The Athlete Hydration Calculator is a free online tool that works in your browser. There is nothing to install, no account to create, and no data sent anywhere. You enter a few numbers, press calculate, and get a clear result you can use in your next training session.
What it measures
Sweat losses during sport can reach a litre or more per hour, and even small dehydration hurts performance, focus, and heat tolerance. Yet many athletes drink by guesswork, either too little or far too much. This calculator builds a simple drinking plan from your body weight, session length, and sweat rate: fluid during the session, a pre-session drink, and a practical amount to sip every 15 minutes, so you can train and race with a plan instead of a guess.
How to use it
Fill in each field with your own numbers, then press Calculate. Use honest measurements: a guessed input gives a guessed result. If a field does not apply to you, use the closest value and note it, so your next test uses the same setup and the comparison stays fair. Results appear below the button at once, and you can change any input and recalculate as often as you like.
Who should use it
This tool suits athletes, coaches, and anyone who trains regularly and wants numbers to guide sessions. Beginners get a starting point and a first target. Experienced athletes get a consistent way to track change over a season. Coaches can test a whole group with the same method and compare results fairly, as long as everyone follows the same test conditions.
How to read your result
Fluid during the session is the headline figure: your sweat rate multiplied by session hours. The pre-session drink of half a litre tops up stores before you start. The 15-minute sip amount turns the total into action you can follow with a bottle in hand. If your sweat rate is a guess, start with 1 litre per hour in moderate heat, then refine it by weighing yourself before and after a session: each kilogram lost is roughly one litre to replace.
Worked example
Take a 75 kg athlete training for 90 minutes with a sweat rate of 1 litre per hour. The calculator plans 1.5 litres during the session, plus half a litre before, for 2 litres in total, and about 250 ml every 15 minutes. On a hotter day the same athlete might sweat 1.5 litres per hour, pushing the during-session figure to 2.25 litres, which shows why heat changes the plan more than weight does.
How to use it in training
Weigh yourself before and after a hot session once a month to learn your true sweat rate. Add electrolytes on sessions over 90 minutes or in heavy heat; plain water alone can dilute blood sodium. Start drinking early in the session rather than waiting for thirst, which already lags behind losses. Keep a simple log of each test date and result. Over eight to twelve weeks, the trend matters far more than any single day, because sleep, heat, and fatigue move daily numbers up and down.
Accuracy and limits
Every estimate on this page is educational, not medical advice. Field formulas trade lab precision for simplicity, so treat results as guides for training, not diagnoses. If you have a heart condition, feel unwell during exercise, or get a result that worries you, stop and talk to a qualified professional before training hard again. Consistent test conditions matter more than chasing a perfect number.
Frequently asked questions
How often should I retest? Every four to eight weeks is enough for most training blocks. Shorter gaps mostly show daily noise, not real change.
Do I need special equipment? Most of these tests need only a timer, a tape measure, or a basic heart rate monitor. A lab gives more precision, but a careful field test is enough to guide training.
Also available in Spanish: Calculadora de Hidratación para Deportistas.