Hydration & Sodium Calculator — English Endurance

English Endurance

Hydration & Sodium Calculator

Set an hourly fluid and sodium target from a measured sweat rate. Intake is bounded by what the gut can absorb, and sodium is coupled to the fraction of fluid actually replaced — so the drink concentration follows the plan rather than a fixed rule of thumb.

Body mass70 kg
40 kg110 kg
Session duration4.0 h
0.5 h30 h
Measured sweat rate1.20 L/h
0.3 L/hWeigh in, weigh out
Sweat sodium920 mg/L
10 mmol/LSalty sweater above 60
Gut ceiling1.00 L/h
0.4 L/hWhat you absorb
Planned fluid target90%
50%Of sweat rate
Compute sweat rate from a weigh-in / weigh-out session
Pre-session mass (kg)
Post-session mass (kg)
Fluid drunk during (L)
Urine passed (L)
Session length (h)
Fill all five fields to compute and load a sweat rate.

Duration regime

Under 6 hmatch concentration
6–12 hscale together
Over 12 hfood + kidney
Fluid—litres per hour
Sodium—mg per hour
Bottle concentration—mg per litre
Sweat rate—litres per hour
Fluid fraction—of sweat replaced
Body-mass loss—projected

Accumulating deficit

Balance sheet

StreamFluidSodium

Athlete card

Drink / hour—millilitres
Bottles / hour—750 mL each
Sodium / hour—milligrams
Total fluid—litres
Total sodium—grams
    Key assumptions & parameters
    • What changed from the flat model. The earlier drink = 0.75 × sweat rate conflated the gut ceiling, which is a cap at high sweat rates, with the 70% sodium tipping point, which is where sodium starts to matter — not a ceiling on fluid. This tool targets min(gut ceiling, target × sweat rate), so a deficit appears only when sweat genuinely outruns the gut.
    • Fuller replacement buys fuelling, not watts. On long moderate rides the benefit is plasma volume, thermoregulation, gastric emptying and fuel absorption, and lower post-session rehydration debt. Performance data for full versus partial replacement in moderate conditions are close to null.
    • Sweat rate. SR = (pre − post + drink − urine) ÷ hours. Predicted fallback: running heat (kW) ≈ 0.00116 × mass × speed, cycling heat (kW) ≈ 0.0033 × power, then SR ≈ k × heat with k ≈ 1.0 cool, 1.2 warm, 1.4–1.6 hot. Latent heat of sweat ≈ 2,426 J/g.
    • Sodium loss. mg/h = SR × sweat [Na] × 23. One mmol of sodium is 23 mg; multiply mg of sodium by 2.54 for mg of table salt.
    • Sodium is prescribed as a concentration first. Required drink concentration rises monotonically with the achieved fluid fraction f — 50% of sweat [Na] at f of 0.50 or below, rising to 100% at f = 1.00. Hourly intake is that concentration multiplied by the volume actually drunk. This reproduces the intended replacement fractions (R = 0.50, 0.65, 0.85, 1.00 at f = 0.70, 0.80, 0.90, 1.00) without the non-monotonic artefact a fraction-first formulation produces.
    • At full replacement the bottle equals sweat concentration. That is the Wijering result falling out of the arithmetic rather than being asserted.
    • Below ~70% replacement, plasma sodium rises on its own because sweat is hypotonic. Sodium there is for palatability, thirst drive and retention, not hyponatremia prevention.
    • Why sodium peaks and then falls. Hourly sodium rises with sweat rate only while the gut is not the limiter. Once intake is capped, drinking more is impossible, the replaced fraction falls, plasma sodium starts rising on its own, and the drink needs less sodium per litre. Since volume is fixed, hourly sodium falls too. The peak sits where the gut ceiling first binds. That concerns plasma sodium, not the sodium deficit, which keeps growing — address that with food afterwards.
    • Long-duration dilution. Past 12 h, respiratory loss ≈ 0.0625 L/h carrying zero sodium and conserving renal loss ≈ 0.083 L/h at about 30 mmol/L both pull the average required concentration below sweat [Na], so the tool stops over-prescribing at ultra duration.
    • Sweat sodium is largely unpredictable. Population mean 35–40 mmol/L, range 10–70, salty sweater above 60. Around 80% of between-athlete variance is unexplained, so it cannot be inferred from thirst, sweat rate or hydration status. Correct a forearm patch reading with 0.57 × forearm + 11.05.
    • Flagged estimates, not measured constants. The short-duration damping on sodium (0.65 at 2 h or less, rising to 1.0 by 4 h) is a synthesised judgement, not a published coefficient — its rationale is that dilutional risk is cumulative and Wijering's fall in plasma sodium developed over 3 h of full replacement. Metabolic-heat-to-sweat coefficients are first-principles derivations. The gut ceiling of 1.0–1.2 L/h and the 2% performance threshold are synthesised consensus round numbers. Respiratory and renal defaults come from a worked 24 h budget, not from any individual athlete. Individual variability dwarfs every group mean here.

