Speed converter
Miles per hour, kilometres per hour, metres per second and knots, all at once.
Britain drives in miles and buys petrol in litres
Speed limits and road signs are in miles per hour and always have been. Fuel is sold in litres. Fuel economy is still quoted in miles per gallon, often against a gallon nobody has bought since 1995.
That combination is why a British driver needs three conversions to work out what a journey costs, and why a page like this gets used.
Converting in your head
Multiply mph by 1.6 for km/h, or add 60%. Going the other way, take 60% and add a little: 100 km/h is about 62 mph.
A neater trick uses consecutive Fibonacci numbers, which approximate the ratio closely: 30 mph is about 50 km/h, 50 is about 80, 80 is about 130. Accurate to within a percent across the range that matters on a road.
Knots, and why they survive
A knot is one nautical mile an hour, and a nautical mile is one minute of latitude. That is the whole reason it persists: a speed in knots converts directly into movement across a chart, with no factor in between.
The name comes from the log line, a rope knotted at intervals and paid out behind a ship against a sand timer. The knots counted off gave the speed.
Common questions
- What is 70 mph in km/h?
- About 113 km/h. The UK motorway limit is therefore slightly above the 110 km/h common on continental motorways and below the 130 km/h used in France. Driving abroad, the useful approximation is that 100 km/h is a little over 62 mph.
- How do I convert mph to km/h in my head?
- Multiply by 1.6, or add 60%. For the other direction, take 60% and add a little: 100 km/h is about 62 mph. A sharper trick uses consecutive Fibonacci numbers, which approximate the ratio: 50 mph is about 80 km/h, and 80 mph is about 130 km/h.
- What is a knot?
- One nautical mile per hour, which is 1.852 km/h or about 1.151 mph. A nautical mile is one minute of latitude, so a speed in knots translates directly into position change on a chart. That is why shipping and aviation kept it.
- Why does Britain use miles per hour?
- Road signage and speed limits were never converted, unlike almost everything else. Vehicles sold in the UK must have speedometers marked in both, with mph as the primary scale, which is a legal requirement rather than a convenience.