Hurricane wind speed: knots to mph and Saffir-Simpson

When a hurricane approaches, the National Hurricane Center publishes forecast cones and sustained-wind projections. But listen to news coverage and you’ll hear the same storm described in miles per hour. Neither number is wrong; they’re just in different units. Knots dominate maritime forecasting and official NHC advisories, while miles per hour is what most Americans understand. This guide explains the conversion, the Saffir-Simpson categories in both units, and why the NHC chose knots in the first place.

The knot-to-mph conversion

The exact conversion factor is simple: one knot equals approximately 1.15078 miles per hour. To convert any knot speed to mph, multiply by this factor. So:

• 50 knots = 57.5 mph
• 75 knots = 86.3 mph
• 100 knots = 115.1 mph
• 150 knots = 172.6 mph

Why this number? A knot is defined as one nautical mile per hour, and a nautical mile is exactly 1,852 meters. A mile, by contrast, is exactly 1,609.344 meters. So the factor is the ratio of those distances: 1,852 ÷ 1,609.344 ≈ 1.15078. This is the same conversion principle as the relationship between kilometers and miles. There is no physicist’s sleight of hand here — it’s just the geometry of how the two units were originally defined.

The Saffir-Simpson Hurricane Wind Scale

The Saffir-Simpson scale categorizes hurricanes into five tiers based on sustained wind speed. Each category carries a threshold in knotsfrom the National Hurricane Center, but you’ll also see the mph equivalents in news reports. Here’s the full scale:

These thresholds are published by the National Hurricane Center, the official US authority on tropical cyclone forecasting. Notice that the same storm — e.g., one with 110 sustained mph winds — sits right on the boundary between Category 2 and 3 depending on whether you're working in knots (95–96) or mph (110–111). The NHC publishes in knots first, and mph is derived by dividing by 1.15078 (or multiplying by the reciprocal).

Why maritime and aviation use knots

The knot didn’t arise from convenience or coincidence. Maritime and aviation navigation work in latitude and longitude, and latitude is divided into degrees and minutes of arc. One minute of latitude is a fixed distance on Earth — exactly one nautical mile. So one knot means your vessel covers one minute of latitude per hour, a direct tie-in to your position on a nautical chart.

A miles-per-hour unit has no such relationship. A mile is an arbitrary 1,609.344 meters — a unit of land distance, not astronomical. Switching to mph for maritime forecasting would mean every navigator has to do an extra mental step to translate between their chart and the weather numbers. The knot stays because it speaks the language of navigation.

Why the NHC publishes in knots — and why news outlets convert

The National Hurricane Center is part of NOAA, the US National Oceanic and Atmospheric Administration, which inherits decades of maritime forecasting tradition. Knots are the international standard for marine weather. An NHC forecast is read not just by Americans, but by ship captains, coast guards, and meteorologists worldwide, so sticking with knots makes the numbers consistent and trustworthy across the global maritime community.

News outlets, by contrast, are writing for a US television audience that thinks in miles per hour — the unit on car speedometers and road signs. A television anchor who says “Hurricane Milton is packing 115-knot winds” will lose the room; “132 miles per hour” registers as immediately dangerous. So news organizations convert, and it’s the correct choice for the audience. If you’re reading an NHC advisory directly, or tracking a marine forecast, prepare to work in knots. If you’re watching broadcast news, mph is what you’ll hear.

How to read a forecast

Next time you see an NHC forecast advisory, it will read something like: “Maximum sustained winds are near 85 knots with higher gusts.” To know what that means in the terms your local news will use, multiply by 1.15. So 85 knots ≈ 98 mph — a Category 2 hurricane. The forecast cone is drawn in the same knots. If the track shows a storm moving toward Florida with 120-knot sustained winds, that’s roughly 138 mph — a Category 4, capable of extreme damage. The conversion logic is identical to how km/h converts to mph: multiply by a constant factor.

Bookmark or memorize the knot-to-mph factor (×1.15, or divide by 0.87) and you can translate any marine forecast on the fly. The same factor works for all maritime speeds: boat reports, Coast Guard warnings, and any wind observation published in the Atlantic or Gulf.