Sweat-rate calculator

A repeatable field routine

Use a measured training session, an accurate scale and measured drinks. Weigh dry and unclothed before and after where practical; otherwise use the same dry clothing mass for both measurements. Record duration, drinking and urine. Approximate loss in litres = mass before minus mass after in kg + drink litres − urine litres. Divide by hours. Example: 70.0 → 69.5 kg, 0.4 L drunk, no urine, 1 hour gives approximately 0.9 L/h. This arithmetic is a planning estimate, not a prescribed intake.

Before starting, place the scale on a firm surface and check that repeat readings agree. Empty your bladder before the initial measurement. Measure bottle contents rather than estimating from mouthfuls. Afterwards, remove wet clothing, towel off and use the same weighing conditions as before (unclothed, or equivalent dry clothing). Record any urine passed during the session. If a measurement is missing, label the result incomplete instead of treating an unknown input as zero.

Validity, error and repeatability

Body-mass methods are useful, but retained sweat, food, breathing and fuel oxidation distort estimates. Error grows in very long sessions. No universal error band applies to all scales and conditions. Repeat in comparable weather and effort; keep a range, not a personal constant.

Keep a small record containing route, elapsed exercise time, temperature, clothing, effort, starting mass, finishing mass, fluid consumed and urine. Heavy rain, swimming, spilled drinks or eating substantial food make interpretation harder. A negative calculated loss should trigger a check of inputs and units. It is not evidence that sweating stopped. Short sessions with very small mass changes may be dominated by scale resolution.

Change the plan, not the target body weight

Author application: check whether carrying capacity and refill points support a comfortable practiced intake. Do not force complete replacement, drink to gain weight, or use this result to diagnose dehydration. Sodium needs cannot be calculated from sweat volume alone.

For example, a relatively high loss estimate with limited refill access can justify carrying more capacity or choosing an earlier refill stop. It does not oblige you to empty every bottle. A much higher estimate on a hotter day may explain why the old carrying plan felt inadequate; it does not prove that fitness deteriorated. Review the plan against comfort and actual route access.

Who needs it, cost and repeat timing

Most useful before hot or logistically difficult ultras; optional for short easy runs with water available. Low cost: scale, bottle and notes. Repeat when climate, effort or equipment changes; there is no evidence-based universal retest interval.

Use an ordinary safe session, not deliberate fluid restriction or a heat challenge. Stop the measurement if you become unwell; completing the data sheet is never the priority. People with prescribed fluid restrictions or conditions affecting fluid balance should discuss testing with their clinician. Retain the date and circumstances of each result so an old cool-weather estimate is not silently reused for a hot race.