Staying well-hydrated during exercise protects performance, safety and recovery. Even a 1–2% drop in body weight from fluid loss starts to impair aerobic capacity and concentration; by 3% you will notice significant fatigue and coordination problems; beyond 5% heat illness becomes a real risk. Yet drinking too much carries its own danger — exercise-associated hyponatraemia (EAH), caused by over-drinking plain water, has led to race fatalities. The answer is a personalised fluid plan based on your body weight, session length, exercise intensity and the temperature you are training in.
How the calculation works
The calculator uses the framework from the American College of Sports Medicine (ACSM) 2017 Position Stand on exercise and fluid replacement, combined with updated guidance from Casa et al. (2019) for the National Athletic Trainers’ Association.
Step 1 — Predict your sweat rate:
Sweat rate (L/h) = 0.75 × intensity factor × heat factor × (body weight kg / 70)
The baseline of 0.75 L/h represents a 70 kg person exercising at moderate intensity in mild conditions (15–24 °C). Intensity factors range from 0.55 (gentle yoga or walking) to 2.1 (elite racing or maximal-effort training). Heat factors range from 0.7 (below 15 °C) to 1.75 (above 32 °C). Body size scales the result linearly — a 90 kg athlete loses more fluid per hour than a 55 kg one at the same pace.
Step 2 — Calculate total sweat loss:
Total sweat (mL) = sweat rate (L/h) × duration (h) × 1000
Step 3 — Target 80 % replacement during activity:
The ACSM advises against trying to replace 100 % of losses during exercise, because that would require drinking at a rate that often exceeds gastric emptying capacity and risks EAH. The target during the session is roughly 80 %; the remaining 20 % is consumed post-exercise.
Step 4 — Sodium:
Sodium to replace (mg) = total sweat (L) × 920 mg/L
The 920 mg/L figure is the mean sweat sodium concentration from large population studies. Individuals range from about 460 to 1,840 mg/L — people who notice a white salt residue on their skin or kit tend to be “salty sweaters” at the high end.
Step 5 — Carbohydrates (sessions over 60 minutes):
The liver holds roughly 90–100 g of glycogen — enough for about 60–75 minutes of moderate exercise. Beyond that, exogenous carbohydrate at 30–60 g/h (depending on intensity) sustains performance. A 6–8 % carbohydrate-electrolyte drink conveniently supplies both fluid and fuel.
Worked example
A 75 kg runner completes a 90-minute run at high intensity on a warm day (28 °C):
- Sweat rate: 0.75 × 1.55 × 1.35 × (75/70) = 1.69 L/h
- Total sweat: 1.69 × 1.5 h = 2,535 mL
- During-run fluid target (80 %): 2,028 mL over 90 min
- Drink schedule: 338 mL every 15 minutes (6 drinks)
- Sodium to replace: 2,333 mg — needs an electrolyte drink, not plain water
- Carbohydrate target: 45 g/h × 1.5 h = 68 g (2 gels plus an isotonic drink)
- Post-exercise top-up: 507 mL
- Pre-exercise: 500 mL 2–4 h before, then 250 mL 15 min before the start
That runner would weigh their water bottle and use two 500 mL isotonic drinks (each providing ~30–35 g carbs and ~500 mg sodium) plus an additional 500 mL plain water during the run.
Tips for accurate personalisation
Use the optional scale-weight section to measure your actual sweat rate. Weigh yourself with minimal clothing (ideally naked) immediately before and after a session in which you do not drink. Each kilogram of weight lost equals roughly 1 litre of sweat. Repeat this in different conditions (cool vs warm, easy vs hard) and you will build a personal sweat-rate profile that no formula can match.