How Do You Convert MPH to Knots?
Multiply miles per hour by 0.868976 to get knots. The formula is: Knots = MPH × 0.868976. This works because a nautical mile is about 15% longer than a statute mile, so the knot value is always smaller than the mph value for the same actual speed.
Tom Brewer checks the weather forecast before his weekend sail. The local TV station reports wind at 18 mph. He converts for his nautical instruments: 18 × 0.869 = 15.6 knots. That is a perfect breeze for his 28-foot sailboat — strong enough for good speed but calm enough for his beginner students from Pinewood Falls.
MPH to Knots Reference Table
| MPH | Knots | Beaufort Scale |
|---|---|---|
| 1-3 mph | 1-3 kt | Force 1: Light air |
| 8-12 mph | 7-10 kt | Force 3: Gentle breeze |
| 13-18 mph | 11-16 kt | Force 4: Moderate breeze |
| 19-24 mph | 17-21 kt | Force 5: Fresh breeze |
| 25-31 mph | 22-27 kt | Force 6: Strong breeze |
| 32-38 mph | 28-33 kt | Force 7: Near gale |
| 39-46 mph | 34-40 kt | Force 8: Gale |
| 47-54 mph | 41-47 kt | Force 9: Strong gale |
| 55-63 mph | 48-55 kt | Force 10: Storm |
| 74+ mph | 64+ kt | Hurricane force |
Practical Uses
Reporting Storm Intensity
American TV weather reports give wind speeds in mph, but the National Hurricane Center's official advisories list wind speeds in both mph and knots. When a hurricane's maximum sustained winds are reported at 145 mph, that is 126 knots — just below the Category 5 threshold of 137 knots (157 mph).
Pilot Communications
Student pilots trained in the US sometimes initially think in mph but must quickly learn knots for all aviation communication. A Cessna 172 cruises at about 122 knots (140 mph). Air traffic control gives all wind and speed information in knots, and flight instruments display airspeed in knots (KIAS — knots indicated airspeed).
Fishing and Recreational Boating
Marine weather forecasts are always in knots. Tom advises his Pinewood Falls sailing students to monitor NOAA weather radio, which reports wind in knots. When they hear "winds southwest 15 to 20 knots," they know that is 17-23 mph — manageable for experienced sailors but too gusty for beginners. Anything above 25 knots (29 mph) means lessons are canceled.