Can I reuse water for wudu?

Leftover water is fine

If you fill a jug, use some of it for wudu, and water remains in the jug, that remaining water is completely fine for the next person. Ibn Abbas reported that the Prophet ﷺ used to bathe with the water left over by his wife Maymunah (Sahih Muslim 323). Sharing a basin or jug was normal in his household, and nothing about it makes the water unusable.

Water that dripped off your body

This is called "used water" (ma' musta'mal): the water that has already washed a limb in wudu or ghusl and run off. Everyone agrees it is pure, not dirty. The Prophet ﷺ once made wudu and poured his used wudu water over Jabir, who was ill (Sahih al-Bukhari 194). So if it splashes on your clothes, there's no problem.

But can it purify again? Scholars differ:

• Hanafi, Shafi'i and Hanbali (main views): it is pure but no longer purifying, so it can't be used for another wudu or ghusl.
• Maliki: it is still purifying, but using it is disliked when other water is available.
• A small amount that drips back into a large basin doesn't spoil the rest of the water.

Saving water without stress

Islam loves moderation, and wasting water in wudu is disliked. Instead of reusing runoff, the simple solution is to use less in the first place: a gentle stream from the tap, or a small jug. The Prophet ﷺ made wudu with roughly one mudd, about two cupped handfuls. See How much water should I use for wudu?

If you're somewhere water is truly scarce and the only water you have is runoff you caught, scholars who allow it (the Malikis) give you room, and otherwise tayammum is the fallback.

Greywater, pools and showers

Water in a bath that several people have washed in, or shower runoff, falls under the same discussion, but in everyday life you'll rarely need it: just use fresh tap water. Large bodies of water like the sea or a pool are a different case. See Can I make wudu with seawater or pool water? And if you're already showering, see Does a shower count as wudu?