Short answer: 1 mph = 0.868976 knots. To convert mph to knots, multiply by 0.869, or just subtract about 13 percent, since a knot is slightly faster than a mile per hour. So 20 mph is about 17 knots, and 30 mph is roughly 26 knots. The full chart below converts every common wind speed and tells you what it means on the water.
Your weather app says 20 mph. Your wind app, your sailing chart, and the marine forecast all say knots. That mismatch is why beach car parks are full of people doing quick mental arithmetic. Knots come from navigation and stuck as the standard for anything to do with wind and water, while mph stayed on land. Here’s the clean way to move between them.
What is a knot compared to mph?
A knot is one nautical mile per hour, and a nautical mile (1,852 m) is longer than a statute mile (1,609 m). That’s the whole story: because the nautical mile is bigger, one knot covers more ground than one mph, so it takes fewer knots to equal a given mph figure. Precisely, 1 mph = 0.868976 knots, and going the other way, 1 knot = 1.15078 mph.
The practical upshot is that knots are always a slightly smaller number than mph for the same wind. If a forecast reads 23 mph and a sailor tells you it’s blowing 20, you’re both right.
The mph to knots chart (with km/h and Beaufort)
These are the wind speeds you’ll actually meet on a forecast, converted from mph to knots, km/h and m/s, with the Beaufort band and what each one means once you’re out in it.
| mph | Knots | km/h | m/s | Beaufort | What it feels like |
|---|---|---|---|---|---|
| 5 | 4.3 | 8.0 | 2.2 | F2 light breeze | Leaves rustle. Too light for most boards; light-wind foils only. |
| 10 | 8.7 | 16.1 | 4.5 | F3 gentle breeze | Flags extend, wavelets. Big kites and light-wind wings wake up. |
| 15 | 13.0 | 24.1 | 6.7 | F4 moderate breeze | Small branches sway, whitecaps. Solid all-round riding. |
| 20 | 17.4 | 32.2 | 8.9 | F5 fresh breeze | Whitecaps everywhere. Powered up on mid-size gear. |
| 25 | 21.7 | 40.2 | 11.2 | F5-6 | Small trees sway. Strong side of fun; size down. |
| 30 | 26.1 | 48.3 | 13.4 | F6 strong breeze | Large branches move, spray. Small kites, experienced hands. |
| 35 | 30.4 | 56.3 | 15.6 | F7 near gale | Whole trees in motion. Experts only, time to depower. |
| 40 | 34.8 | 64.4 | 17.9 | F8 gale | Hard to walk into. Beyond almost everyone’s limit. |
Numbers rounded to one decimal. Source: standard nautical conversions (1 nautical mile = 1.852 km) and the Beaufort wind scale bands from the US National Weather Service. For the force-by-force breakdown, see our Beaufort scale in knots.
Quick answers: how many knots is X mph?
The exact conversions for the speeds people ask about most. Each is a real figure, not rounded guesswork: multiply mph by 0.868976 for knots.
- 10 mph = 8.7 knots. Gentle breeze; big kites and light-wind foiling.
- 15 mph = 13.0 knots. Moderate breeze; solid all-round riding.
- 20 mph = 17.4 knots. Fresh breeze, Force 5, the powered-up sweet spot.
- 25 mph = 21.7 knots. Strong side of fun; time to size down.
- 30 mph = 26.1 knots. Strong breeze; small gear, experienced riders.
- 35 mph = 30.4 knots. Near gale; most riders come in.
- 40 mph = 34.8 knots. Gale; watch from the beach.
Need a value in between? The wind speed converter does any number instantly, in all four units at once.
How do you convert mph to knots in your head?
Take off about an eighth, roughly 13 percent. A knot is 15 percent faster than a mph, so the knots figure comes out that much smaller. Drop 12 or 13 percent from the mph and you land within a hair of the real number. 24 mph minus 3 is 21 knots, which is exactly right.
For a rougher gut check, just remember that the two are close: knots and mph are within about 15 percent, so a “20-something” wind is a 20-something in either unit. It’s the opposite mistake from km/h, where the numbers are nearly double. Mix those up and you’ll badly over- or under-read the day.
Set your alarm in whatever unit you think in. Download WindUp free and set your wind threshold in mph, knots, km/h or m/s. It watches your spot and rings the second the wind is right, even on Do Not Disturb. No converting at 6am.
When will you need this?
Any time your weather source and your wind source disagree on units. A few that come up constantly:
- Checking a US weather app before a session. Phone weather apps default to mph; your kite or wing app almost certainly speaks knots. Convert to compare the forecast against your ride threshold.
- Sailing or paddling. Charts, the VHF marine forecast and most boat instruments are in knots, even where road signs are in mph. Our sailing guide lists the knots each boat type likes.
- Flying a drone. Manufacturer wind limits are often quoted in mph, but live wind stations and pilot apps report knots, so you convert to check you’re inside the ceiling.
Get fluent in the swap and a forecast in either unit tells you the same thing at a glance. Or skip it entirely: pick your unit once in WindUp and let the alarm do the reading.
FAQ
The short version: a knot is about 13 percent fewer than a mph for the same wind, so 20 mph is 17 knots, 30 mph is 26 knots. Memorise 1 mph = 0.869 knots and you can convert any forecast on the fly. For the reverse direction, our knots to mph and km/h chart has it, and the wind speed converter handles any value in all four units.
