Calculating petrol expense is a four-step process: measure the distance, find your vehicle's real mileage, note today's petrol price, and multiply through the formula. Below is the math, two worked examples (one in km/L, one in MPG), and the rules of thumb that turn the formula into a real-world fuel budget.
The petrol cost formula
Cost = (Distance ÷ Mileage) × Price per unit
That single equation handles every petrol calculator on this site. Distance and mileage must share the same unit. The calculator normalizes everything to a common base — kilometres and litres — and then applies the price per litre, gallon, or whatever the user picked.
The exact international-mile and US MPG conversion factors used here are documented in the NIST Guide to the SI. European consumer fuel information also uses L/100 km, km/L, and compatible mile-and-gallon units under Directive 1999/94/EC.
Step 1 — Measure your distance
Distance is the simplest input. For a daily commute, measure one-way and tick the round-trip option to double it. For a single journey between two cities, use the figure your maps app gives you. For a delivery rider tallying a day's work, sum every leg.
Step 2 — Find your vehicle's real mileage
A rated fuel-economy figure is measured under a defined test procedure and may not represent a particular route. Traffic, speed, temperature, load, maintenance, and driving style can all change observed use. For planning, calculate an average from consistent full-tank records:
- Fill the tank to the click. Reset the trip meter.
- Drive normally for a few hundred kilometres. Don't empty the tank — refill at the same pump and brand for consistency.
- Fill to the click again. Note the litres on the receipt and the trip meter reading.
- Divide trip distance by litres added. That's your real km/L. If your bike has a 50 km/L sticker but two fill-ups give you 600 km on 14 L, your real mileage is 42.9 km/L.
Step 3 — Note today's petrol price
Fuel prices and pricing schedules vary by jurisdiction and station. Enter the value shown at the pump or on a dated receipt, using the same litre, US-gallon, or Imperial-gallon unit selected in the calculator. The tool does not fetch live prices or exchange rates.
Step 4 — Apply the formula
Plug the three numbers in. The calculator does it live as you type, so you can sweep distance up and down to see the cost react.
Worked example 1 — metric commute
- Daily distance: 30 km (one-way)
- Bike mileage (real): 50 km/L
- Illustrative petrol price: 2.00 local currency units per litre
Petrol used = 30 ÷ 50 = 0.6 L. Daily cost = 0.6 × 2.00 = 1.20 local currency units. Monthly (30 days) = 36.00. With round-trip selected (60 km/day), the result is 2.40/day and 72.00/month.
Worked example 2 — MPG commute
- Daily distance: 25 miles
- Car mileage: 35 US MPG
- Illustrative petrol price: 3.60 local currency units per US gallon
Petrol used = 25 ÷ 35 = 0.71 US gal. Daily cost = 0.7142857 × 3.60 = 2.57. Monthly (22 working days) = 56.57. Annual (220 working days) = 565.71.
Common mistakes
- Using manufacturer mileage instead of real mileage. A rated figure can differ from observed use, and the gap compounds across an annual budget.
- Forgetting round-trip. A 30 km one-way commute is actually 60 km each day.
- Using last month's petrol price. Price moves weekly in most countries.
- Mixing units. Don't enter mileage in km/L and distance in miles. The calculator's unit toggles prevent this but it's easy to do on paper.
Try it on your trip
Published 1 May 2026. Reviewed and corrected 17 September 2026 by the Petrol Expense Calculator editorial team. Report reproducible errors through the contact page.
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