56.781
20	16.653	75.708
50	41.633	189.271
55	45.796	208.198
100	83.267	378.541

> All values rounded to 3 decimal places for readability; the converter uses the full NIST precision.

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Typical Cases

Case 1 – Fuel tank for a road trip

A US pickup truck has a 26-gallon (US) tank. A driver crossing into Canada wants to know the capacity in liters to compare pump prices:

26 × 3.785411784 = 98.421 L

The tank holds 98.4 liters. At CAD $1.60/L, a full fill-up costs approximately CAD $157.47.

Case 2 – British homebrew recipe

A classic British ale recipe calls for 5 Imperial gallons of water. The American brewer's kettle is marked in liters:

5 × 4.54609 = 22.730 L

The brewer needs 22.73 liters — notably more than the 18.93 L a US-gallon interpretation would give, a difference of nearly 3.8 liters that would throw off malt concentration significantly.

Case 3 – Industrial chemical order

A factory orders 200 US gallons of solvent. The EU supplier invoices in liters:

200 × 3.785411784 = 757.082 L

The order corresponds to 757.08 liters. If the purchasing agent mistakenly used the UK gallon factor, they'd calculate 909.2 L — an over-order of more than 152 liters.

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Common Mistakes

1. Assuming all gallons are equal. The US and UK gallon differ by ~20.1%. Using the wrong factor in fuel calculations, for example, leads to errors of roughly 1 liter per 5 liters converted — significant at scale.

2. Confusing the US liquid gallon with the US dry gallon. The US dry gallon (≈ 4.40488 L, used for grain and produce) is legally distinct and larger than the liquid gallon. This converter handles the liquid gallon, which is the standard for fuel, beverages, and most consumer goods.

3. Rounding the conversion factor to 3.78 or 4.55. Using 3.78 instead of 3.785411784 introduces an error of ~0.14% per conversion. Over 10,000 gallons (e.g., a swimming pool or fuel depot), that shortfall is ~14 liters — enough to matter for billing or compliance.

4. Applying US-gallon MPG directly to liters-per-100km. The formula is L/100km = 235.214 ÷ MPG_US, not a simple multiplication. Using the liters-per-gallon factor directly without the distance component gives a dimensionally incorrect result.

5. Ignoring temperature effects in legal metrology. For trade purposes (fuel dispensing), volumes are often corrected to 15 °C (60 °F). The raw gallon-to-liter conversion does not include temperature correction; always check whether a thermal volume correction is required for official or commercial use.

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Related Calculators

Liters to Cubic Meters Converter

Miles to Kilometers Converter

Fuel Efficiency: MPG to L/100km

Fluid Ounces to Milliliters

Frequently asked questions

What is the exact value of a US gallon in liters?

The US liquid gallon equals exactly 3.785411784 liters. This is derived from the definition of the US gallon as 231 cubic inches, combined with the exact conversion 1 inch = 25.4 mm. NIST Handbook 44 (Appendix C) lists this as the official factor used in US commerce and legal metrology.

What is the exact value of a UK (Imperial) gallon in liters?

The Imperial gallon equals exactly 4.54609 liters by definition, as established in the UK Weights and Measures Act 1985 and confirmed by the international yard and pound agreement. This value is exact — not an approximation — and is the standard used in the UK, Canada (historically), and Commonwealth countries for certain applications.

How much bigger is the UK gallon compared to the US gallon?

The Imperial gallon is approximately 20.095% larger than the US liquid gallon (4.54609 ÷ 3.785411784 ≈ 1.20095). This means 1 UK gallon = ~1.201 US gallons, or conversely 1 US gallon ≈ 0.8327 UK gallons. This gap is large enough to cause serious errors in fuel economy comparisons between US and UK vehicle specs.

Does Canada use US gallons or Imperial gallons?

Canada officially switched to the metric system (liters) for fuel and trade in the 1970s–1980s. Before metrication, Canada used Imperial gallons. Today, Canadian gas stations pump liters exclusively. However, some older Canadian appliances, well-water systems, and rural water tanks may still reference Imperial gallons in documentation, so the distinction remains relevant.

Why do US and UK fuel economy numbers look so different for the same car?

Because UK MPG uses the larger Imperial gallon while US MPG uses the smaller US gallon. A car rated at 40 US MPG ≈ 48 UK MPG (multiply by ~1.2009), even though both measure the same physical fuel efficiency. To compare fairly, always convert both figures to L/100km: L/100km = 235.214 ÷ MPG_US or 282.481 ÷ MPG_UK.

Is there a US dry gallon, and should I use it here?

Yes — the US dry gallon ≈ 4.40488 liters and is used for agricultural commodities like grains and berries (e.g., a US dry gallon of corn). It is legally distinct from the US liquid gallon and is NOT used for fuel, beverages, or most industrial liquids. This converter handles the US liquid gallon (3.785411784 L), which covers the overwhelming majority of everyday use cases.

How do I convert gallons per minute (GPM) to liters per minute (LPM)?

Apply the same factor to the numeric value: LPM = GPM × 3.785411784 (US) or LPM = GPM × 4.54609 (UK). For example, a US pump rated at 10 GPM delivers 37.85 LPM. Flow rate conversions are linear — the time unit (minutes, hours) cancels out and does not affect the gallon-to-liter factor.

What conversion factor should I use for quick mental math?

A reliable mental shortcut: 1 US gallon ≈ 3.785 L (or roughly 'multiply by 3.8'). For Imperial gallons, use 1 UK gal ≈ 4.546 L (or roughly 'multiply by 4.5'). The error with these shortcuts is under 0.5%, acceptable for everyday estimates. For billing, engineering, or legal compliance, always use the full NIST-certified factors.

Do US and UK gallons differ for gases or only for liquids?

The gallon — in all its definitions — is a unit of volume, applied equally to liquids and gases when measured at a specified pressure and temperature. In practice, gas volumes in the US are more commonly expressed in cubic feet or cubic meters at standard conditions (14.696 psi, 60 °F per API standards), so gallons are rarely used for gases commercially. The liquid-to-liter conversion factors remain the same regardless of the substance.