MPG Calculator

Background.

This MPG calculator turns a fill-to-fill distance and a fuel volume into every fuel-economy unit a driver might be asked for: US miles per gallon, UK (imperial) miles per gallon, kilometres per litre, litres per 100 km, and US gallons per 100 miles. The arithmetic is one division — distance divided by fuel — but the unit story is where most people get tripped up. The US gallon (3.785411784 L, fixed exactly by NIST SP 811) and the UK (imperial) gallon (4.54609 L, fixed exactly by the UK Weights and Measures Act 1985) are not the same volume; the imperial gallon is roughly 20% larger, which means a car advertised at 50 mpg in a British magazine is actually delivering about 41 mpg by the US definition, with no change in real-world efficiency. That single mismatch is responsible for more confused new-car buyers than any other number on the window sticker.

The EU and most of the rest of the world side-step the gallon problem entirely by publishing fuel economy in litres per 100 km (L/100km) — a 'consumption' unit rather than an 'economy' unit, where lower is better. The EU prefers L/100km for a reason that matters here: it is linear in fuel cost. Doubling the L/100km figure doubles your fuel bill and doubles your CO2 emissions. MPG, by contrast, is a reciprocal unit, which produces the well-documented 'MPG illusion' (Larrick and Soll, Science, 2008): consumers consistently overvalue a 10-mpg jump at the top of the scale (40 → 50 mpg) and undervalue the same 10-mpg jump at the bottom (15 → 25 mpg), even though the second case saves more than three times as much fuel and CO2 over the same distance.

That finding is the reason the EPA added 'gallons per 100 miles' as a co-equal metric on the redesigned 2013 window sticker, and it is why this calculator outputs every standard unit side by side — so you can read the one that maps to the question you are actually trying to answer (how much fuel will I burn this year, how much will it cost, how does it compare to the car on the next lot, how does it compare to the rating my friend in London just quoted).

The conversion math itself is anchored on three exact constants: 1 mile = 1.609344 km (NIST SP 811, by international yard-and-pound agreement of 1959), 1 US gallon = 3.785411784 L (NIST SP 811), and 1 UK gallon = 4.54609 L (UK statute). Every output is computed in Decimal arithmetic from those exact factors — no float drift, no rounding-induced disagreement between the US MPG and the L/100km figures.

Below the calculator you will find sections on the MPG illusion (and why the EPA fixed it with gallons-per-100-miles), how CAFE standards translate to a fleet-average MPG target (54.5 mpg by 2026 in the original Obama-era rule, with subsequent NHTSA revisions), why hybrid combined MPG is the number that actually matters for cost-of-ownership math (and why the city/highway split flips for hybrids vs conventional cars), how the EPA's MPGe figure converts kWh of electricity into an MPG-equivalent for EVs (the 33.7 kWh-per-gallon energy reference), and the well-documented gap between EPA-rated and real-world fuel economy in cold weather, on short trips, and at sustained highway speeds above the EPA test cycle's 48 mph average. The goal is the same as the EPA window sticker's: give you a number you can plug into a budget or a CO2 estimate without first having to recompute the units in your head.

What is mpg calculator?

MPG — miles per gallon — is the ratio of distance driven to fuel consumed, the unit the United States has used to rate vehicle fuel economy since the EPA began publishing window-sticker ratings under the Energy Policy and Conservation Act of 1975. A car that travels 300 miles on 12 US gallons of gasoline has an MPG of 25. The reciprocal is L/100km, the unit used by the EU, Canada, Australia, and most of the rest of the world: a car that consumes 12 litres of fuel over 100 km has a rating of 12 L/100km (and an equivalent US MPG of about 19.6). Mathematically MPG and L/100km are inverses — MPG × L/100km ≈ 235.215 — and that inverse relationship is the entire reason the EPA added gallons-per-100-miles to the window sticker in 2013: it makes fuel savings track linearly with the rating change, where the raw MPG number does not. CAFE — Corporate Average Fuel Economy — is the regulatory programme NHTSA administers that requires each manufacturer's new-vehicle fleet to meet a sales-weighted MPG floor; CAFE has been the principal driver of US fuel-economy improvement since 1978, when the original passenger-car standard of 18 mpg took effect. UK and EU window-sticker MPG figures, when they appear, are measured against the imperial gallon (4.54609 L), which is about 20% larger than the US gallon, so a British 'mpg' figure is roughly 20% higher than the US figure for the same car burning the same amount of fuel over the same distance. None of these ratings is a measurement of how efficiently your car turns chemical energy into motion — that is thermodynamic efficiency, measured in percent — they are simply distance-per-fuel ratios, useful precisely because distance and fuel are the two numbers a driver can actually observe at the pump.

How to use this calculator.

