Sweat Rate & Workout Hydration Planner
Turn one test session into a simple drink plan
Sweat rate, hydration targets and common questions
What is “sweat rate” in this planner?
Sweat rate here is an estimate of how much fluid you lose through sweat per hour in a specific set of conditions. It uses your body weight change plus what you drank, divided by the length of the workout, to get a ballpark litres-per-hour number.
Why weigh before and after a workout?
For steady sessions, most of the short-term weight change is fluid. If your weight drops and you also drank during the workout, you can back-calculate roughly how much fluid left your body as sweat. It is not perfect, but it is a practical field method many athletes use.
Why doesn’t this tool include urine volume?
Some lab-style calculations also track urine. To keep the form simple and easier to repeat, this planner assumes you empty your bladder before weighing and does not add a urine box. If you are working with a sports scientist, they may use a more detailed version.
Should I drink exactly my sweat rate every hour?
Not usually. Many guidelines suggest aiming for a fraction of sweat rate so you avoid both large fluid deficits and over-drinking. That is why the planner shows a range instead of a single exact target. Your clinician or coach may give you different rules.
Can I use this for illness, heat stroke or very long races?
No. Serious illness, heat illness and extreme events need individual medical plans. This page is for everyday training in generally healthy adults, not for treating dehydration or managing medical emergencies.
How often should I repeat a sweat test?
Sweat rate can change with temperature, humidity, pace, clothing and fitness. Many people test a few typical scenarios: warm easy runs, very hot sessions, indoor intervals, etc. That way you can build a few drink ranges instead of one single number.
What if my weight goes up after the session?
Gaining weight despite sweating usually means you drank more fluid than you lost. In that case this simple method is less useful, and it may be a sign to talk with a coach or clinician about how much you drink during longer or hotter workouts.
How to use this sweat rate and hydration planner
This tool turns one weighed workout into a rough sweat rate and drink range you can reuse on similar days. You enter your pre- and post-workout weight, how much you drank and how long you trained, and the calculator estimates litres per hour plus a simple target range.
1. Choose units and do a weighed workout
Start by picking US (lb, fl oz) or metric (kg, mL). Then, for a typical session in the conditions you care about, weigh yourself right before and soon after the workout, ideally with dry clothes and after using the bathroom.
2. Enter pre- and post-workout weight
Put your before and after weights into the boxes. The planner checks that the numbers are in a typical adult range and that weight did not jump up in a way that makes the estimate meaningless. If something looks off, it asks you to adjust and try again.
3. Add your drinks and session time
In the drinks box, enter the total fluid you consumed during the workout in fl oz or mL. Then add the active session length in minutes. That lets the tool convert everything to litres and litres-per-hour behind the scenes.
4. Read sweat rate and a drink range
When you tap Estimate sweat rate and drink range, the calculator shows:
- Your pre and post weight in the chosen unit.
- Estimated sweat loss in litres for that workout.
- Estimated sweat rate in L/h and mL/h.
- A suggested drink range per hour (fraction of sweat rate).
The aim is not precision; it is to give you a number you can actually use when filling bottles.
5. Tag the test and copy the summary
The note box lets you label the scenario, such as “hot tempo run” or “indoor bike, cool room”. That note appears in the copyable summary so you can paste the result into a training log, spreadsheet or message to a coach or clinician.
Remember that this is a planning helper, not a safety guarantee. Heat, altitude, illness, medications and clothing can all change what you actually need to drink. Follow any rules you have been given by health professionals or event organisers ahead of numbers from this page.
How the sweat rate and hydration math works
The math here is a standard field method used by many endurance athletes and coaches. It turns changes in body mass and recorded drink volume into a simple litres-per-hour number.
1. Convert weight change into litres
First, the tool converts your weights into kilograms and works out the change:
Body mass change (kg) = Pre weight − Post weight
It then assumes 1 kg ≈ 1 L of fluid for this timeframe, so a 0.8 kg drop is treated as about 0.8 L of fluid lost.
2. Add the fluid you drank
Your drinks during the session are converted to litres and added to the mass-based estimate:
Sweat loss (L) ≈ Mass change (L) + Drink volume (L)
This assumes that most of the fluid you drank either replaced sweat or contributed to the weight change.
3. Divide by time to get sweat rate
Workout duration in minutes is turned into hours, then the tool calculates:
Sweat rate (L/h) = Sweat loss (L) ÷ Time (h)
It also shows the same number as mL per hour for easier bottle planning.
4. Suggest a drink range
Instead of telling you to match sweat rate exactly, the planner suggests a range based on a fraction of sweat rate, for example:
Drink range ≈ 0.6–0.8 × Sweat rate
That idea reflects guidance not to over-drink during exercise, especially over long events, while still chipping away at fluid losses.
Real-life hydration needs depend on medical conditions, medicines, climate, clothing and pace. Use these numbers as a starting point, then adjust with your coach or clinician if you have specific health considerations.
References and further reading on sweat rate and hydration
These resources discuss sweat rate, fluid replacement during exercise and practical hydration guidance:
- American College of Sports Medicine — Exercise and fluid replacement (position stand) — classic position stand outlining practical guidance on fluid replacement before, during and after exercise, including example drinking rates per hour.
- German Journal of Sports Medicine — Fluid replacement in sports (position paper) — reviews sports drink composition, sweat losses and suggests typical fluid intake ranges (e.g. around 0.4–0.8 L/h) during intense endurance activity.
- British Nutrition Foundation — Sport and exercise — explains how to manage hydration around training and events, including rehydration advice based on body weight changes after exercise.
Use these as general background and pair them with personalised advice from your healthcare or sports medicine team, especially if you have health conditions or are training for very long or hot events.