You walk out of the sauna feeling lighter. Step on the scales and you might actually be lighter. But that's not fat you've lost. It's water, and your body wants it back.
Here's what the research says about how much you sweat in a sauna, when to drink, and how to stay on top of it.
How much do you sweat in a sauna?
More than you might think. Researchers in Poland measured 674 young adults before and after two 10-minute dry sauna sessions. Body mass loss ranged from about 0.24 kg in underweight women to 0.82 kg in men with a higher BMI, all in around 20 minutes of heat (Podstawski et al., 2014). Since about 1 kg of body weight lost as sweat is roughly 1 litre of fluid, that's up to the best part of a litre.
A second study had overweight men do four 10-minute rounds at about 90°C. They lost an average of 0.65 kg over the session (Podstawski et al., 2019).
Who sweats more?
In both studies, bigger bodies lost more. People with a higher body weight, BMI and body surface area lost more fluid, and the researchers suggested they should pay particular attention to replacing it. Longer sessions, more rounds and hotter rooms will all push the number up too.
Is sauna weight loss real?
The number on the scale drops, but it's water. Drink it back and the weight comes back. That's not a bad thing: sauna isn't a fat-loss tool, and treating it as one is a good way to end up dehydrated. Go for the heat, the relaxation and the ritual, and keep your water topped up.
When should you drink?
Before
Turn up hydrated. A glass or two of water in the hour before is a simple habit, especially after a workout, when you've already lost fluid.
During
This is the one most people skip. A Finnish review of the sauna and body fluid balance recommended replacing fluid with small drinks repeated often during bathing, describing it as the most convenient and effective approach. It also noted that if you sauna after heavy exercise, topping up fluid is especially important (Ahonen & Nousiainen, 1988). Take a few sips between rounds instead of waiting until you're done.
After
Keep drinking after your last round. If you want to get precise, weigh yourself before and after: each kilogram lost is roughly a litre to replace.
A simple sauna hydration routine
- Before: drink 1–2 glasses of water in the hour beforehand.
- Every break: a few good sips between rounds.
- After: finish your bottle and keep sipping until you're back to normal.
- Long or post-workout sessions: bring more water. Some people add electrolytes for long sessions; talk to a health professional if you're unsure what's right for you.
Signs you need to get out
Leave the sauna, cool down and drink if you feel:
- dizzy or light-headed
- nauseous
- a headache coming on
- confused, or unusually weak or tired
Don't push through it. The heat will still be there next round.
Bring a bottle that's built for the heat
The easiest way to drink during a session is to have water on the bench. Problem is, most bottles aren't made for 90°C. Plastic gets warm and bare metal gets too hot to hold.
We built the Lucky Sauna Water Bottle for exactly this. Double-wall vacuum-insulated stainless steel keeps your water cold through every round. The removable cork sleeve means you can grab it straight off the bench without burning your hand. (Here's why we chose cork.) A bamboo lid and carry hoop finish it off, and there are no plastics and no BPA.
It comes in 600ml and 1000ml. If you're doing multiple rounds or a longer session, go for the litre.
Why it's worth getting right
Regular sauna bathing has been linked with some impressive long-term outcomes. A large Finnish study linked frequent sessions with lower risk of fatal heart disease (Laukkanen et al., 2015), and the same research group has linked it with lower dementia risk (Laukkanen et al., 2017). These are associations, not proof, but they're why so many of us keep coming back. Staying hydrated helps you enjoy every session and keep showing up for the next one.
Make it easy: pick up the Lucky Sauna Water Bottle, and pair it with one of our wool sauna hats and the Lucky Sauna Scrub for the full sauna setup.
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FAQ
How much water should I drink after a sauna?
Roughly replace what you lost. Studies show 0.2–0.8 kg in a 20-minute session; weigh yourself before and after to know your number.
Should I drink during a sauna?
Yes. Small, frequent sips between rounds are the most practical way to keep up.
Is it bad to sauna dehydrated?
Starting dehydrated makes the heat harder on your body. Drink beforehand, and if you feel dizzy or unwell, get out.
This article is general information, not medical advice. If you have a heart condition, low blood pressure, are pregnant or take medication, check with your doctor before using a sauna.