Self Speed
mph to knots
Divide by 1.150779. To go the other way, multiply knots by 1.150779.
| mph | knots | km/h |
|---|---|---|
| 1 | 0.87 | 1.6 |
| 5 | 4.34 | 8.0 |
| 10 | 8.69 | 16.1 |
| 15 | 13.03 | 24.1 |
| 20 | 17.38 | 32.2 |
| 25 | 21.72 | 40.2 |
| 30 | 26.07 | 48.3 |
| 40 | 34.76 | 64.4 |
| 50 | 43.45 | 80.5 |
| 60 | 52.14 | 96.6 |
| 80 | 69.52 | 128.7 |
| 100 | 86.90 | 160.9 |
| 150 | 130.35 | 241.4 |
| 200 | 173.80 | 321.9 |
The formula
knots = mph ÷ 1.150779 mph = knots × 1.150779
The factor is the ratio of the two miles. A statute mile is 1,609.344 m; a nautical mile is exactly 1,852 m. Divide one by the other and you get 1.150779.
Two different miles, and why
The statute mile descends from the Roman mille passus, a thousand double paces. English law fixed it at eight furlongs in 1593, giving the memorably awkward 5,280 feet.
The nautical mile is geometric rather than historical: it is one minute of latitude. The Earth's circumference divided by 360 degrees of 60 minutes gives roughly 1,852 m.
That definition is why the unit survives at sea. On a chart, one minute of latitude on the side scale is one nautical mile, so a navigator measures distance with dividers against the latitude scale and never converts anything. No other unit has that property, which is why two centuries of metrication have failed to displace it.
Which unit gets used where
| Domain | Unit | Why |
|---|---|---|
| Road vehicles, UK and US | mph | Legal requirement on signs |
| Road vehicles, elsewhere | km/h | Metric standard |
| Ships and boats | knots | Charts are in nautical miles |
| Aircraft | knots | Aviation charts share marine geometry |
| Wind, marine forecasts | knots | Beaufort scale is defined in knots |
| Wind, land forecasts | km/h or mph | Matches local road units |
| Physics and engineering | m/s | SI unit |
Speed over ground versus through water
If you are converting because you are on a boat, the more important distinction is what is being measured. GPS gives speed over ground; a paddlewheel log gives speed through water. The difference between them is the current — see boat speedometer and knots to km/h.
Self Speed reads out in knots and reports distance in nautical miles when you select those units.
Open the speedometer → Free, no install, works offline. Nothing leaves your device.