Wind Speed Conversion: mph, Knots, and the Beaufort Scale

By StormAuditor Editorial Team · Published 2026-09-24 · Updated 2026-09-24

Based on public NOAA and National Weather Service records, published under StormAuditor's methodology and data-source standards. Radar-derived hail sizes are estimates unless attributed to a ground report.

Damage-threshold ranges in this article are general guidance synthesized from industry and federal research — IBHS building-performance research, NWS severe criteria, and product standards such as UL 2218 (hail impact resistance) and ASTM D3161/D7158 (shingle wind ratings). Actual performance depends on product rating, age, installation quality, exposure, and impact characteristics — thresholds describe when damage becomes possible, not when it occurs.

One wind, three numbers. This reference guide shows how mph, knots, and the Beaufort scale relate, why storm archives mix them, and how to convert figures before comparing them to NWS and NHC thresholds.

The National Weather Service calls a thunderstorm gust severe at 58 mph. A marine forecast reports that same wind as 50 knots. On the Beaufort scale, it lands at force 10 — storm force at sea. One wind, three numbers. Anyone researching storm damage will meet all three in the public record. This guide shows how the units relate, how to convert between them, and where each one appears.

The three systems, and where you meet them

Miles per hour (mph) is the everyday unit in the United States. Television forecasts, NWS public warnings, and most damage-survey write-ups use mph.

Knots are nautical miles per hour. One knot equals about 1.15 mph. Aviation observations, marine forecasts, buoy data, and many raw station reports use knots. The NCEI Storm Events database also records wind magnitude in knots. That single fact trips up more claim researchers than any other unit issue.

The Beaufort scale is not a unit at all. It is a 0-to-12 ranking that ties wind ranges to visible effects: smoke drift, whitecaps, breaking twigs, structural damage. It survives in marine forecasts and in words like "gale" and "storm force." Older records and adjuster narratives sometimes lean on it.

Quick conversion math

Two constants cover nearly everything:

1 knot = 1.15 mph (1.15078 to be precise)
1 mph = 0.87 knots

For fast mental math, add 15 percent to knots to get mph. Going the other way, knock about 13 percent off mph to get knots.

| Knots | mph (rounded) |

|---|---|

| 34 | 39 |

| 40 | 46 |

| 50 | 58 |

| 58 | 67 |

| 64 | 74 |

| 65 | 75 |

Those rows are not random. Each one maps to a threshold that matters in storm research.

Reference thresholds in every unit

|---|---|---|---|

| Tropical storm (NHC) | 39 | 34 | Force 8 |

| StormAuditor screening threshold | 45 | ~39 | Force 8 |

| NWS severe gust criterion | 58 | ~50 | Force 10 |

| Hurricane (NHC) | 74 | 64 | Force 12 |

| NWS significant severe gust | 75 | ~65 | Force 12 |

A few notes on that table. The National Weather Service sets its severe thunderstorm criterion at gusts of 58 mph or greater. The National Hurricane Center defines tropical storms at 39 mph and hurricanes at 74 mph. StormAuditor applies a lower screening threshold of 45 mph — a screening level, not a severe designation — because property damage reports often begin well below the NWS severe line.

The Beaufort ladder, forces 6 through 12

|---|---|---|---|

| 6 | Strong breeze | 25–31 | 22–27 |

| 7 | Near gale | 32–38 | 28–33 |

| 8 | Gale | 39–46 | 34–40 |

| 9 | Strong gale | 47–54 | 41–47 |

| 10 | Storm | 55–63 | 48–55 |

| 11 | Violent storm | 64–72 | 56–63 |

| 12 | Hurricane | 73+ | 64+ |

Why storm records mix their units

Automated surface stations report wind in knots. Storm Prediction Center reports convert those values to mph. NCEI Storm Events keeps knots. The same measured wind can therefore appear as 50 knots in one archive and 58 mph in another.

Rounding leaves fingerprints, too. Reported gusts cluster at values like 58, 60, 63, and 67 mph because they began life as round knot figures. A 61-knot report becomes 70 mph after conversion. Spotting these patterns helps you recognize when two documents describe the same observation rather than two different winds.

Sustained wind and gusts are a separate trap. A sustained wind is an average over a period, typically one or two minutes. A gust is a brief peak, often well above the sustained value. Thresholds differ: NWS severe criteria use gusts, while hurricane categories use one-minute sustained winds. Converting units correctly still misleads if you compare a gust to a sustained-wind threshold.

How StormAuditor estimates it

StormAuditor's SAWE-2 wind engine blends public model analyses, station observations, and storm reports into a location-specific wind estimate, expressed in mph. Every value is an estimate of the public weather record, not a measurement at the roofline. The full approach is documented on the wind methodology page, with a parallel write-up for hail on the hail methodology page.

Practical guidance for owners and researchers

Check the unit before you quote a number. If a database column says "MAGNITUDE" with no label, assume knots until the documentation says otherwise.

Convert before you compare. A 55-knot gust exceeds the NWS 58-mph severe criterion; the raw digits alone suggest the opposite.

Keep the original figure alongside your conversion. Engineers and adjusters reviewing a date of loss research file can then check your math in seconds.

Translate Beaufort language when it appears. "Gale-force" in a narrative means roughly 39 to 46 mph — a range, not a point value. A wind report built from converted, dated figures reads far more credibly than one built from labels.

Limitations

Conversions are exact; the underlying winds are not. Station gusts, model analyses, and spotter reports each carry their own uncertainty, and rounding during unit conversion adds a small layer more. A converted figure inherits every limitation of its source, and an estimate at a property address remains an estimate. The limitations page explains where these numbers can mislead and how to read them with care.

FAQ

How do I convert knots to mph?

Multiply knots by 1.15. So 50 knots is about 58 mph, and 64 knots is about 74 mph.

Is 50 knots a severe wind?

Fifty knots converts to roughly 58 mph, which matches the NWS severe thunderstorm gust criterion. Winds at or above that level meet the NWS definition of severe.

What Beaufort force is hurricane strength?

Force 12, the top of the scale. It begins at 73 mph, or 64 knots — essentially the NHC hurricane threshold.

Why does one report say 61 knots and another say 70 mph for the same storm?

They describe the same wind. Multiplying 61 knots by 1.15 gives about 70 mph. Different archives store the same observation in different units.

Does StormAuditor report wind in mph or knots?

All StormAuditor wind estimates are expressed in mph, with source data converted before screening against the 45 mph screening threshold.