Pre-weight, post-weight, duration, what you drank. Out comes your hourly fluid loss, sodium-loss range (low / average / high), sweat-rate category, and how much body weight you actually shed. Run this on a representative session, then build your race-day fluid plan from real numbers.
Weigh yourself nude before and after the session, track exactly how much you drank during. Best run on a session that mimics race-day intensity, duration, and conditions, results vary 2× from a cool morning to a hot afternoon.
Five-minute protocol, if you skip the bits in bold the numbers are noise.
Empty bladder. Towel-dry sweat from the warm-up. Nude on a scale, recorded to 0.1 kg. Clothes hold sweat and skew the result.
Same heat, same effort, same gear. Pre-weigh the bottles, post-weigh, measure exactly how much went in. 1 mL = 1 g for water-based drinks, just weigh.
Right after stopping, before a shower, before a meal. Subtract bathroom breaks from duration if any (or just don’t go).
Run the test on at least 2–3 different sessions, one cool, one hot, one race-pace. Sweat rate doubles between cool and hot conditions, so a single number won’t cover race day if conditions change.
The 600 / 800 / 1200 mg/L numbers in the calculator bracket the realistic range, but where you sit on that range is genetic and can shift with heat acclimation. If multiple of these apply, anchor your race-day plan to the high column, not the average.
The calculator tells you the number; this tells you whether to worry about it.
Within normal noise. Slight performance impairment possible; bloating if you over-drank.
Most well-fueled training falls here. Thirst kicks in around 2%. Performance largely unaffected.
Older studies show clear performance decline; recent data are more mixed. Either way, you under-drank for the conditions.
Performance hit in most studies. Heat tolerance drops, cognition starts to slip. Rebuild fluid intake before the next session.
Significant performance and decision-making impairment. Real heat-illness risk, heat stroke, rhabdo. Stop, cool down, rehydrate slowly with sodium.
What to drink, when, with how much sodium.
500–750 mL of a sodium-containing drink with 300–700 mg sodium per liter. Gives the body time to absorb fluid + salt before the session loads up the gut.
Frequent small sips with sodium, promotes gastric emptying and helps carb absorption. Match intake to your measured sweat rate; don’t force-drink past 1 L/h unless your number says you need it.
Replace ~150% of fluid lost with sodium-containing drink (300–700 mg/L). Stimulates thirst, promotes retention, recovers electrolyte balance for the next session.
Reference: 1 tsp table salt ≈ 6 g ≈ 2 325 mg sodium (table salt is ~40% Na by weight). Salty foods that work: pretzels, pickles, olives, salted nuts, broth, vegetable juice.
For an individualized profile beyond the population averages this calculator uses, including a real measured sweat sodium concentration, book a hydration session at the lab.
Pair your sweat number with a carb plan, total g/h, food equivalents, DIY ratios.
Open → All-in-one converterConcentration tab tells you whether your DIY bottle is too dilute or too thick.
Open → All calculatorsBack to the full catalog, 30+ endurance calculators across performance, fueling, swim, bike, run.
Browse →The 600 / 800 / 1200 mg/L brackets cover most athletes, but some lose 1 600+ mg/L. A sweat patch test or in-lab hydration session measures your sodium concentration, so race-day fluid plans are anchored in real numbers.
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