Hydration for Training: Build a Session Record

Updated 17 October 2026 · Reviewed by Tools by Layna Editorial Team · General information only

A daily water estimate cannot describe a particular run, ride or gym session. Useful session planning starts with what happened during similar exercise: duration, weather, drink access, comfort and approximate fluid losses.

Record a comparable session

Choose ordinary training rather than a race-day experiment. Note the activity, duration, temperature, clothing and intensity. Measure body mass before and after using the same scales and dry, minimal clothing; wet kit can distort the comparison. Record drinks consumed and any urine passed during the interval. Food intake also needs accounting for where relevant.

This is a field estimate, not a hydration diagnosis. The Sports Dietitians Australia position statement discusses body-mass methods and their limitations, particularly during very long exercise.

Work through the fluid-loss arithmetic

For a simple one-hour session with no food or urine during the measurement period, imagine these observations:

Using approximately 1 kg per litre of water, estimated loss is 0.6 L from the body-mass change plus 0.4 L consumed: about 1.0 L over the hour. The finishing body-mass difference is 0.6 ÷ 70 × 100 = about 0.9%. These calculations describe this session. They do not establish that 1.0 L per hour must be drunk next time.

Test a practical plan, not exact replacement

Use the record to discuss realistic drinking opportunities and tolerance. A bottle that is inaccessible for half the session is not a workable plan. Repeat the observation under similar conditions before treating one estimate as typical. A cooler, easier workout is a separate comparison from a hot, hard one.

Do not assume that fully replacing estimated losses during exercise is always necessary or comfortable. Avoid finishing heavier because a fixed target encouraged excessive drinking. For endurance events, limited access to drinks or large losses, seek an individual plan from a sports dietitian.

Keep fuel and sodium decisions separate

A sports drink supplies carbohydrate and electrolytes as well as fluid. Decide whether those additions suit the session rather than counting every drink as interchangeable water. An estimated sweat rate does not reveal the sodium concentration of sweat. The electrolyte label guide explains how to calculate what a product actually supplies.

Know when this method is too rough

Long events, eating during exercise, toilet stops and wet clothing complicate body-mass estimates. Illness and medical fluid restrictions need different assessment. The daily water calculator has no fields for these session measurements, so do not expect its activity preset to reproduce this calculation.