Fluid and sodium arithmetic
Weigh yourself before and after a training run to estimate how much you sweat per hour and how much sodium that may carry, then check your race drinking plan against it.
Weigh without clothes, towelled dry, before and after a 1–2 hour run at race effort.
Compare your planned drinking and a sweat test result with this sweat rate.
Printable sweat-test log
120 minutes · 70.0 → 68.9 kg · drank 800 mL
Per hour, in these conditions
1 kg of body mass counts as 1 L
-1.6% of starting weight
Share of sweat loss you drank back
Untested: sweat holds 230–1,610 mg/L
500 mL an hour against this sweat rate
Mass-balance estimate (Baker 2017). Scale accuracy, sweat trapped in clothing and unmeasured drinks or toilet stops all shift the result.
The calculation is the mass-balance method from Baker's review: one kilogram of lost body mass is treated as one litre of sweat. It becomes less reliable over long efforts, because body mass then reflects fuel use as well as sweat.
Sweat sodium ranges from about 10 to 70 mmol/L, roughly 230–1,610 mg per litre, so the untested estimate spans a factor of seven. Enter a lab or patch-test result to narrow it. The ISSN discusses 500–700 mg of sodium per litre of fluid and roughly 300–600 mg an hour in hot or humid conditions. McCubbin's model finds that targeted sodium replacement matters most when you replace more than 80% of your sweat and your sweat holds at least 40 mmol/L; the calculator flags that combination.
Use the sweat rate as a ceiling check: a drinking plan above it means weight gain and a higher risk of hyponatremia. Do not multiply a two-hour test across a full day. The hydration and sodium guide explains how to read race weigh-ins, and the running calorie calculator fits your drink into the hourly carbohydrate plan. On a hot day, the heat-adjusted pace calculator shows how much to slow down.
Confusion, repeated vomiting or collapse during a race needs urgent medical assessment, whatever the scale says. If you have a heart or kidney condition, or a history of hyponatremia, plan fluid and sodium with a clinician.
Weigh yourself without clothes before and after a 1–2 hour run, then use (kg before − kg after + litres drunk − litres of urine) ÷ hours. Weighing 70.0 kg, drinking 0.8 L, passing 0.2 L of urine and weighing 68.9 kg after 2 hours gives 0.85 L an hour.
Drink to thirst. The ISSN racing reference for ultras is 450–750 mL an hour, less on cool nights. Use your sweat rate to check that a plan stays below your losses, not to replace every millilitre.
Sweat holds about 230–1,610 mg of sodium per litre, so at 0.85 L an hour the loss is roughly 200–1,370 mg an hour. Only a sweat sodium test narrows that range; salt marks on clothing cannot.
No. A sweat test estimates losses; it does not prescribe replacing all of them or stopping at a fixed percentage. Drinking beyond your losses is the main cause of exercise-associated hyponatremia, and extra salt does not cancel it.
At least once in cool conditions and once in warm conditions, at race effort, because sweat rate rises with heat. Repeat it when the season or your race changes.
Only with care. Over long races body mass also falls with fuel use; a model of a 161 km mountain ultra estimated 1.2–3.5% loss while body water stayed in balance. Weight gain is the clearer warning sign.