Loading page
About this tool
Find your hourly sweat rate from the weigh-in weigh-out method, plus fluid replacement and sodium loss estimates.
A sweat rate calculator turns a before-and-after weigh-in into litres of sweat lost per hour, using the standard field formula: body-mass change plus fluid drunk minus urine passed, divided by session time. It is aimed at runners, cyclists, footballers, gym-goers and anyone training in heat who wants a drinking plan based on their own body rather than a generic number. The tool also reports body-mass loss as a percentage — the marker ACSM uses, where losses beyond 2% start to hurt endurance and heat tolerance.
Open Sweat Rate Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Sweat rates range from under 0.5 L/h to over 2.5 L/h between people in the same session.
Converts the hourly rate into a practical millilitres-per-15-minutes drinking target.
Estimates sodium lost from your sweat concentration, so electrolyte choices are informed.
Sweat loss in litres equals your pre-exercise weight minus your post-exercise weight in kilograms, plus everything you drank in litres, minus any urine passed; divide that by the hours you exercised to get litres per hour. Weigh in minimal dry clothing after emptying your bladder, and towel off fully before the second weigh-in. One kilogram of body-mass change is treated as one litre of body water.
Most people fall between 0.5 and 2.0 litres per hour during moderate to hard exercise, with well-trained athletes in hot, humid conditions reaching 2.5 to 3.0 litres per hour. Anything much above 3 L/h is rare and usually points to a measurement error rather than an unusual physiology.
ACSM guidance is to drink roughly 1.25 to 1.5 litres for every kilogram of body mass lost when your next session is less than about 12 hours away. You need more than the deficit itself because a share of any large drink is passed as urine rather than retained, and including sodium in the drink or the next meal improves how much you hold on to.
Sweat sodium concentration typically runs between 20 and 80 mmol per litre, which is about 460 to 1,840 mg of sodium per litre of sweat. Salt crusting on your skin or kit after training suggests you sit at the higher end. This is general information, not medical advice — anyone on a sodium-restricted diet or with a heart, kidney or blood-pressure condition should discuss fluid and salt intake with their doctor.
Add the Sweat Rate Calculatorwidget to your blog or website — free, responsive, no signup. Just keep the “Widget by AltFTool” credit link visible.
<iframe src="https://www.altftool.com/embed/widget/sweat-rate-calculator"
title="Sweat Rate Calculator — free AltFTool widget"
width="100%" height="640" style="border:0;border-radius:12px;overflow:hidden"
loading="lazy" referrerpolicy="no-referrer-when-downgrade"></iframe>
<p style="font-size:12px;margin:4px 0 0">Widget by <a href="https://www.altftool.com/tools/all/sweat-rate-calculator?utm_source=embed&utm_medium=widget">AltFTool — free online tools</a></p>