Recovery calculator
Sweat sodium calculator
Turn a measured sweat rate and a sweat test into the sodium you lose over a session — in milligrams, per hour, and as grams of table salt. Free, instant, no sign-up.
These bands say when the question is worth asking. They do not say how much to take.
Fuelling is decided by the session in front of you. The coach works from your actual week.
Why your own numbers are the point
The two inputs vary far more between people than anyone expects. Sweat sodium spans roughly 10 to 90 mmol/L (Baker, Sports Med 2017), so two athletes side by side at the same sweat rate can differ nearly tenfold in the salt they lose. Whole-body sweating rates run about 0.5 to 2.0 L/h, with roughly 2% of athletes above 3.0. That spread is the honest case for measuring yourself rather than reading a chart, and it is also why an average-based estimate is close to useless for any particular person.
The arithmetic
Litres of sweat times millimoles per litre times 22.99 mg per millimole. That is the whole calculation, and it is only unit conversion. The salt figure is the same sodium expressed as sodium chloride at 58.44 g/mol — one millimole of sodium arrived as one millimole of salt, and salt is about 2.5 times the mass of the sodium in it. Quoting one for the other is a common way to overstate a loss by 150%.
Why there is no recommended dose here
The jump from "you lost 3 g of sodium" to "therefore take 3 g of sodium" is not supported. Sodium losses are replaced across meals and days, not one-for-one during a session, and the controlled trials of sodium supplementation during endurance exercise have mostly found no performance effect. The evidence for individualised sodium replacement is weaker than the supplement industry implies — the one-for-one story gets repeated because it sells capsules. This page reports the loss and stops there.
When sodium is worth thinking about at all
The defensible point is about duration rather than dose. Nothing in the guidelines asks a 45-minute gym session to be salted. The ACSM position stand puts sodium in the drink beyond an hour. Past four hours, fluid and sodium balance becomes a real problem — though the dangerous end of it, exercise-associated hyponatraemia, is reported mostly beyond four to six hours and is caused by drinking too much water rather than by losing too much salt. The failure mode at the long end is over-drinking, not under-salting.
What the inputs will and will not accept
Concentrations above 100 mmol/L are refused. Sweat is always hypotonic to plasma, which sits around 140 mmol/L sodium, so a reading of 120 is an error in the test rather than a finding about the athlete. The method behind your number matters too: local patch tests typically read about 10 to 90 mmol/L and whole-body collection about 10 to 70, so the same person can produce two different figures depending on how the sweat was collected.