Headwind vs. Crosswind Component
Wind rarely blows straight down the runway. Whatever direction it comes from, you can split it into two parts: the headwind component (along the runway, which shortens your ground roll) and the crosswind component (across the runway, which pushes you sideways and is the part that demands rudder, aileron, and proficiency).
The split depends entirely on the angle between the wind and the runway. A wind straight down the runway is all headwind and no crosswind. A wind 90° to the runway is all crosswind. Everything in between is a mix — and the math below tells you exactly how much of each. The calculator at the top of this page runs it for every runway end at your airport from the live METAR, so the runway with the best headwind and the smallest crosswind is already flagged before you taxi.
The Crosswind Formula
First find the wind angle — the difference between the wind direction and the runway heading — then split the wind with sine and cosine.
Step 1 — the wind angle
Runway heading is the runway number × 10 (Runway 09 = 090°). Subtract the wind direction from it, or the other way round, and take the difference. Anything over 180° means you subtracted the wrong way — take 360 minus your answer.
Step 2 — split the wind
Crosswind = wind speed × sin(angle)
Headwind = wind speed × cos(angle)
A negative headwind is a tailwind, and it is worth naming out loud rather than treating as a small number: a 5-knot tailwind lengthens the ground roll far more than a 5-knot headwind shortens it, which is why the calculator marks a tailwind rather than just showing a minus sign.
Worked example — Runway 09, wind 120° at 20 kt
Runway heading 090°, wind from 120° at 20 knots, so the angle is 120 − 90 = 30°.
Crosswind = 20 × sin 30° = 10 kt
Headwind = 20 × cos 30° ≈ 17 kt
So you are landing with a 17-knot headwind and a 10-knot crosswind from the right. The clock rule gets you the same crosswind in your head: 30° is half the wind, and half of 20 is 10.
The Clock Rule (no calculator needed)
Pilots estimate the crosswind in their head with the clock rule: read the wind angle like minutes on a clock face and take that fraction of the wind speed. It is the check you run in the run-up area when the tower gives you a different runway than the one you planned.
15°
¼
a quarter of the wind speed
30°
½
half the wind speed
45°
¾
about 70% of the wind speed
60°+
all
treat as the full wind speed
The rule reads slightly high at 45° (the true figure is 71%, not 75%) and slightly high at 60° (87%, not 100%). Both errors are on the conservative side, which is the right direction for a mental estimate — but it is also why the exact number is worth computing before a marginal landing rather than after.
Gusts, and Which Number You Check
A METAR reporting 18015G27KT is describing two different airplanes to fly. Compute the crosswind at 15 knots and again at 27, and treat the gust figure as the number that matters — controllability has to hold at the worst moment of the flare, not the average one. The calculator above does both at once and checks the gust value against your limit.
The gust factor is the spread between the two, and it also affects approach speed: adding part of the gust factor to your final approach speed is a common convention, but the authority is your POH and your own procedure, not a rule of thumb from the internet. Whatever you add, remember it lengthens the landing roll — cross-check the result against takeoff and landing distances for the runway you picked.
True Wind vs. Magnetic Wind
Runway numbers are magnetic. So are the winds a tower controller or an ATIS broadcast reads to you — they come off the field’s own sensors and are given in magnetic degrees. Winds in a METAR, a TAF, or a winds-aloft forecast are referenced to true north.
Mix the two and your angle is off by the local magnetic variation, which runs past 15° in parts of the country — enough to turn a comfortable crosswind into an uncomfortable one, or the reverse. Convert first, or compare like with like.
Maximum Demonstrated Crosswind
Your POH lists a maximum demonstrated crosswind component — the strongest crosswind a test pilot handled during certification, usually in Section 4. It is not a regulatory limit. But exceeding it puts you beyond what the manufacturer proved, and usually beyond what you have practiced, at the exact moment the airplane is slowest and closest to the ground.
Decide with three numbers side by side: the computed crosswind, the gust value, and the POH demonstrated figure. Enter your aircraft’s number in the calculator above and it checks every runway against it. Then take the whole picture — wind, weather, runway, and yourself — into Go / No-Go.
A personal-minimums crosswind figure is the more useful number day to day, and it is allowed to be well below the POH value and to move as you gain or lose currency. The FAA’s Airplane Flying Handbook (FAA-H-8083-3) covers crosswind takeoff and landing technique; the preflight-action rule, 14 CFR 91.103, is what obliges you to have runway lengths and performance data in hand before you go.
Picking the Runway
At a non-towered field the runway choice is yours, and the smallest crosswind is not always the right answer: a shorter runway, a displaced threshold, rising terrain off the departure end, or a NOTAM’d closure can all outrank a couple of knots of wind. The calculator ranks every runway end by headwind and flags closures where FlightKit has them, but the decision is a judgement call about the whole field, not just the wind.
Density altitude belongs in that same call — thin air lengthens the roll on the same runway the crosswind is pushing you off of. Run the density altitude calculator before you commit, and check current weather for the trend rather than the snapshot: a wind that has been backing all afternoon will not be the wind you land in.
Frequently Asked Questions
What is a crosswind component?
The crosswind component is the part of the wind that blows across the runway, perpendicular to your direction of travel. The rest of the wind is the headwind (or tailwind) component, blowing along the runway. You care about the crosswind because it is what pushes the airplane sideways during takeoff and landing.
How do you calculate the crosswind component?
Find the angle between the wind and the runway, then: crosswind = wind speed × sine(angle), and headwind = wind speed × cosine(angle). Example: a 30° angle with a 20-knot wind gives a crosswind of 20 × sin 30° = 10 knots and a headwind of 20 × cos 30° ≈ 17 knots. A crosswind calculator or a wind-component chart does this instantly.
What is the crosswind clock rule of thumb?
Treat the wind angle like minutes on a clock face: 15° ≈ a quarter of the wind, 30° ≈ a half, 45° ≈ three-quarters, and 60° or more ≈ the full wind speed as crosswind. It is close enough for a quick mental check in the run-up area before you commit to a runway.
What is maximum demonstrated crosswind component — is it a limit?
It is the strongest crosswind a test pilot demonstrated during the aircraft’s certification, published in the POH. It is not a regulatory limitation — but it is a strong guideline, and exceeding it means you are operating beyond what the manufacturer proved and your own demonstrated skill. Treat it, your personal minimums, and gusts together when deciding.
True wind or magnetic wind — which do I use?
Runway numbers and tower/ATIS winds are referenced to magnetic north, so they match directly. Winds in a METAR or a forecast are referenced to true north. If you mix the two, correct for local magnetic variation first, or the angle — and your crosswind — will be off.
How do gusts change the crosswind number?
Run the calculation twice: once at the steady wind and once at the gust. The gust figure is the one to check against your limit, because the airplane has to be controllable at the worst moment of the landing, not the average one. The gust factor — gust speed minus steady speed — is also what many pilots add to approach speed, half of it being a common convention; use your POH and your own procedure.
How much crosswind is too much?
There is no universal number — it depends on the aircraft, your proficiency, the gust spread, and the runway. Build a personal-minimums crosswind limit, check it against the POH max demonstrated value, and add margin for gusts. When in doubt, pick a more favorable runway or wait.