1. 01Fill the tank completely at the first stop. Click the pump off, wait a few seconds for the fuel to settle, and top up gently until the auto-shutoff trips a second time. Reset the trip odometer to zero. This is the start of your fill-to-fill measurement window.
2. 02Drive normally for at least one full tank — ideally two — so that variation in starting fuel level averages out. Long single-tank measurements (mixed city and highway) match the EPA combined rating most closely.
3. 03Refuel at the same pump (or at least the same pump model) using the same shutoff technique. The number on the pump display when it auto-stops is your fuel consumed, in gallons or litres. Record the trip-odometer reading before resetting it — that is your distance driven.
4. 04Enter the distance and select its unit (miles for US or UK drivers, kilometres for metric). Enter the fuel volume and select the matching unit — US gallons for a US pump, UK gallons for a UK pump, or litres if your receipt is metric.
5. 05Read the five outputs side by side. US MPG is the headline figure for an American window-sticker comparison. L/100km is the one to plug into a fuel-cost or CO2 estimate (because it is linear). UK MPG is the figure to use when comparing against British magazine reviews — and the gap from US MPG to UK MPG should be roughly 20%, confirming the calculator is reading the units correctly.
6. 06For trend tracking, log the result for every tank in a spreadsheet. The single-tank figure is noisy (weather, traffic, terrain, fuel batch), but the rolling 10-tank average is the cleanest real-world rating you will get without dyno equipment — and it is the number to compare against the EPA combined rating, not the highway-only figure.

The formula.

The core relationship is:

US MPG = distance (mi) / fuel (US gal)

From the US MPG figure every other standard unit is one exact conversion away. Using the NIST SP 811 (2008 ed.) constants — 1 mile = 1.609344 km exactly, 1 US gallon = 3.785411784 L exactly — the canonical conversions are:

L/100km = 235.214583 / US MPG km/L = US MPG × 1.609344 / 3.785411784 = US MPG × 0.4251437 UK MPG = US MPG × (4.54609 / 3.785411784) = US MPG × 1.20095 gal/100mi = 100 / US MPG

The constant 235.214583 in the L/100km conversion is not magic — it is (3.785411784 L/gal × 100) / 1.609344 km/mi, the exact product of the two NIST constants. The 1.20095 ratio between UK and US MPG is the same imperial-to-US gallon ratio (4.54609 / 3.785411784), fixed by UK statute and NIST. The calculator's engine does the math internally in Decimal arithmetic to avoid float drift, but a desk-calculator estimate from these formulas will match the on-screen figure to four significant digits in every case. The reverse direction — going from L/100km to MPG — is the EU's preferred frame, because L/100km is linear in fuel cost and CO2: doubling the L/100km figure doubles your fuel bill. MPG is reciprocal, which is why two cars rated five mpg apart at the bottom of the scale (15 vs 20 mpg) differ in actual fuel consumption by three times more than two cars rated five mpg apart at the top (40 vs 45 mpg) — the well-documented MPG illusion. The L/100km frame removes that illusion entirely; the gallons-per-100-miles output on this calculator is the US equivalent of the same fix.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Environmental Protection Agency. Fuel Economy Guide (model year 2025). The authoritative annual publication of EPA combined, city, and highway MPG ratings for every US-market vehicle, plus the gallons-per-100-miles secondary metric introduced in 2013.
2. [2]U.S. National Highway Traffic Safety Administration. Corporate Average Fuel Economy (CAFE) Standards — official regulatory text and history. The federal programme that sets the sales-weighted MPG floor each automaker's new-vehicle fleet must meet, beginning with the 18-mpg passenger-car standard of 1978.
3. [3]Thompson, A. & Taylor, B. N. (2008). NIST Special Publication 811: Guide for the Use of the International System of Units (SI). U.S. Department of Commerce. The authoritative source for the exact conversion constants used in this calculator: 1 mile = 1.609344 km and 1 US gallon = 3.785411784 L, both fixed by international agreement.
4. [4]International Energy Agency. Global Fuel Economy Initiative — annual report on light-duty vehicle fuel-economy trends across IEA member countries. The authoritative cross-country reference for how the US, EU, Japan, China, and India each express and regulate vehicle fuel economy.
5. [5]Larrick, R. P. & Soll, J. B. (2008). 'The MPG Illusion'. Science, vol. 320, no. 5883, pp. 1593–1594. The peer-reviewed paper that documented systematic consumer misperception of fuel savings from MPG improvements, and which motivated the EPA's 2013 addition of gallons-per-100-miles to the window sticker.
6. [6]United Kingdom. Weights and Measures Act 1985, Schedule 1. The statute that fixes the imperial gallon at 4.54609 litres exactly — the legal definition behind every UK window-sticker MPG figure and the source of the roughly 20% gap between UK and US MPG ratings.