To convert miles per hour to kilometers per hour, multiply by 1.609344. So 30 mph is 48.3 km/h, 60 mph is 96.6 km/h and 70 mph is 112.7 km/h. For the reverse, multiply km/h by 0.621371: 100 km/h is 62.1 mph.
Why is the mph to km/h factor 1.609344? #
An hour is an hour in both units, so the speed factor is just the distance factor. A mile is exactly 1,609.344 m, which is 1.609344 km. One mile per hour is therefore exactly 1.609344 km/h, or exactly 0.44704 meters per second (NIST SP 811 lists both as exact).
The mental shortcut #
mph to km/h: add 60%. Take the speed, add six tenths of it. 50 mph becomes 50 + 30 = 80 km/h (exact: 80.5). The error is about 0.6%, always a little low.
km/h to mph: multiply by 5/8. Divide by 8, multiply by 5. 80 km/h becomes 10 × 5 = 50 mph (exact: 49.7). This runs about 0.6% high. Plain “multiply by 0.6” is simpler but 3.4% low, so 120 km/h comes out as 72 mph when it’s really 74.6.
mph to km/h chart #
| mph | km/h | mph | km/h | |
|---|---|---|---|---|
| 5 | 8.0 | 50 | 80.5 | |
| 10 | 16.1 | 55 | 88.5 | |
| 15 | 24.1 | 60 | 96.6 | |
| 20 | 32.2 | 65 | 104.6 | |
| 25 | 40.2 | 70 | 112.7 | |
| 30 | 48.3 | 75 | 120.7 | |
| 35 | 56.3 | 80 | 128.7 | |
| 40 | 64.4 | 100 | 160.9 | |
| 45 | 72.4 | 200 | 321.9 |
km/h to mph chart #
| km/h | mph | km/h | mph | |
|---|---|---|---|---|
| 10 | 6.2 | 90 | 55.9 | |
| 20 | 12.4 | 100 | 62.1 | |
| 30 | 18.6 | 110 | 68.4 | |
| 40 | 24.9 | 120 | 74.6 | |
| 50 | 31.1 | 130 | 80.8 | |
| 60 | 37.3 | 150 | 93.2 | |
| 70 | 43.5 | 200 | 124.3 | |
| 80 | 49.7 | 300 | 186.4 |
Worked examples #
You’re driving a rental in Canada and the sign says 100. That’s km/h: 100 × 0.621371 = 62.1 mph. The 5/8 shortcut gives 62.5.
A European visitor asks how fast 65 mph is. 65 × 1.609344 = 104.6 km/h. The add-60% shortcut gives 65 + 39 = 104.
Your treadmill shows 10 km/h. That’s 6.2 mph, a pace of about 9:39 per mile.
A tennis serve is clocked at 200 km/h. 200 ÷ 1.609344 = 124.3 mph.
If you’re driving somewhere that uses the other unit, learn a few key numbers before you set off: 50 km/h ≈ 31 mph, 80 ≈ 50, 100 ≈ 62, 120 ≈ 75. Do the converting before the trip, not while you’re behind the wheel.
How do km/h, m/s and knots relate? #
Other speed units show up in weather forecasts, science classes and boating. Here’s 100 km/h in each:
| Unit | 100 km/h equals | Conversion |
|---|---|---|
| Miles per hour | 62.14 mph | km/h × 0.621371 |
| Meters per second | 27.78 m/s | km/h ÷ 3.6 |
| Knots | 53.99 kn | km/h ÷ 1.852 |
| Feet per second | 91.13 ft/s | m/s ÷ 0.3048 |
The ÷ 3.6 rule for meters per second is exact and worth remembering: an hour has 3,600 seconds and a kilometer has 1,000 meters, so 1 m/s = 3.6 km/h. Wind speeds in many forecasts are in m/s, and 10 m/s is 36 km/h or 22.4 mph.
A knot is one nautical mile per hour, exactly 1.852 km/h, or about 1.151 mph. Ships, aircraft and many marine forecasts use it. For the speed of sound and Mach numbers, see how fast is Mach 1.
Why do the US and UK use mph? #
Road speed is one of the last places imperial units hold on. The US uses customary units for road signs, and the UK kept miles and mph on its roads even after metricating most other things. Canada, Australia, Ireland and most of the world post limits in km/h. Speedometers in many cars sold in North America show both scales, with km/h in smaller figures. The history is in why the US uses the imperial system.
Converting speeds on your phone #
Convert Units Calculator has a Speed category with meters per second, km/h, mph, feet per second and knots, and version 1.0.5 adds Mach. Type 100 with km/h selected and it shows 62.137119 mph, with every other speed unit listed below it. It converts as you type and works offline, which makes it useful for travel. Check it before you drive, not on the road.
For the distance side of the same conversion, see km to miles.
Frequently asked questions #
What is 70 mph in km/h? #
70 mph is 112.7 km/h. Multiply 70 by 1.609344.
What is 120 km/h in mph? #
120 km/h is 74.6 mph. Multiply by 0.621371, or use the 5/8 shortcut: 120 ÷ 8 × 5 = 75.
Is 100 km/h the same as 60 mph? #
Close, but no. 100 km/h is 62.1 mph, and 60 mph is 96.6 km/h. The two are often treated as equivalent in car magazines (“0 to 60” versus “0 to 100”), but 100 km/h is slightly faster.
How do you convert km/h to m/s? #
Divide by 3.6. So 36 km/h is 10 m/s, and 90 km/h is 25 m/s. To go from m/s to km/h, multiply by 3.6.
Is a knot faster than a mile per hour? #
Yes. One knot is one nautical mile per hour, and a nautical mile (1,852 m) is longer than a statute mile (1,609.344 m). One knot is about 1.15 mph.