This free speed converter changes miles per hour (mph) into knots (kn) and back again, updating live as you type. It is built for sailing, aviation, and weather reports that quote wind or vessel speed in knots while you think in mph.
How it works
A knot is one nautical mile per hour. Since a statute mile is 1609.344 m and a nautical mile is 1852 m, the converter uses the ratio of those definitions:
- kn = mph × (1609.344 ÷ 1852) = mph × 0.868976242
- mph = kn ÷ 0.868976242
The factor is computed at full floating-point precision, and the conversion is bidirectional — edit either field and the other updates instantly.
Example
A weather forecast gives a wind speed of 30 mph and you want it in knots.
30 × 0.868976242 = 26.07 kn.
| Miles per hour (mph) | Knots (kn) |
|---|---|
| 1 mph | 0.86898 kn |
| 30 mph | 26.0693 kn |
| 60 mph | 52.1386 kn |
| 100 mph | 86.8976 kn |
All calculations run entirely in your browser with no data sent to any server.
Why knots exist and who uses them
A knot is one nautical mile per hour. A nautical mile is defined as one minute of arc along a meridian of longitude — roughly 1,852 metres. This elegant connection to Earth’s geometry made nautical miles (and knots) the natural unit for ocean and air navigation long before GPS, because speed and distance aligned directly with the coordinates on a chart.
Today, knots remain the standard unit in:
- Marine navigation — all vessel speeds, wind speeds, and current speeds in nautical charts and VHF weather broadcasts
- Aviation — airspeed indicators, ATC communications, and flight plans use knots worldwide
- Meteorology — surface wind speeds in official synoptic reports (METARs, SYNOPs) are given in knots
- Military — maritime and airborne operations coordinate in knots
The US uses mph for road speed but knots at sea and in the air — which is why converting between the two is a practical daily need for sailors, pilots, and anyone reading aviation or marine weather.
Quick conversion reference
| Speed context | mph | Knots |
|---|---|---|
| Light breeze | 10 mph | 8.7 kn |
| Typical sailing wind | 15 mph | 13.0 kn |
| Gale-force wind | 40 mph | 34.8 kn |
| Hurricane threshold | 74 mph | 64.3 kn |
| Commercial jet cruise | 575 mph | 499.7 kn |
| Fast ferry | 40 mph | 34.8 kn |
| Olympic sprint speed | ~25 mph | ~21.7 kn |
The exact conversion factor
The factor 0.868976242 comes from the definitions of the statute mile and nautical mile:
- 1 statute mile = 1,609.344 metres (exact, by definition since 1959)
- 1 nautical mile = 1,852 metres (exact, by international standard since 1929)
- Therefore: 1 mph = 1,609.344 / 1,852 knots = 0.868976242… knots
This is not an approximation — both source lengths are exact by international agreement, making the ratio mathematically exact to full precision. The tool uses floating-point arithmetic which is accurate to 15+ significant figures, far beyond any practical measurement need.