Speed Converter

Road signs say kilometres per hour in most of the world and miles per hour in a few countries, ships use knots, and physics problems want metres per second. This converter takes one speed and shows it in all of them at once, along with feet per second, Mach and the speed of light, so you never have to chain two conversions together in your head.

How to use it

1. Enter the speed you have.
2. Choose the unit you're starting from and the one you want as the headline answer.
3. Read the headline result, then glance down the list for every other unit.
4. Press Swap units to reverse the conversion.

When it helps

• Making sense of speed limits when driving abroad.
• Converting a runner's or cyclist's pace into another unit.
• Understanding a ship's or aircraft's speed in knots.
• Checking physics or engineering homework.

Good to know

• A knot is one nautical mile per hour, about 1.852 km/h, so a 20-knot boat is doing roughly 37 km/h.
• Mach here assumes the speed of sound at sea level and 15 degrees Celsius. At altitude, where it's colder, the same Mach number is slower.
• For a quick mental estimate, multiply miles per hour by 1.6 to get km/h.

How the conversion works

Every speed is first turned into metres per second, the standard scientific unit, and then into whatever you asked for. A kilometre per hour is 1,000 metres divided by 3,600 seconds, so 100 km/h is about 27.8 m/s. A mile per hour is 1,609.344 metres per 3,600 seconds, which is where the 0.447 factor comes from.

Going through one common unit keeps the results consistent, so converting from miles per hour to knots gives the same answer as going through kilometres per hour. The full table at the bottom of the result is generated from the same number, which is why every line agrees.

Worked example: a road trip

A sign shows 120 km/h and your car's speedometer is in miles per hour. Enter 120 with km/h selected and mph as the target: the answer is about 74.6 mph. The same table shows 33.3 m/s and 64.8 knots, which is how fast a small boat would need to travel to match it.

Going the other way, a US speed limit of 65 mph comes to about 104.6 km/h. Many people round these in their heads, 60 mph to 100 km/h being the usual shortcut, but the exact value is 96.6 km/h, so it's worth a quick check before you rely on it.

Speeds worth knowing

A brisk walk is around 5 km/h, a casual cycle ride 15 to 20, and a sprinter at top speed about 36. Highway driving is typically 100 to 130 km/h, a commercial jet cruises near 900 km/h, and sound in air travels at roughly 1,235 km/h at sea level. Light moves at 299,792,458 m/s, which is exactly why that value defines the metre.

Having a few of these in mind helps you spot a typo. If your conversion says a car is travelling at 2,000 km/h, something has gone wrong with the unit or the decimal point.

Choosing the right unit

Different fields settled on different units for historical reasons. Roads use km/h or mph, ships and aircraft use knots because a knot relates directly to nautical miles and degrees of latitude, athletics uses minutes per kilometre or mile for pace, and science uses metres per second.

When you write for an international audience, give both units, such as 100 km/h (62 mph). It costs a few characters and saves readers from doing arithmetic. Always include the unit next to the number, because a bare number can be misread.

Questions