Wind forecasts may describe the same breeze in knots, miles per hour, kilometers per hour, or meters per second. Knowing how to read those units—and what the number leaves out—helps you compare forecasts and plan a windsurfing day with fewer surprises.
Start with the unit, not the number
A forecast value such as “15” is incomplete until you know its unit. Fifteen knots is a different wind speed from 15 miles per hour or 15 kilometers per hour. Look for the unit label beside the wind value, in the map legend, or in the forecast settings. Some services let you choose preferred units, while others use a default that may differ from the units you usually discuss at the beach.
The common wind-speed units are:
- Knots (kt or kn): One nautical mile per hour. Knots are widely used in marine and aviation forecasts and are common in wind sports.
- Miles per hour (mph): Often used in the United States and in everyday weather reports there.
- Kilometers per hour (km/h or kph): Used in many countries and consumer weather apps.
- Meters per second (m/s): Common in scientific data, some specialist forecasts, and some wind-sport tools.
A forecast that shows a number without a clear unit is a reason to open its legend or settings before using it. If the unit still cannot be found, compare the same location and time in another clearly labeled source rather than guessing.
Convert wind speeds with a quick reference
Use this approximate table for mental comparisons. Values are rounded, so they are suited to planning and discussion, not precision calculations.
| Knots | Approx. mph | Approx. km/h | Approx. m/s |
|---|---|---|---|
| 5 | 6 | 9 | 2.6 |
| 10 | 12 | 19 | 5.1 |
| 15 | 17 | 28 | 7.7 |
| 20 | 23 | 37 | 10.3 |
| 25 | 29 | 46 | 12.9 |
For more exact conversion, use these factors:
- Knots to mph: multiply by about 1.15.
- Knots to km/h: multiply by about 1.85.
- Knots to m/s: multiply by about 0.514.
- To reverse a conversion, divide by the same factor. For example, divide mph by 1.15 to estimate knots.
Work through one forecast value
Suppose a beach forecast shows 18 knots. Multiply 18 by 1.15 to get about 21 mph. Multiply by 1.85 to get roughly 33 km/h, or by 0.514 to get about 9.3 m/s. Those are different expressions of nearly the same speed, not four different forecasts.
For quick on-the-water conversations, round sensibly: 18 knots is about 21 mph. Keep the original unit beside the converted value in notes—“18 kt (about 21 mph)”—so that nobody mistakes the estimate for a separate observation.
Read sustained wind and gusts separately
Forecasts often show a typical or sustained wind speed plus a gust value. A display such as 14 kt, gusting to 20 kt means the forecast expects the wind to average around 14 knots over the provider’s defined period, with stronger brief peaks possible. The averaging period and gust definition can vary by source, so check its help text if close comparisons matter.
Do not treat the gust number as the steady wind you will have throughout a session. Also do not ignore it: gusts can change how manageable a rig feels, especially during a launch, landing, or lull-to-gust transition. When comparing two locations or apps, check that you are comparing the same fields—sustained wind with sustained wind, gust with gust—and the same time window.
A useful planning note includes both numbers, for example: “16 kt sustained, 23 kt gusts, afternoon.” If a forecast presents only one value, find out whether it means a sustained average, a model snapshot, or a gust. The label and provider documentation matter.
Put the speed in a windsurfing context
A unit conversion tells you how fast the air is moving; it does not tell you whether conditions suit a particular sailor or setup. The same forecast can feel very different depending on board and sail choice, body weight, technique, water state, wind direction, and how sheltered the launch is. Treat generic wind ranges as orientation, not as a personal go/no-go rule.
Before choosing equipment, consider:
- Your current ability and comfort: A speed you can manage in familiar, forgiving water may be unsuitable in unfamiliar or exposed conditions.
- Sail and board range: Check the equipment maker’s guidance and the range you have actually used successfully. Do not assume a conversion makes one setup interchangeable with another.
- Wind quality: A steady breeze and a highly variable one can share the same average speed but produce very different sessions.
- Water and launch conditions: Chop, current, shore break, obstacles, and the distance back to shore add demands that a wind-speed number cannot express.
- A safe return plan: Consider whether the wind direction and forecast trend could make returning to the launch harder.
