Knots versus miles per hour in marine forecasts

A knot is one nautical mile per hour, while mph uses statute miles. A nautical mile is longer than a statute mile, so the same numerical value represents more speed when it is expressed in knots. NOAA conversion material lists 1 nautical mile as 1.151 statute miles, which makes 1 knot about 1.15 mph. For planning, multiply knots by 1.15, or add roughly 15 percent when making a quick mental check.

Marine forecasts use knots because nautical miles are the distance unit used in navigation and on nautical charts. The unit itself does not change the weather; 20 knots and about 23 mph describe the same wind. The practical risk is a unit mix-up between a road-weather habit and a marine forecast. Reading the displayed number as mph creates a calmer picture than the forecast actually communicates, and that gap becomes more consequential as wind increases.

The conversion difference is small enough to overlook at light wind but grows with every additional knot. Ten knots is about 12 mph, 15 knots is about 17 mph, 20 knots is about 23 mph, 25 knots is about 29 mph, 30 knots is about 35 mph, and 34 knots is about 39 mph. A forecast of 25 knots is therefore not a forecast of 25 mph; it is nearly 29 mph before any gusts are considered.

Convert the gusts as carefully as the sustained wind. A forecast of 15 to 20 knots with gusts to 25 knots means winds around 17 to 23 mph with gusts near 29 mph. The sustained range describes the expected prevailing wind, while gusts identify stronger bursts that can arrive during an exposed crossing, launch, dock approach, or return. Build the trip around the full range and the gust value, not just the lowest sustained number.

A nearby land forecast, airport report, or sheltered marina observation may not represent the water you will cross. Terrain, buildings, trees, coves, and islands can alter the wind near shore. Marine zones are designed for the forecast waters, so start with the official marine forecast covering the route and time of your outing. Then read the wind direction and expected changes through the day rather than treating one launch-time value as the whole trip.

Wind is only one part of the open-water picture because it contributes to waves and rough surface conditions. How the water responds depends on the wind, how long it blows, the distance over water available for it to act, existing swell or waves, current, and local geography. A correctly converted wind forecast can still be a poor trip fit when waves, a changing direction, or an exposed return route reduce your margin. Use the wind conversion to understand the forecast, not to replace the rest of it.

A correct mph conversion is never permission to go. Check official marine warnings, advisories, statements, and any local access or navigation restrictions before relying on your own interpretation. National Weather Service Small Craft Advisory thresholds are geographically specific and can be based on wind, frequent gusts, seas, waves, ice, or other locally hazardous conditions. There is no single universal definition of a small craft because vessel design, loading, seaworthiness, and operator experience matter.

A Gale Warning covers sustained winds or frequent gusts from 34 through 47 knots, approximately 39 through 54 mph. That headline illustrates why unit awareness matters, but lesser conditions can still be hazardous for a particular boat or route. Also account for visibility, thunderstorms, water temperature, daylight, tide or current where relevant, and the outlook for the return. If an official product conflicts with a Fishing Weather interpretation, the official product and local restrictions take priority.

First, confirm the units beside every wind value rather than assuming they are mph. Second, convert both the sustained range and gusts when the forecast is in knots. Third, identify the marine zone, wind direction, timing of increases or shifts, and the forecast seas or waves. Fourth, compare that forecast with the latest available buoy, Coastal-Marine Automated Network station, harbor, or other marine observation. Observations report conditions already measured at one place and time; forecasts describe expected conditions and can change.

Make the decision from the least favorable credible portion of the outing: the farthest exposed point, the crossing back, and the period when wind or storms are expected to increase. GO fits when no official hazard conflicts with the plan and conditions remain comfortably inside your own conservative margin. MAYBE fits when the outlook is workable but variable, with a shorter sheltered option and a firm turn-back point. SKIP fits when warnings, increasing wind, gusts, rough water, storms, poor visibility, or a unit-corrected forecast leave too little safety margin.

These references were consulted when this guide was generated. Check current official alerts and local rules before every trip.

• ocean.weather.gov (ocean.weather.gov)
• fisheries.noaa.gov (fisheries.noaa.gov)
• nauticalcharts.noaa.gov (nauticalcharts.noaa.gov)
• weather.gov (weather.gov)
• weather.gov (weather.gov)
• weather.gov (weather.gov)