Can I reuse coffee grounds?

You can put the grounds through your brewer a second time, but you will not enjoy the result. The first brew already pulls out most of the good stuff: the sweetness, the bright acids, and the pleasant flavors come out early. A second pass mostly drags out what is left, which is weak, flat, and on the bitter, papery side. So reusing grounds to save coffee is a false economy. Save them for something else instead.

Why the second cup is bad

Brewing is extraction: hot water dissolves flavor compounds out of the grounds in a rough order. The pleasant, sweet, fruity ones leave first; the harsh, bitter ones come last. By the time your first cup is done, the easy flavor is gone and the grounds are largely tapped out. Run water through again and you get a thin, hollow liquid that is also weirdly bitter, the worst of both worlds. This is the same idea behind a weak cup and is explained in extraction-theory.

To brew a genuinely stronger or second cup, you need fresh grounds, not recycled ones. If your coffee is too weak, the answer is more coffee, a finer grind, or a longer brew, not a second run through old grounds.

What used grounds are actually great for

Do not bin them. Spent coffee grounds are handy:

• Compost: they are a nitrogen-rich “green” material. Mix them in, do not dump a thick layer on its own.
• Garden: lightly worked into soil or sprinkled thinly as a mulch. Worms in a worm bin like them in moderation.
• Around the house: a bowl of dried grounds can soften fridge or freezer smells, and they make a gritty scrub for pots and hands (skip delicate or light-colored surfaces, since they can stain).
• Dry them first if you are storing grounds for later, so they do not go moldy.

One thing they are not great at: brewing more coffee. For that, start fresh, and keep your unused coffee airtight and away from heat, air, and light, as covered in how-to-store-coffee.