MPH to Knots
Convert mph to knots for aviation planning, sailing, and weather reporting. Enter any mph — get knots for flight plans, ATC communications, and maritime use.
Enter your values above to see the results.
Tips & Notes
- ✓Quick estimate: multiply mph by 0.87 (0.12% error). Example: 100 mph × 0.87 = 87 knots (exact: 86.90 knots). For rough flight planning, multiply mph by 0.87 consistently.
- ✓Flight planning: FAR/AIM (US flight regulations) accept airspeeds in knots. A general aviation pilot flying 120 mph indicated = 120 × 0.868976 = 104.3 knots. ATC communications use knots; "maintain 250 knots or less below 10,000 feet" is a US airspace rule.
- ✓Sailing: boat speeds are always discussed in knots. A 15-mph wind = 15 × 0.868976 = 13.0 knots — close to the small craft advisory threshold of 15 knots. Hull speed for a 30-foot boat ≈ 1.34 × √30 = 7.3 knots = 8.4 mph.
- ✓Weather reporting: surface observations (METAR/SPECI) for aviation use knots. Reporting wind "25 mph" to a pilot should be converted: 25 × 0.869 = 21.7 knots. NWS forecasts for marine areas use knots; inland forecasts use mph.
- ✓The Saffir-Simpson hurricane scale uses mph for public communication but meteorologists work in knots internally. Category 1 hurricane 74 mph = 64.3 knots; Category 5 minimum 157 mph = 136.4 knots. The "64 knots" tropical storm definition = 73.7 mph.
Common Mistakes
- ✗Applying the mph-to-knot conversion to land-based navigation — on land, you use mph; at sea or in the air, you use knots. Reporting a car speed of 60 mph as 52.1 knots to a marine context is technically correct but unusual and likely to cause confusion.
- ✗Confusing mph-to-knots with mph-to-km/h — mph × 0.869 gives knots; mph × 1.609 gives km/h. These are completely different outputs. 60 mph = 52.1 knots = 96.6 km/h. Always verify which target unit you need.
- ✗Using statute miles for nautical calculations after converting — if you convert mph to knots for speed, all subsequent distance calculations must also use nautical miles. Mixing knot speed with statute mile distances gives wrong time/range results.
- ✗Forgetting that ATC speed restrictions are in knots — below 10,000 ft MSL in US airspace, 250 knots IAS maximum = 287.7 mph. Class B airspace entry often requires speed reductions. These limits are always in knots regardless of the aircraft type.
- ✗Assuming wind gust speed in mph converts directly to sustained knot speed for marine advisories — gusty wind at 30 mph = 26.1 knots. Marine advisories are based on sustained wind, not gusts. A 30-mph gust with 20-mph sustained wind = 26.1-knot gust and 17.4-knot sustained — only the sustained matters for advisory thresholds.
MPH to Knots Overview
Converting mph to knots is the bridge between the US land-based speed world and the global nautical/aviation standard. Pilots, sailors, and weather observers all work in knots — making this conversion essential whenever mph-referenced information enters a nautical or aeronautical context.
