True vs Apparent Wind Calculator

The wind you feel on a moving boat isn't the real wind. True-Wind Calculator turns the apparent wind you measure into the true wind you need — in two taps, no wind instruments required. In use since 2013, completely rebuilt in 2026. HOW IT WORKS Enter boat speed, apparent wind speed, apparent wind angle and heading. The app does exact vector math — no lookup tables, no approximations — and gives you true wind speed and true wind direction instantly. Boat speed and heading can be filled automatically from GPS and compass with one tap. FREE • True wind speed, direction and angle — unlimited calculations • One-tap GPS fill for boat speed and heading • The formula explained, so you can trust the numbers • Works completely offline — no account, no ads, no tracking PRO (one-time purchase, no subscription) • Performance — live VMG, target boat speed, % of target VMG and point of sail, driven by your boat's polar • Tactics — tack preview, windward-mark laylines, and whether you can lay the mark • Boat polars — pick a preset or build your own in the editor • Compass rose view — heading, apparent and true wind on one dial • Calculation history — every result logged with its inputs • All speed units — knots, m/s, km/h, mph Bought the app back when it was paid? Pro is already yours — it unlocks automatically. PRIVACY Everything stays on your device. Location is read only to fill the input fields and never leaves your phone. No analytics, no third-party SDKs.

Apparent Wind Calculator takes the maths out of one of sailing's most important concepts. Feed in your true wind angle, true wind speed, and boat speed — and instantly see your apparent wind angle and speed, visualised as a live vector diagram. Whether passage planning, under sail or just getting your head around why the wind always seems to come from forward when you bear away, this app makes it immediately clear. BOAT POLAR Load a polar table and the app sets your boat speed automatically based on your true wind angle and wind speed. Built-in presets cover small, medium, and large monohulls and catamarans — or enter your own target speeds for a custom polar. With auto-fill enabled, every diagram you see reflects realistic performance numbers for your boat, not a guess. FEATURES • Live vector diagram showing true wind, apparent wind, and boat velocity as scaled arrows — updates in real time as you move the sliders • Monohull and catamaran boat graphics that respond to your apparent wind angle • Boat polar table with presets for monohulls and catamarans, fully editable • Auto-fill boat speed from your polar for realistic results • Point of sail readout — from Close Hauled to Deep Run • Port and starboard tack display • Works entirely offline, no account required WHO IT'S FOR Sailing students learning the theory, racers checking their VMG angles, cruisers planning passages, and coaches explaining wind shifts to crew. The apparent wind is what your sails actually feel. This app helps you understand it.