MPG Calculator

Work out your car's true miles per gallon from what you put in the tank, and track it fill-up by fill-up.

Free, no sign-up
Runs in your browser
Updated 4 Nov 2026

Distance driven

Fuel to refill the tank

Price per litre (optional)

pence

For cost per mile. Petrol average: 172.78p (UK average, w/c 2 Nov 2026 (DESNZ)).

Dashboard MPG (optional)

MPG

See how far your trip computer is out.

For an accurate figure

Fill until the pump first clicks off, reset the trip, then measure the miles and the litres needed to fill to the click again.

Your fuel economy

MPG

Litres per 100 km
Miles per litre
US MPG equivalent
Cost per mile
Cost of this tank
Trip computer

Untick Full tank for a top-up. Its litres are added to the next full fill so the MPG stays correct.

No fill-ups yet

Start with a full tank and its odometer reading. Your MPG appears from the second full fill.

Average MPG

Best / worst

Miles logged

Cost per mile

MPG by fill-up

Date	Odometer	Litres	Paid	Fill	Miles	MPG	Delete

Saved only in this browser on this device. Nothing is sent to us.

How to use the mpg calculator

1 Fill the tank until the pump clicks off, then reset the trip meter or note the odometer reading.
2 Drive as normal until you next need fuel.
3 Fill to the click again. Enter the litres from the receipt and the miles since the last fill.
4 For a running average, add each fill-up to the log. Mark top-ups as partial so the maths stays brim-to-brim.

Worked example

One tank, brim to brim

Miles since last fill: 412
Litres to refill: 41.3

41.3 litres ÷ 4.546 = 9.08 gallons; 412 ÷ 9.08

= 45.4 MPG (6.23 L/100km)

Assumptions and limitations

Accurate MPG needs a consistent fill: same pump if possible, stop at the first click, no topping off.
One tank is a snapshot. Average three or more fills for a reliable figure, especially in winter.
Trip computers often read a few percent high, so expect your measured figure to be a little lower.
Your fill-up log is stored only in this browser (local storage). We never see it, and clearing site data removes it.

Why your MPG is lower than the brochure

Official figures come from the WLTP laboratory test: a warm engine, steady temperatures, no roof bars and no passengers. Real journeys add cold starts, queues, motorway speeds, heaters and air conditioning, so most drivers see less than the official number.

Short trips hurt most. An engine uses noticeably more fuel for the first few miles while it warms up, so a car that does lots of school runs can return far less than the same car on a long run.

Brim-to-brim is the only honest method

Fuel gauges are not linear and the dashboard average is an estimate, so the reliable way to measure MPG is to fill to the brim, drive, then measure exactly how much fuel it takes to fill to the brim again. The fuel you put back in is the fuel you used.

If you top up part-way through a tank, log it as a partial fill. The calculator adds those litres to the next full fill, so the average stays correct.

Using MPG to save money

Once you know your real MPG, use the fuel cost calculator for any journey, or put it into the running cost calculator to see what the car costs per month. A sudden drop in MPG can also be an early sign of under-inflated tyres, a dragging brake or a sensor fault, so it is worth keeping the log going.

Official guidance and sources

Run a free vehicle check

Check a car's recorded mileage at every MOT before you buy.

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All free tools

Fuel Cost Calculator Cost of any journey, return trip or a year of driving from miles, MPG and pence per litre. Car Running Cost Calculator Fuel, tax, MOT, servicing, insurance, parking and finance added up per month, per year and per mile. EV vs Petrol Cost Comparison Electric against petrol or diesel over a year and your ownership period, with break-even on the price gap.

Frequently Asked Questions

Fill the tank to the first click and note the mileage. When you next fill up, again to the first click, divide the miles driven by the litres used and multiply by 4.54609. For example, 412 miles on 41.3 litres is 412 ÷ (41.3 ÷ 4.54609) = 45.4 MPG.

Divide 282.48 by the L/100km figure to get UK MPG. So 6 L/100km is about 47.1 MPG, and 8 L/100km is about 35.3 MPG. The calculator shows both figures for every fill-up.

Yes. A UK (imperial) gallon is 4.546 litres and a US gallon is 3.785 litres, so the same car shows a higher MPG in the UK. Multiply a US MPG figure by 1.2 to get the rough UK equivalent. This calculator uses UK gallons.

Dashboard figures are estimated from injector timing rather than measured at the pump, and many read a few percent optimistic. Brim-to-brim measurement over several tanks is the more accurate figure, which is why it is the one to use for budgeting.

Add it to the log with Full tank unticked. The calculator holds those litres and adds them to the next full fill, so the MPG for that stretch still compares the full distance with all the fuel used.

Only in your own browser, using local storage on this device. Nothing is sent to our server. You can download it as a CSV file at any time, and clearing your browser's site data deletes it.

Figures and guidance last checked 4 November 2026. An independent free tool from Free Car Tax MOT Check; not affiliated with DVLA, DVSA or GOV.UK. Results are estimates to help you plan, not financial, legal or mechanical advice.