Sailing Wind Calculator
Calculate apparent wind angle and speed from true wind conditions and boat speed. Determine your velocity made good (VMG) toward the wind.
About this calculator
The wind a sailor actually feels on a moving boat — the "apparent wind" that fills the sails and needs trimming for — is never quite the same as the true wind reported by a weather station, because the boat's own motion adds a vector to the mix. This calculator does that vector addition explicitly: it decomposes the true wind into components along and across the boat's heading using the true wind angle, adds the boat's own speed to the along-heading component (since the boat is effectively generating a headwind equal to its own speed), and then recombines those components with the Pythagorean theorem to get apparent wind speed, with the angle found via the arctangent of the two components. The practical effect is that apparent wind is always stronger than true wind when sailing upwind and often lighter than true wind downwind, which is exactly why sail trim differs so much between points of sail.
Velocity made good (VMG) is calculated separately and more simply, as boat speed times the cosine of the true wind angle — it isolates the component of your boat speed that's actually carrying you toward (or away from) the wind direction, which is the number that matters when tacking upwind to a mark, since raw boat speed alone can be misleading if you're sailing a wide angle relative to the wind. One assumption to note: this model treats wind and boat velocity as constant, steady-state vectors, so it won't capture the effects of gusts, waves, or current, which shift apparent wind and VMG moment to moment on the water.
Inputs
Results
Apparent Wind Speed
19.7 kts
How to Use This Calculator
- Enter the true wind speed in knots from a weather station or instrument.
- Set the true wind angle in degrees — 0° is directly into the wind, 90° is a beam reach, 180° is a dead run.
- Enter your current boat speed in knots.
- Read Apparent Wind Speed (AWS) in knots — this is what your wind instruments and sails respond to.
- Check Apparent Wind Angle (AWA) to know how to trim your sails for the actual wind your boat is experiencing.
- Use Velocity Made Good (VMG) in knots to evaluate how efficiently you are making progress toward an upwind or downwind mark.
How the result changes with True Wind Speed
| True Wind Speed | Apparent Wind Speed |
|---|---|
| 7.5 | 12.49 kts |
| 11 | 15.82 kts |
| 23 | 27.57 kts |
| 38 | 42.46 kts |
What each input means
- True Wind Speed
- The actual wind speed measured relative to the ground or water surface, typically from weather reports or an anemometer.
- True Wind Angle
- Angle between the boat's heading and the true wind direction. 0° = head-to-wind, 90° = beam reach, 180° = dead downwind.
- Boat Speed
- Current speed of the boat through the water, measured by a knotmeter or GPS.
What each result means
- Apparent Wind Speed
- The wind speed as felt on the boat, combining true wind with the boat's own motion.
- Apparent Wind Angle
- The angle of the wind relative to the bow as experienced on the moving boat.
- Velocity Made Good
- The component of boat speed directly toward (or away from) the wind. Positive = upwind progress, negative = downwind.
How this is calculated
Worked example, using the default values
- Identify Input ParametersTrue Wind Speed = 15, True Wind Angle = 45, Boat Speed = 6 = 3 input(s) provided
- Calculate Apparent Wind SpeedApparent Wind Speed19.7 = 19.7
- Calculate Apparent Wind AngleApparent Wind Angle = Math32.6 = 32.6
- Calculate Velocity Made GoodVelocity Made Good4.24 = 4.24
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why is apparent wind speed calculated using both sine and cosine of the wind angle?
The calculator treats true wind and boat speed as two vectors and adds them component by component: it uses cosine of the true wind angle to find the component along the boat's heading (adding boat speed to that component) and sine to find the component across the boat's heading, then combines both with the Pythagorean theorem. This vector approach is necessary because wind and boat motion rarely point in exactly the same or opposite direction, so a simple addition or subtraction of speeds would be wrong except at 0° and 180°.
Why is apparent wind stronger upwind and often weaker downwind compared to true wind?
When sailing upwind (small true wind angle), the boat's own speed adds almost directly to the wind's along-heading component, so the vectors reinforce each other and apparent wind speed increases. Downwind (large true wind angle, closer to 180°), the boat is moving away from the wind source, so its speed partially cancels the wind's component, which is why apparent wind is often noticeably lighter than true wind on a dead run.
How is velocity made good (VMG) different from just reading my boat speed?
VMG multiplies your boat speed by the cosine of the true wind angle, isolating only the component of your motion that's actually directed toward or away from the wind source. Two boats can have identical boat speed but very different VMG if one is sailing a tighter angle to the wind than the other, which is why VMG — not raw speed — is the number that matters when tacking upwind toward a mark.
Does this calculator account for gusts, waves, or current?
No — it treats true wind speed, true wind angle, and boat speed as constant, steady-state values and computes apparent wind and VMG from that single snapshot. Real conditions on the water shift moment to moment, so treat the output as the apparent wind and VMG for the specific conditions you entered, not a continuously updating live reading.
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