Journey details
Extras (tolls, ferry, Eurotunnel, parking; leave blank if none)
How the cost is worked out
UK fuel is sold in litres but economy is quoted in miles per imperial gallon, and one imperial gallon is 4.546 litres. So the calculation is litres = (distance ÷ MPG) × 4.546, then cost = litres × price per litre. That gallon conversion is the step most people miss when trying to do this in their head, and it's also why comparing against American MPG figures misleads: a US gallon is only 3.785 litres, so a US-rated 35 MPG equals about 42 MPG in UK terms.
Which MPG figure to use
Use your real-world average, not the brochure figure. Most cars since 2010 show a long-term average on the trip computer, which is close enough for planning, though computers typically flatter by 5 to 10%. For an honest number, brim the tank, drive normally until the next fill, brim it again, and divide miles covered by gallons added (litres ÷ 4.546). Motorway runs will beat your average; town driving will undercut it, sometimes by a third.
Cost per mile is the number to remember
Once you know your pence-per-mile figure, every journey decision gets easier: whether the cheaper supermarket across town actually saves money, what to charge a mate for a lift, or whether the train beats driving once parking is added. For a typical UK car it currently lands between 12p and 20p per mile in fuel alone. Remember that's only the marginal cost; depreciation, tyres, servicing and insurance roughly double the true per-mile cost of running a car.
Crossings and tolls belong in the total
Fuel is rarely the whole cost of a longer trip. UK toll roads and crossings, a ferry, or the Eurotunnel can dwarf the fuel figure on some routes, which is why they're line items here rather than an afterthought. Enter them once and the per-person split includes them too, handy for settling who owes what on a shared drive. For round trips, most tolls and crossings are paid in each direction, so that's the default; untick it for anything bought as a return.
Small habits, measurable savings
Because the maths is linear, savings scale directly. Improving from 38 to 42 MPG cuts every fuel bill by 9.5%. The reliable ways to get there: correct tyre pressures (a 10 psi shortfall costs a few percent), removing roof bars and boxes when unused, gentle throttle away from lights, and lifting off early rather than braking late. Speed matters most of all on motorways, where fuel use rises steeply past 60 mph because aerodynamic drag grows with the square of speed.