Wind components
For a runway heading h and a wind from direction d at speed V:
headwind = V cos(d − h)
crosswind = V sin(d − h)
A negative headwind is a tailwind. The reciprocal runway simply flips both signs. These are the same identities as the crosswind calculator, which is built for one runway and a wind diagram; this page compares a list of runways and names a preferred heading.
Magnetic and true
Runway numbers are magnetic. A METAR wind is magnetic. Winds aloft, and some charted or true-north sources, are true. When the references differ, convert the wind with variation: east is least, west is best — magnetic direction = true direction − east variation. This page will do that conversion when you mark the wind as true.
Sources
FAA Pilot’s Handbook of Aeronautical Knowledge (wind components, runway selection); AIM (runway numbering, magnetic variation). For a single-runway diagram including both ends and gusts, use the crosswind calculator. For a wind-correction angle and groundspeed, use the E6B wind triangle.
Frequently asked questions
Is a METAR wind magnetic or true?
Magnetic, at a land airport in the United States, and runway numbers are magnetic as well, so you can usually compare them directly. Winds aloft are true. If you mix a true wind with magnetic runways, enter the variation.
What if every runway is over my crosswind limit?
The page still names the into-wind runway with the greatest headwind (and a warning). That is not a recommendation to go. Consider another airport, a longer wait, or a different airplane.
How is this different from the crosswind calculator?
The crosswind page is built around one runway, both ends, and a wind diagram. This page takes a list of runways, optional true-to-magnetic conversion, and a crosswind limit, and highlights a preferred heading.