In many ways, balmy summer temperatures help take the guesswork out of doing laundry. When you’re drenched in sweat after running a quick errand, for example, tossing your T-shirt in the hamper is a no-brainer. During colder months, it’s not always quite so straightforward—you might feel like your favorite chunky sweater can go weeks without a wash.

But just because it doesn’t show any visible coffee stains doesn’t mean it’s not dirty. So just how often should you wash your sweaters? Madewell posed the question to experts from Madame Paulette, a luxury apparel cleaning service in New York, and they recommended every four wears. Elaine Cella, a Procter & Gamble fiber scientist, gave Glamour some similar advice: every three or four wears for thick sweaters, and every wear for thin sweaters.

The general rule is that if a top touches your skin directly, it should get cleaned between each use. This may seem surprisingly often, especially if you step right out of the shower and into your shirt, but you might not realize just how much stuff your skin emits in a day; according to Glamour, we produce 10 grams of skin flakes, 40 grams of oil, and 1 liter of sweat on average (but that volume can vary a lot).

Since people like to layer cozy sweaters over other shirts, those garments are spared from some of our epidermal emissions and can therefore skip some laundry days. That said, you should also factor in how long—and where—you wore the sweater in question. If you donned it for a backyard bonfire or during a brisk walk on an unexpectedly hot day, it’s not a great idea to try to eke out multiple wears. If your fourth wear was just to pick up coffee around the corner, on the other hand, you can probably shoot for a fifth.

As for how to wash your sweaters, it varies by fabric. According to Bustle, most are actually fine to throw in the washing machine, and your safest bet is a gentle or delicate cycle with cold water. For certain wools or other tricky materials, here are some hand-washing tips from Madewell.

[h/t Bustle]