What Dehydration Does to Your Training
Start a session at 80.0 kg, finish at 79.1, drink half a litre along the way, pass no urine, and your sweat rate for that hour was 1.4 litres. Pass 0.2 litres and it drops to 1.2. That figure belongs to you, it costs one hour and a set of scales, and it beats every generic hydration rule you will ever read.
Working out your sweat rate
For a session with drinks only, take your pre-exercise weight in kg, subtract your post-exercise weight, add the litres you drank, and subtract the litres you passed. Divide that by the session length in hours. Weigh yourself the same way both times, and allow for wet clothing.
The answer is approximate. Food, breathing losses, fuel use, scale precision and clothing all shift the sum, and a change in body mass covers slightly more ground than sweat alone. Repeat it in the conditions you train in, and treat one session as one session.
What moves the number
Duration, intensity, temperature, clothing, acclimatisation and where your hydration started all matter. The NATA position statement on fluid replacement covers inadequate replacement and excessive intake both. Research has not landed on a single percentage of body-weight loss that is harmless across every activity, so treat “2%” as a loose landmark that moves with the activity. One small trial with one null result does not close the question either.
The water-intake calculator works from a fixed exercise allowance. It handles everyday intake, and your sweat rate and your race-day drinking plan both sit outside what it does.
Use the estimate as context
Knowing you lose 1.4 litres an hour tells you the size of the hole. How much of it you fill, and when, is a separate decision, and it turns on the event, the chances you get to drink, how long you have to recover and your medical needs. Drinking past your losses to hit a number on a spreadsheet helps nobody.
Sodium matters in prolonged, sweaty activity, and it does not protect you from drinking too much. Read the electrolyte guide before you treat a sports drink as insurance.
Symptoms outrank the calculator
Thirst, urine changes and weight trends are clues, and vitamins, medication and illness all interfere with them. A symptom that worries you is worth acting on even when a urine colour chart looks fine.
Confusion, collapse, seizures or severe illness during exercise calls for urgent medical help. The CDC heat-illness guidance explains the warning signs. Move the person out of the heat and get emergency assistance, and do not force drinks into anyone who is confused or cannot swallow safely.
For everyday intake assumptions, see the daily water guide. For short-term changes on the scale, see weight fluctuations.
Scope and correction note, 13 September 2026: This guide provides general adult education. It has been revised to distinguish estimates from measurements and to remove unsupported guarantees. It has not received independent clinical review. See the editorial policy and medical disclaimer.