A sauna makes you sweat, and sweat is mostly water. On days you use a sauna, your usual daily water target is not quite enough. The good news is that the extra amount is easy to estimate with a little math and a bathroom scale.

Start With Your Normal Daily Baseline

Before thinking about the sauna, know roughly how much water you need on an ordinary day. A common starting point is about half your body weight in pounds, in ounces. A 160-pound adult would aim for around 80 ounces from drinks and food combined. Our water intake calculator gives a more tailored number based on weight and activity.

How Much Do You Lose in a Sauna?

Sweat loss varies a lot from person to person. Heat, humidity, session length and your own sweat rate all matter. As a rough guide, many adults lose somewhere between half a pound and a full pound of water during a 15 to 20 minute session.

Session Length Typical Sweat Loss Extra Water to Plan
10 minutes About 0.3 to 0.6 lb 6 to 10 oz
15 minutes About 0.5 to 0.8 lb 8 to 14 oz
20 minutes About 0.6 to 1.0 lb 10 to 16 oz
Two rounds of 15 minutes About 1.0 to 1.6 lb 16 to 26 oz

These ranges are estimates. Your own number can be higher or lower, which is why weighing yourself is the best guide.

Measure Your Own Sauna Sweat Loss

  1. Weigh yourself with minimal clothing right before the session.
  2. Do your normal sauna session without drinking.
  3. Towel off and weigh yourself again in the same clothing.
  4. Each pound lost equals about 16 ounces of water.

The sweat loss calculator does this math for you, and our guide on measuring your sweat rate at home explains the method in more detail.

Putting the Math Together

Your sauna day target is simply your baseline plus what you lose in the heat. Many people drink a little more than they lose, about 1.25 times the amount, because some of what you drink after a session passes through before your body holds on to it.

Example: a 160-pound adult with an 80-ounce baseline loses 0.75 lb in a 20-minute session. That is 12 ounces of sweat. Multiplied by 1.25, the extra target is about 15 ounces, for a total near 95 ounces that day.

When to Drink

  • Before: a glass of water in the hour before you go in, so you start well hydrated.
  • During: small sips between rounds if you do more than one session.
  • After: most of the extra amount over the next hour or two, not all at once.

If you sweat heavily or do long sessions, a drink with some sodium and potassium can help replace what plain water does not. Check with a doctor first if you have heart or kidney concerns or take medicine that affects fluid balance.

Signs You Need More

Dark yellow urine, a headache, dizziness when you stand up or feeling unusually tired after a session can all point to low fluids. Leave the sauna right away if you feel lightheaded. For guidance on how long and how hot to go, see the sauna session calculator.

Final Thoughts

Sauna days call for your normal water target plus the sweat you lose in the heat. Weigh yourself before and after a few sessions, turn each pound into 16 ounces, and add a little extra. After a week or two you will know your own number and can plan it without thinking.

FAQs

Should I drink water inside the sauna?

Small sips are fine, especially between rounds. Bring a water bottle and avoid glass containers in the heat.

Do I need electrolytes after a sauna?

Not usually for one short session. Long or repeated sessions with heavy sweating may call for a drink with some sodium.

Does coffee count toward sauna day water?

It counts toward your daily fluids. Plain water is still the better choice right before and after a session.