Apparent Wind Calculator
Wind and boat inputs
Apparent wind result
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True wind vs. apparent wind
True wind here means wind over the water—the wind that would be observed aboard if the boat stopped moving through the water. Apparent wind is what the moving boat, crew, sails, and wind sensor experience.
Forward motion creates an equal-speed airflow from the bow. Adding that motion-induced component to the true-wind vector usually makes the apparent wind stronger and farther forward when sailing upwind, and weaker when sailing downwind.
How to use this calculator
- Choose one speed unit for every speed input.
- Enter true wind speed over the water.
- Enter true wind angle as the angle from the bow to where the wind comes from.
- Choose port or starboard and enter forward boat speed through the water.
- Review the apparent wind speed, angle, components, and assumptions.
Apparent wind formulas
Let TWS be true wind speed, BS boat speed, and TWA true wind angle. Resolve the wind into bow-axis and lateral “from” components:
forward = TWS × cos(TWA) + BS
lateral = TWS × sin(TWA)
AWS = √(forward² + lateral²)
AWA = atan2(|lateral|, forward)
Using atan2 preserves whether apparent wind is forward or aft of the beam. The displayed angle remains between 0° and 180°.
Keep the reference frames consistent
This calculator pairs water-referenced true wind with forward speed through water, matching common sailing-instrument terminology. If you instead start with wind over ground, the boat-motion vector must also be over ground and must include its direction relative to the bow.
Mixing true wind over water with speed over ground can introduce a current-related error. The error can affect both apparent wind speed and angle.
Frequently asked questions
What is apparent wind?
Apparent wind is the airflow experienced on the moving boat. It is the vector combination of true wind and the motion-induced wind created by the boat moving forward.
How do you calculate apparent wind speed?
Square true wind speed and boat speed, add twice their product multiplied by the cosine of true wind angle, then take the square root: AWS = √(TWS² + BS² + 2 × TWS × BS × cos TWA).
Why does apparent wind move forward as boat speed increases?
Boat motion adds an airflow component from directly ahead. As this component grows relative to the true wind, the combined apparent-wind vector normally rotates toward the bow.
What do 0°, 90°, and 180° mean?
Angles describe where wind comes from relative to the bow. Zero degrees is directly ahead, 90° is abeam, and 180° is directly astern. Port or starboard identifies the windward side for angles between those centerlines.
Should I use speed through water or speed over ground?
Use speed through water with true wind over the water, as this calculator does. To model wind over ground, use a consistently ground-referenced boat vector and account for the angle between heading and course over ground.
Can apparent wind come from ahead while sailing faster than the true wind downwind?
Yes. If the boat moves directly downwind faster than the true wind, boat-induced wind exceeds the following true wind and the remaining apparent wind comes from the bow.
Why might my masthead instrument show something different?
Sensor calibration and alignment, mast and sail flow distortion, upwash, heel, pitch, roll, mast motion, leeway, current, and averaging can all alter a real reading. Many systems also apply proprietary corrections.
Are my inputs uploaded or tracked?
No. Calculations run locally in the browser, and the tool does not send, store, or attach entered values to analytics events.
Assumptions and safety limits
- The boat moves forward along its bow axis with no lateral velocity, leeway, yaw, or acceleration.
- Wind is steady, horizontal, and uniform at the measurement point. Vertical wind and gust dynamics are omitted.
- True wind angle is a “from” angle relative to the bow, not a compass bearing or the direction air travels toward.
- The calculator accepts speeds up to 300 knots (or the equivalent) and preserves full floating-point precision before display rounding.
- If equal boat and true-wind speeds cancel exactly while running dead downwind, apparent wind is calm and its angle is undefined.
Safety disclaimer: This educational vector calculator does not replace calibrated instruments, weather information, seamanship, vessel guidance, sailmaker limits, or the skipper's judgment. Reefing and sail-choice decisions should account for gusts and actual conditions.
Methodology and sources
Last reviewed: August 2, 2026. Formula signs, wind-angle conventions, vector reference frames, and instrument terminology were checked against: