You probably don’t notice how much shampoo you squeeze into your palm when you’re washing your hair in the shower. Unless the bottle is about to run out and you’re struggling to get every last drop, it’s an action we don’t think twice about.

But when you wash with too much shampoo, you’re not just wasting product and water. You could also be impacting the health of your hair and scalp in the process.

So, how much shampoo do you actually need to get the job done? Lucy Chen, a board-certified dermatologist at Riverchase Dermatology, has the answer. And it’s way less than you think.

How Much Shampoo Do You Need?

Just like when you’re figuring out how often to wash your hair, the amount of shampoo you need is also dependent on a few factors, including hair type and length.

We all want super clean hair, so most of us tend to over-shampoo. But while most people squeeze a big glob into their hand, Dr. Chen says the reality is that you should only be applying a quarter-sized amount.

“The best way to get your hair clean while also saving water is by limiting the amount of shampoo you use. You should only be applying a quarter-sized amount of shampoo,” Dr. Chen says. “If you stick to this small amount, you won’t need to rinse your hair as much.”

Even though that might not seem like a lot, it’s more than enough when you lather it and massage it into your scalp. Especially when you apply it to the correct areas—primarily your scalp.

“You don’t need to shampoo the tips of your hair—just the roots,” she says. “If you spend less time applying the shampoo, you won’t need to keep the water running as long.”

That smaller amount is also better for the planet. Aside from saving water, it also saves money, making every bottle of shampoo last longer.

Can You Shampoo Too Often?

In theory, it would make sense that more shampoo equals cleaner hair. Unfortunately, that’s not the case.

Using too much—and shampooing too often—strips your hair of natural oils and dries out your scalp. “If you’re someone who shampoos your hair too often, you’ll notice that your head is itchy, dry, and flaky,” Dr. Chen says. On the other hand, if you don’t wash your hair enough, you risk oil buildup, which could lead to odor and irritation.

As a rule of thumb, Dr. Chen says the average individual can typically go 2 to 3 days without shampooing their hair. If you want to make your shower routine even more eco-friendly, you can also switch to a shampoo bar or shampoo concentrate, which takes the plastic bottle out of the equation.