Speed conversions matter most exactly when you have the least time for them: a rental car abroad, a speed limit sign in the wrong unit, a treadmill that thinks in miles while your training plan thinks in kilometers. The factor is the same exact 1.609344 that governs distance, the shortcuts are excellent, and one of them is secretly better than the other. Our free speed converter covers every unit pair, pace included.
In this guide
The factor and the two shortcuts
Speed inherits its factor from the mile: 1 mph = 1.609344 km/h, exactly (the 1959 definition story is in our unit conversion pillar). The mental shortcuts, with their honest costs:
- mph → km/h: multiply by 1.6. Error 0.6% low; 60 mph → 96 vs the true 96.6. Excellent.
- km/h → mph: multiply by 0.6. Error 3.4% low; 100 km/h → 60 vs the true 62.1. Usable but the weakest shortcut on this page.
- km/h → mph, better: multiply by 5/8. 100 km/h → 62.5 vs 62.1, error only 0.6% high. Eighths sound harder than they are: halve, halve again, halve once more, then multiply by five. For 120 km/h: 15 × 5 = 75 vs the true 74.6.
The asymmetry is worth noticing: ×1.6 and ×5/8 are a matched pair (they are inverses), while the popular ×0.6 is just a rougher cousin. Drivers converting limits should use the better pair; a 3.4% misreading of a speed limit is a ticket-sized error.
The speed limit tables
The conversions drivers actually meet:
| US sign (mph) | In km/h | Metric sign (km/h) | In mph |
|---|---|---|---|
| 30 | 48.3 | 50 | 31.1 |
| 55 | 88.5 | 80 | 49.7 |
| 60 | 96.6 | 100 | 62.1 |
| 65 | 104.6 | 120 | 74.6 |
| 70 | 112.7 | 130 | 80.8 |
Two patterns make the table easy to carry mentally. City limits roughly pair off: 30 mph and 50 km/h are near neighbors. And the classic highway pairs are 60 mph ≈ 100 km/h (true within 4%) and 75 mph ≈ 120 km/h. A rental car usually shows both scales on the speedometer; the skill is for reading road signs, which never do.
Speed vs pace: the runner’s inversion
Runners flip the fraction: instead of distance per time, pace is time per distance, so pace = 60 / speed. A treadmill at 12 km/h is a 5:00 min/km pace (and 7.46 mph, for treadmills that think in miles). The inversion has one trap: speed and pace move in opposite directions, so “increasing the pace” on a treadmill means lowering the minutes, and more than one interval session has gone wrong on that wording. The full pace toolkit, race predictions, training zones, treadmill tables, lives in our pace guide and the pace calculator. For perspective while you run: a world-class marathon is held at about 21 km/h, a number that makes every treadmill display more humbling.
Knots, the third wheel
Aviation and sailing use knots: 1 knot = 1.852 km/h exactly (one nautical mile per hour), which is 1.151 mph. The nautical mile is a minute of latitude, which is why navigation never gave it up: charts and angles line up. Quick anchors: 10 knots is about 18.5 km/h, a 500-knot airliner cruise is about 926 km/h. The same ×1.852 factor and its friends live in the converter, which is less romantic than a chart table but faster.
Frequently asked questions
Is the mph to km/h factor exact?
Yes: the mile is defined as exactly 1.609344 km, so the speed factor is exact by inheritance. Any digits past those are display rounding, not extra precision.
Why do UK cars use mph but UK fuel comes in liters?
The UK metricated partially: road distances and speeds stayed imperial, most of trade went metric. The result is the only country where drivers buy liters of fuel to cover miles, and fuel economy comes in miles per gallon using a gallon the US does not share.
What about meters per second?
Science and weather use m/s. The factor to km/h is exactly 3.6 (since an hour is 3,600 seconds), so a 10 m/s wind is 36 km/h. From m/s to mph, multiply by roughly 2.24.
How fast is a knot compared to running?
A brisk 10 km/h run is about 5.4 knots. If a sailing friend brags about a 7-knot passage, they were moving at about 13 km/h, a comfortable cycling pace, which is either deflating or charming depending on the friend.