Crosswind Calculator

How to calculate crosswind

1. Turn the runway number into a heading by adding a zero: runway 27 = 270°.
2. Find the angle between the wind and the runway: 300° − 270° = 30°.
3. Crosswind = wind speed × sin(angle): 20 × sin 30° = 10 kt.
4. Headwind = wind speed × cos(angle): 20 × cos 30° = 17.3 kt.

If the angle is more than 90°, the cosine is negative and the wind is behind you – a tailwind. A wind from the right of the runway heading gives a right crosswind, and vice versa.

Quick rule of thumb

Pilots often estimate crosswind with the clock method: at 15° off the runway, use a quarter of the wind speed; 30°, half; 45°, three-quarters; 60° or more, all of it. It slightly overestimates at 15° and 45° (true values 26% and 71%), which errs on the safe side.

Gusts, true vs magnetic, and limits

• Gusts: work out the crosswind for the gust value too; 300/18G28 on runway 27 gives 9 kt steady and 14 kt in gusts.
• True vs magnetic: METAR and TAF winds are given relative to true north, while tower and ATIS winds and runway numbers are magnetic. Convert with the local magnetic variation before using a written report.
• Limits: aircraft manuals list a maximum demonstrated crosswind – for example 15 kt for many Cessna 172 models. It is a tested value, not a structural limit, and your personal limit may be lower.

The same maths works outside aviation – for cyclists judging side-wind on a route, sailors and drone pilots, or anyone working out how much of a wind pushes across a direction of travel.

Worked example with a tailwind

Runway 36 is in use and the wind is 210° at 12 kt. The angle is 210° − 360° = −150°, so the wind is from behind and to the left. Crosswind = 12 × sin 150° = 6 kt from the left. Headwind = 12 × cos 150° = −10.4 kt, meaning a 10.4 kt tailwind. A tailwind of that size would make most light aircraft use the opposite runway, 18, where the same wind becomes a 10.4 kt headwind with the same 6 kt crosswind.

Why headwind matters too

A headwind shortens take-off and landing ground rolls because the aircraft reaches flying speed with less ground speed. A tailwind does the opposite, and many flight manuals add 10% or more to take-off distance for every few knots of tailwind. That is why runways are chosen to face into the wind wherever possible.

Sources

1. FAA – Airplane Flying Handbook, Chapter 8: crosswind approach and landing — faa.gov
2. FAA – Pilot's Handbook of Aeronautical Knowledge (wind and runway numbers) — faa.gov