The GPS Speedometer team · Apprexy StudioKilometres per hour, miles per hour and knots all answer the same question: how much distance you would cover in one hour at your current pace. They differ only in the length of the distance unit. One mph is about 1.61 km/h, and one knot is exactly 1.852 km/h, or about 1.15 mph.
Three units built on three different distances
Every speed unit is a distance divided by a time. Here the time is always one hour, so the whole difference comes down to which "mile" or "kilometre" sits on top.
- Kilometre per hour (km/h). A kilometre is 1,000 metres. It is the metric unit most people meet on the road, and it is sometimes written kph.
- Mile per hour (mph). The mile used on land, often called the statute mile, is defined as exactly 1,609.344 metres. That makes it roughly 60 percent longer than a kilometre.
- Knot (kn). A knot is one nautical mile per hour, and a nautical mile is exactly 1,852 metres. It is longer than both of the others.
The nautical mile has a practical origin. It was tied to one minute of latitude, so on a nautical chart the latitude scale along the side doubles as a distance scale. That is a large part of why it has stayed in use at sea and in the air, where people navigate by charts and coordinates.
The word "knot" comes from an old method of measuring a ship's speed: a line with knots tied at regular intervals was let out behind the moving vessel, and sailors counted how many knots ran out in a set time. Because a knot already means "per hour", saying "knots per hour" is a common slip. It would describe acceleration, not speed.
Where each unit shows up
The unit you see depends mostly on where you are and how you are travelling.
On the road. The large majority of countries post their road signs and print their car speedometers in km/h. The United States and the United Kingdom are the best-known places that use mph on road signs, along with a small number of others. Cars sold in mph markets often show km/h as a second, smaller scale on the dial, and the reverse is common too.
At sea. Boats and ships measure speed in knots almost everywhere, whatever unit the country uses on land. Marine weather forecasts usually give wind speed in knots as well, so sailors can compare wind and boat speed directly.
In the air. Aviation largely works in knots for airspeed and wind, with nautical miles for distance. Ground speed shown to passengers on seat-back maps is often converted to km/h or mph instead, depending on the airline.
Everywhere else. Running, cycling and train timetables generally follow the local road unit. Scientific work prefers metres per second, which you will rarely see on a dashboard.
The exact conversion factors
When you need a precise answer, these are the numbers to use. The first two pairs come straight from the definitions above; the rest follow from them.
| From | To | Multiply by |
|---|---|---|
| km/h | mph | 0.6214 |
| mph | km/h | 1.6093 |
| knots | km/h | 1.852 |
| km/h | knots | 0.5400 |
| knots | mph | 1.1508 |
| mph | knots | 0.8690 |
A worked example: a boat doing 12 knots is moving at 12 × 1.852 = 22.2 km/h, or 12 × 1.1508 = 13.8 mph. Going the other way, 65 mph is 65 × 1.6093 = 104.6 km/h.
For most everyday purposes, rounding to two decimal places (0.62, 1.61, 1.85, 0.54, 1.15, 0.87) is plenty.
Doing the maths in your head
You rarely need three decimal places. A rough figure you can produce in two seconds is usually more useful than an exact one you have to stop and calculate.
Between km/h and mph
- km/h to mph: halve it, then add a tenth of the original. 80 km/h becomes 40 + 8 = 48 mph. The true answer is 49.7, so this lands slightly low.
- For a closer answer, multiply by 5 and divide by 8. 80 × 5 = 400, and 400 ÷ 8 = 50 mph. This method is only about half a percent high.
- mph to km/h: add 60 percent. 50 mph becomes 50 + 30 = 80 km/h. The true figure is 80.5, so this one is very close.
- Use the Fibonacci shortcut. Neighbouring numbers in the sequence 3, 5, 8, 13, 21, 34, 55, 89 have a ratio close to 1.6, so 5 mph is about 8 km/h, 34 mph is about 55 km/h, and 55 mph is about 89 km/h.
Bringing knots into it
- Knots to km/h: double it, then take off a little. 15 knots doubled is 30; subtract about 7 percent and you get 28 km/h. The exact figure is 27.8.
- km/h to knots: halve it, then add a little. 100 km/h halved is 50; add about 8 percent and you have 54 knots, which is right.
- Knots to mph: add 15 percent. 20 knots plus 3 is 23 mph. The exact answer is 23.0.
- mph to knots: take off about 13 percent. 40 mph minus 5 is 35 knots, against an exact 34.8.
A useful anchor: a knot is a little faster than a mile per hour, and a mile per hour is a lot faster than a kilometre per hour.
A quick reference table
These values are simple conversions, rounded to one decimal place, so you can scan across a row rather than calculate.
| km/h | mph | knots |
|---|---|---|
| 10 | 6.2 | 5.4 |
| 20 | 12.4 | 10.8 |
| 30 | 18.6 | 16.2 |
| 50 | 31.1 | 27.0 |
| 80 | 49.7 | 43.2 |
| 100 | 62.1 | 54.0 |
| 120 | 74.6 | 64.8 |
Notice that the knots column is always a little more than half the km/h column. That pattern alone covers most quick checks on a boat.
Common mix-ups to watch for
A few errors come up again and again when people move between units.
- Mixing up nautical and statute miles. A distance on a sea chart is in nautical miles, which are about 15 percent longer than the miles on a road map. Plans that ignore this end up short.
- Reading the wrong scale on a dual dial. On a speedometer with two rings of numbers, it is easy to glance at the inner scale by mistake, especially in an unfamiliar car. Check which ring the needle is meant to be read against before you set off.
- Assuming a rental car matches the road signs. A car brought in from another market may show a different main unit from the local signs. Some cars let you switch the digital display between units in their settings menu; where that option sits varies by make and model.
- Comparing wind in knots with speed in km/h. If the forecast says 20 knots of wind, that is about 37 km/h, not 20.
- Saying "knots per hour". As above, a knot already includes the hour.
Choosing your unit in GPS Speedometer
GPS Speedometer reads your speed straight from your phone's GPS and lets you show it in km/h, mph or knots. You choose the unit alongside your vehicle type and gauge theme, so you can match whatever the road signs, chart or forecast in front of you is using.
That makes the boat profile and knots a natural pair on the water, while a drive abroad might call for switching from mph to km/h for the length of the trip. The conversion is done for you, so you never need the mental tricks above while you are moving.
The live speed readout is free with the Digital Clean gauge, and it works in any of the three units. Premium adds the other gauge themes, HUD mode, trip recording and history, the speed-limit alerts, the drag timer and the Picture-in-Picture widget.
FAQ
How do I convert km/h to mph quickly?
Multiply by 5 and divide by 8 for a close answer, or halve the number and add a tenth of the original for a quicker, slightly low one. For an exact figure, multiply by 0.6214.
What is a knot in speed?
A knot is one nautical mile per hour, which is exactly 1.852 km/h or about 1.15 mph. It is the standard unit for boats and is widely used in aviation and marine weather forecasts.
Is a knot faster than a mile per hour?
Yes, a little. One knot is about 1.15 mph, because a nautical mile is longer than the mile used on land, so 20 knots is about 23 mph.
Why do some countries use mph and others km/h?
It follows each country's measurement system. Most of the world adopted metric units and uses km/h on the road, while a few countries, most visibly the United States and the United Kingdom, kept miles for road distances and speeds.
Written by the GPS Speedometer team with the help of AI. What the article says about GPS Speedometer comes from the app itself; menu names in other apps can change between versions.