MPH to knots formula:
knots = mph × 0.868976 | 1 mph = 0.868976 knots
EX: Aircraft cruise speed 150 mph → 150 × 0.868976 = 130.3 knots for ATC communication. Wind 25 mph → 25 × 0.868976 = 21.7 knots — inside small craft advisory threshold (15-25 kt)Inverse — knots to mph:
mph = knots × 1.15078 | 1 knot = 1.15078 mph
EX: FAA 250 KIAS limit → 250 × 1.15078 = 287.7 mph. Ferry service 28 knots → 28 × 1.15078 = 32.2 mphCommon mph speeds in knots:
| mph | Knots | Context |
|---|---|---|
| 15 mph | 13.0 kts | Small craft advisory wind begins at 15 kts |
| 25 mph | 21.7 kts | Upper small craft advisory range |
| 40 mph | 34.8 kts | Gale warning threshold (34 kts) |
| 60 mph | 52.1 kts | Storm warning range |
| 73.7 mph | 64.0 kts | Hurricane force wind threshold |
| 120 mph | 104.3 kts | GA aircraft typical cruise |
| 287.7 mph | 250.0 kts | FAA max speed below 10,000 ft |
| Waterline (ft) | Hull Speed (kts) | Hull Speed (mph) | Typical Boat Type |
|---|---|---|---|
| 20 ft | 5.99 kts | 6.89 mph | Small daysailer |
| 30 ft | 7.34 kts | 8.45 mph | Cruising sailboat |
| 40 ft | 8.47 kts | 9.75 mph | Performance cruiser |
| 50 ft | 9.47 kts | 10.90 mph | Offshore racer/cruiser |
| 100 ft | 13.40 kts | 15.42 mph | Large yacht |
Frequently Asked Questions
Multiply mph by 0.868976. Examples: 50 mph = 43.45 knots; 75 mph = 65.17 knots; 100 mph = 86.90 knots; 120 mph = 104.28 knots; 150 mph = 130.35 knots; 200 mph = 173.80 knots; 300 mph = 260.69 knots; 500 mph = 434.49 knots. Quick approximation: multiply by 0.87 (within 0.1%). Reverse: knots × 1.15078 = mph.
US general aviation uses knots for airspeed in flight planning and ATC communication. Convert your planned cruise speed: 120 mph = 104.3 knots; 150 mph = 130.3 knots; 180 mph = 156.4 knots. For fuel planning with range in nautical miles: if flying at 100 knots for 2.5 hours, range = 250 nautical miles = 287.7 statute miles. FAA examinations and pilot certificates express speeds in knots. If your older aircraft instrument shows mph, convert mentally for all ATC-required readbacks. Stall speeds, approach speeds, and Vne (never exceed speed) in POH may be in mph for older aircraft — always verify.
NOAA marine weather forecasts use knots. Converting for sailors: wind 10 mph = 8.7 knots (light); 15 mph = 13.0 knots (small craft advisory begins at 15 knots); 20 mph = 17.4 knots; 25 mph = 21.7 knots (small craft advisory); 35 mph = 30.4 knots; 40 mph = 34.8 knots (gale warning begins at 34 knots); 50 mph = 43.4 knots; 55 mph = 47.8 knots (storm warning begins at 48 knots). Hurricane force: 73.7 mph = 64 knots — the threshold for issuing hurricane force wind warnings in marine forecasts.
Hull speed is the theoretical maximum speed for a displacement hull sailboat or motorboat before it begins climbing its own bow wave. Hull speed (knots) = 1.34 × √(waterline length in feet). Examples: 25-foot waterline → hull speed = 1.34 × √25 = 1.34 × 5 = 6.7 knots = 7.7 mph. 36-foot waterline → 1.34 × 6 = 8.04 knots = 9.25 mph. 64-foot waterline → 1.34 × 8 = 10.72 knots = 12.34 mph. Exceeding hull speed in a displacement hull requires dramatically more power — the reason planing hulls are used for high-speed boats.
FAA airspeed regulations use knots exclusively. Key limits: maximum airspeed in Class B airspace 250 KIAS (knots indicated airspeed) = 287.7 mph below 10,000 ft MSL; in Class C/D airspace 200 KIAS = 230.2 mph within 4 nm of primary airport; noise abatement procedures commonly specify 160 KIAS = 184.1 mph. VFR traffic pattern speed: 90-100 KIAS = 103.6-115.1 mph; final approach speed typically 65-80 KIAS = 74.8-92.1 mph for light aircraft. Minimum controllable airspeed (Vmc) for multi-engine aircraft is specified in KIAS in POH/AFM.
For ETA calculations using a nautical chart: (1) measure distance in nautical miles (nm) using the latitude scale. (2) Use speed in knots. (3) Time = distance (nm) ÷ speed (knots) in hours. Example: passage of 150 nm at 7 knots: time = 150/7 = 21.4 hours. If your log shows 8 mph: convert to knots first (8 × 0.869 = 6.95 knots), then calculate: 150/6.95 = 21.6 hours. Mixing mph with nm gives the wrong answer: 150 nautical miles ÷ 8 mph = 18.75 hours (incorrect, understates by 13%).