Estimate sweat rate from a recorded session

Use measurements from one session to estimate sweat loss and hourly sweat rate. Select metric (kg and litres) or US (pounds and US fluid ounces) before entering values. Record all drinks and, where applicable, urine during the same measured period.

This is a measurement worksheet, not a prediction of dehydration risk. It does not prescribe how much to drink, sodium intake or a post-run replacement schedule.

Measurement method and limits

The equation follows Table 2 of the NATA fluid-replacement position statement:

Estimated sweat loss (L) = body mass before (kg) − body mass after (kg) + fluid consumed (L) − urine (L).

Estimated sweat rate (L/hour) = estimated sweat loss ÷ duration in hours.

This field estimate treats a kilogram of the relevant mass change as approximately a litre of water. Use consistent measurement conditions; wet clothing, scale error, unrecorded intake and other mass changes affect the estimate. The NATA statement recommends repeated assessments in relevant conditions rather than assuming one rate applies to everyone or every run. Both insufficient and excessive fluid intake matter; the result is not itself a drinking prescription.

Record the inputs

Input	What to record	Common entry error
Before and after body mass	Consistent measurements around the same session	Mixing kg and pounds
Drinks	Volume consumed between measurements	Entering millilitres into a litres field
Duration	Minutes in the recorded session	Entering hours as minutes
Urine	Measured volume during that period, if any	Treating an unknown amount as confidently zero

Blank urine is visibly labeled assumed zero in the output. If you did pass urine but do not know its volume, the result has an unresolved input and should not be treated as a precise measurement.

Worked example

An illustrative record contains 70.0 kg before, 69.5 kg after, 0.5 L consumed, 0.1 L urine and 60 minutes:

Estimated loss: 70.0 − 69.5 + 0.5 − 0.1 = 0.90 L.
Duration: 60 ÷ 60 = 1 hour.
Estimated rate: 0.90 ÷ 1 = 0.90 L/hour.
Net body-mass change: 69.5 − 70.0 = −0.50 kg.

Net mass change and sweat loss are different because the record includes intake and urine. The example is arithmetic, not a target sweat or drinking rate.

Unit conversions

The calculator converts US inputs internally using 1 lb = 0.45359237 kg and 1 US fl oz = 0.0295735295625 L. A fluid ounce measures volume; it is not treated as an ounce of body mass. Results show both litres and US fluid ounces after calculation. Round measurement entries only to the precision your equipment supports.

When the record needs checking

A negative estimated sweat loss means these inputs cannot support the calculation as entered. Check the measurements instead of replacing the result with a reassuring risk grade. A body-mass gain can coexist with positive estimated sweat loss when intake is included; the tool reports that gain explicitly and does not use it to recommend more drinking.

The form supports sessions of 1–1440 minutes and calculated rates up to 10 L/hour. These are tool limits, not definitions of normal physiology or safety. An out-of-range result prompts an input check, not a diagnosis.

Repeat-measurement log

Date	Conditions / effort	Before / after and units	Drinks / urine and units	Minutes	Estimate	Measurement uncertainties
Your session
Comparable session
Different conditions

Keep the conditions with the number. Copy or print the worksheet for a discussion about individual hydration planning; this page does not save the entries to your account. This measurement task is separate from choosing an intake strategy, and no automatic recommendation is inferred from its output.

For organizing the surrounding decisions, use the hydration planning guide. If you already have a chosen volume, the separate carrying and refill planner calculates logistics without selecting your intake.

Contents

Measured Sweat Rate Calculator