A sauna is pure-heat sweat — so the honest answer is "it depends."
You lose water and some sodium sitting still in the heat, without the big losses of a hard workout. For one short session, that's mostly a rehydration job with a little salt. For a daily heavy habit, it adds up. Here's the honest amount — and why we won't pretend a 15-minute sauna is a marathon.
Estimate your sauna loss →What you actually lose.
Sweat is sweat: water carrying sodium, potassium and a little magnesium. The difference in a sauna is that you're generating it from heat alone, usually for 10–20 minutes rather than an hour of effort, so a single session's total is typically smaller than a hard workout's. The catch is the usual one — how much and how salty varies about tenfold between people, so "grab a stick" is a guess. Weigh yourself before and after once and you'll know your own figure exactly. How to weigh in →
If the sauna is a daily practice.
Heavy sauna users, contrast (hot–cold) routines, and hot-yoga-plus-sauna stacks are a different story: repeated, near-daily heat means the fluid and sodium turnover is real and worth managing rather than winging. Repeated heat exposure is also a genuine stimulus, not just a cost — the same hormetic angle covered on the hot-yoga page — and the thing that lets heat become an adaptation instead of strain is staying on top of the fluid you're losing.
Keep reading
Hot yoga, Pilates & sculpt
Same room, bigger losses — and the mitochondria angle, honestly caveated.
Your sweat rate
A before/after weigh-in works for a sauna too — one number, no guessing.
Why salt, not just water
Why a little sodium helps you hold the water you drink.