True Wind Calculator
Calculate true wind speed and angle from apparent wind measurements.
True wind vs apparent wind
Apparent wind is what you feel and what your masthead instruments read on a moving boat — the combination of the real wind and the wind created by your own motion. True wind is the actual wind over the water, as if you were stopped. Sailing into the wind makes the apparent wind feel stronger and shifts it forward; sailing downwind makes it feel lighter. Knowing the true wind tells you the genuine conditions for choosing a course and trimming sails.
How to use this calculator
- Enter the apparent wind speed (AWS) in knots from your instruments.
- Enter the apparent wind angle (AWA) in degrees off the bow.
- Enter your boat speed through the water in knots.
- Read off the true wind speed and angle.
The true wind formula
The calculator resolves the apparent wind into fore-aft and athwartships components, removes the boat's motion, then recombines them:
TWS = √[(AWS·cos(AWA) − boat speed)² + (AWS·sin(AWA))²]
The true wind angle is the inverse-tangent of those same components, measured from the bow.
Worked example
Beating upwind you see 18 knots of apparent wind at 30° off the bow while making 6 knots. The true wind works out to about 13 knots at 41° — lighter and further aft than the masthead suggests, which is exactly why the breeze feels stronger upwind than it really is.
Related tools
Already know the true wind and want to predict what you'll see while sailing? Use the apparent wind calculator to work in the other direction.
Frequently Asked Questions
What is the difference between true wind and apparent wind?
Apparent wind is the wind you feel on a moving boat — a combination of the true wind and the wind created by your own movement. True wind is the actual wind over the water, independent of boat speed. Sailing upwind makes apparent wind stronger and further forward than the true wind; sailing downwind makes it weaker.
How do you calculate true wind from apparent wind?
Resolve the apparent wind into components along and across the boat's heading, subtract your boat speed from the along-track component, then recombine. True wind speed = √[(AWS·cos(AWA) − boat speed)² + (AWS·sin(AWA))²]. This calculator does the vector maths for you from apparent wind speed, apparent wind angle and boat speed.
Why do sailors need to know the true wind?
True wind tells you the real conditions and direction the wind is blowing, which is essential for choosing a course, planning tacks and gybes, comparing forecasts to what you measure, and trimming sails correctly. Instruments usually display apparent wind, so converting to true wind gives a clearer picture of the actual weather.