If you are learning, ask a qualified local instructor or experienced local sailor about site-specific conditions and suitable equipment. A forecast unit conversion is a useful first step, not a substitute for local knowledge or sound judgment.
Compare forecasts without mixing up the numbers
Different forecast products may use different units, update times, grid sizes, and averaging methods. Two apps showing “20” may not agree at all if one means 20 mph and the other 20 km/h. Even after converting units, the forecasts can differ because they draw on different models or represent different points and time intervals.
Use this comparison routine:
- Set one location. Check that map pins or forecast coordinates refer to the actual launch, not a nearby inland town or offshore grid cell.
- Set one time zone. Confirm whether the time shown is local time or another zone, especially when planning across borders.
- Match the forecast period. Compare the same hour or time block, rather than one app’s midday value with another’s daily peak.
- Match the measure. Check sustained speed against sustained speed and gusts against gusts.
- Convert to a common unit. Choose the unit you know best and apply the same conversion factor consistently.
- Read direction and trend too. Speed alone cannot show whether the wind is onshore, offshore, cross-shore, strengthening, or fading.
- Check again near departure. Forecasts change. Revisit the current forecast and local observations before committing to a session.
If one source shows 12 knots and another shows 14 mph, convert before concluding they disagree: 12 knots is about 14 mph. If values remain different, note the range and uncertainty instead of choosing the most appealing number.
Prepare for the beach with a short checklist
A forecast is a plan for possible conditions, not a promise about what will happen at the water. Make a simple note that keeps units and context together:
- Location and local time
- Sustained wind speed and unit
- Gust speed and unit, if available
- Wind direction and expected trend
- Source and time you checked it
- A backup plan if conditions change or do not suit your group
At the beach, compare the forecast with what you can safely observe from shore. Look at flags, trees, water texture, and other local indicators, but remember that they are qualitative and may be affected by terrain. Wind can vary between shore and the sailing area, and a reading at one point does not describe every part of a bay.
If conditions appear stronger, weaker, or more erratic than expected, pause and reassess. Do not let a neatly converted number override changing conditions, local restrictions, or advice from site staff. If you are unsure whether the conditions fit your ability and equipment, choose a safer alternative such as waiting, using a sheltered location, or not launching.
Use the same method when traveling
Trips add opportunities for unit mistakes: an app may change its default by region, a forecast may use local conventions, and a rental shop may discuss wind in a different unit from the one you use at home. Before traveling, set a preferred unit if the service allows it, but verify the label each time you open a new forecast source.
Save a small conversion reference on your phone or remember a few anchors: 10 kt is about 12 mph; 20 kt is about 23 mph; 10 m/s is about 19 kt. These approximate equivalents help decode conversation and forecast summaries without a calculator. For a specific decision, use the full value and check the source’s definition.
When asking about local conditions, repeat the unit in your question: “Is the forecast 18 knots sustained, with gusts near 24 knots?” This is clearer than asking whether “18” is windy. Also ask what the forecast covers—at the beach, offshore, or over a broader area—and whether local effects commonly change the wind near the launch.
Troubleshoot confusing displays
The number seems much too high or low. Check whether the display is in mph, km/h, or knots, then verify whether it shows a gust, a daily maximum, or a sustained value. A unit label may be in a settings menu rather than next to every number.
The map and hourly forecast disagree. They may show different times, locations, or forecast measures. Open the details for the same point and hour before comparing. Map colors are not enough; read the legend and numerical value.
A wind arrow is confusing. Speed units do not tell you the arrow convention. Some displays point toward where wind is going; others describe the direction it comes from. Read the app’s legend or help. If direction remains unclear, confirm it with a reliable local source before using it to plan a route or launch.
One source uses Beaufort numbers. The Beaufort scale describes wind force in categories rather than a single exact speed. Look up the source’s stated speed range and unit for that category; the range is approximate, and sea-state descriptions may not match a sheltered shoreline.
The forecast says one thing but the beach feels different. A forecast grid represents an area and time period, while your launch is a specific place at a specific moment. Terrain, buildings, and local weather can affect wind. Check a current local observation and reassess rather than forcing the forecast value to fit what you see.
A forecast becomes more useful when every number travels with its unit, time, and meaning. Keep sustained wind separate from gusts, convert only after checking labels, and make the final plan around the whole setting: direction, variability, water, equipment, and the people sailing.