km/h to mph
Convert kilometres per hour to miles per hour and back, with the common speed limits in both systems and the mental shortcuts that work at driving speeds.
100 km/h is about 62 mph. Multiply km/h by 0.6 for a quick road-sign estimate.
| km/h | mph |
|---|---|
| 10 | 6.21 |
| 30 | 18.64 |
| 50 | 31.07 |
| 60 | 37.28 |
| 80 | 49.71 |
| 100 | 62.14 |
| 120 | 74.56 |
| 130 | 80.78 |
| 200 | 124.27 |
| 300 | 186.41 |
The signs are round. The conversions never are.
The conversion itself is exact — a mile is exactly 1609.344 metres, so nothing is approximated on the way through. What is approximate is every limit you have ever seen signposted, because countries pick numbers that are round in their own units:
| Limit | In mph | Nearest signable 5 | Change to the actual limit |
|---|---|---|---|
| 30 km/h | 18.64 | 20 | +7.3% |
| 50 km/h | 31.07 | 30 | -3.4% |
| 60 km/h | 37.28 | 35 | -6.1% |
| 70 km/h | 43.50 | 45 | +3.5% |
| 80 km/h | 49.71 | 50 | +0.6% |
| 90 km/h | 55.92 | 55 | -1.7% |
| 100 km/h | 62.14 | 60 | -3.4% |
| 110 km/h | 68.35 | 70 | +2.4% |
| 120 km/h | 74.56 | 75 | +0.6% |
| 130 km/h | 80.78 | 80 | -1.0% |
Not one is round, and the other direction is no better — 20 mph is 32.19 km/h. So a limit "translated" onto a sign in the other system is not the same limit. Rounding 30 km/h to the nearest 5 mph gives 20 mph, which is 7.3% faster. Averaged over km/h limits from 10 to 200 the rounding moves the limit by 3.9%, and at the low end it reaches 19.5%.
And the two grids are not equally fine
This is the part worth checking rather than guessing, and it goes the opposite way to the obvious answer. Signs in miles step by 5; signs in kilometres step by 10. Those are not the same size:
| One step | In km/h | In mph |
|---|---|---|
| 5 mph | 8.05 | 5.00 |
| 10 km/h | 10.00 | 6.21 |
Ten is a bigger number than five, so the metric grid looks finer — and it is in fact coarser. One metric step is 1.24 imperial ones, spacing 24% wider. A driver crossing between the systems is not just relearning numbers; every notch up or down is a bigger change in actual speed than they are used to.
Which is why a dual-unit sign is always odd
There is no set of limits that is round in both systems. Checked across every whole km/h value up to 500: not one is a whole number of miles per hour. The two grids share zero and nothing else, so any sign showing both has an unround number on it somewhere.
The near misses are the ones worth knowing, and they are not the ones most people would guess. Ranked by how far each lands from a whole number of mph, only 4 values under 200 come within a tenth:
| km/h | In mph | Distance from whole |
|---|---|---|
| 140 | 86.992 | 0.008 |
| 190 | 118.061 | 0.061 |
| 50 | 31.069 | 0.069 |
| 90 | 55.923 | 0.077 |
140 km/h wins by a wide margin: 86.992 mph, eight thousandths from 87. That is the one conversion in this range you can do in your head and be right.
Worth naming the one that is not on that list, because it caught the author out. 120 km/h is 74.56 mph — four tenths from 75, and further out than anything in the table. It reads as close only when the error is quoted as a percentage. Those are different questions: 0.6% of a large number is a bigger absolute miss than 0.2% of a small one, and on a speedometer it is the absolute miss you can see.
How to use
- Enter a speed in km/h or mph.
- Read the converted speed instantly.
- Check the table for the speed limits you are likely to meet.
- Remember that a speedometer usually reads slightly high by design.
Frequently asked questions
How do I convert km/h to mph?
Multiply by 0.6214, or divide by 1.609. For a quick mental version, take 60% and add a touch: 100 km/h is about 62 mph, and 120 km/h is about 75 mph.
What are the common speed limits in both units?
A 30 mph urban limit is about 48 km/h, which is why 50 km/h is the usual metric equivalent. Motorway limits of 70 mph and 120 km/h come out at 113 km/h and 75 mph respectively — close, but the metric limit is slightly higher.
Why does my speedometer read high?
By regulation. Most jurisdictions require a speedometer never to read below the true speed, so manufacturers build in a margin — typically a few percent. A car indicating 70 may well be doing 67. GPS speed is generally closer to the truth, though it lags on hills and in tunnels.
Is there a shortcut using Fibonacci numbers?
There is, and it is oddly reliable. Consecutive Fibonacci numbers approximate the mile-kilometre ratio: 50 km/h is near 30 mph, 80 near 50, and 130 near 80. Each pair is accurate to about 1%, which is better than most mental arithmetic.
How fast is 100 km/h in mph?
62.1 mph. This is the standard benchmark for acceleration figures in metric markets, where cars are timed from 0 to 100 km/h, against 0 to 60 mph in the US — so the two figures are not directly comparable, with the metric test being marginally harder.
What about metres per second?
Divide km/h by 3.6. So 100 km/h is 27.8 m/s. Physics problems and wind speeds often use m/s, and the conversion is one of the more useful to remember because it turns an unfamiliar unit into an intuitive one.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.