    Sodium conversion

    Sweat [Na]mg Na / Lmg salt / L
    20 mmol/L4601,170
    30 mmol/L6901,755
    40 mmol/L9202,340
    50 mmol/L1,1502,925
    60 mmol/L1,3803,510
    70 mmol/L1,6104,095
    80 mmol/L1,8404,680

    References

    • McCubbin AJ. Sodium intake for athletes before, during and after exercise: review and recommendations. Performance Nutrition 2025;1:11. doi:10.1186/s44410-025-00011-9
    • Wijering LAJ, Cotter JD, Rehrer NJ. Beverage sodium concentration, plasma sodium and plasma volume during prolonged exercise in the heat. Eur J Appl Physiol 2023;123:81–89. doi:10.1007/s00421-022-05025-y
    • Gagnon D, Jay O, Kenny GP. Evaporative requirement for heat balance determines whole-body sweat rate. J Physiol 2013;591(11):2925–35. PMID 23459754
    • Montain SJ, Coyle EF. Influence of graded dehydration on hyperthermia and cardiovascular drift. J Appl Physiol 1992;73(4):1340–50. PMID 1447078
    • Sawka MN, et al. ACSM Position Stand: Exercise and Fluid Replacement. Med Sci Sports Exerc 2007;39(2):377–90. PMID 17277604
    • McDermott BP, et al. NATA Position Statement: Fluid Replacement for the Physically Active. J Athl Train 2017;52(9):877–95. PMID 28985128
    • Hew-Butler T, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Conference. Clin J Sport Med 2015;25(4):303–20. PMID 26102445
    • Baker LB. Sweating rate and sweat sodium concentration in athletes. Sports Med 2017;47(Suppl 1):111–28. PMID 28332116
    • Baker LB, et al. Normative data for regional sweat sodium and whole-body sweating rate. J Sports Sci 2016;34(4):358–68. PMID 26070030
    • Goulet EDB, Hoffman MD. Ad libitum versus programmed drinking and endurance performance: meta-analysis. Sports Med 2019;49(2):221–32. PMID 30659500
    • Hoffman MD, Hew-Butler T, Stuempfle KJ. Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Med Sci Sports Exerc 2013;45(4):784–91. PMID 23135369
    • Mitchell JB, Voss KW. Influence of volume on gastric emptying and fluid balance during exercise. Med Sci Sports Exerc 1991;23(3):314–19. PMID 2020269
    • Rehrer NJ. Fluid and electrolyte balance in ultra-endurance sport. Sports Med 2001;31(10):701–15. PMID 11547892
    • Miller KC, et al. An evidence-based review of exercise-associated muscle cramps. J Athl Train 2022;57(1):5–15. PMID 34185846

    Disclaimer. Educational tool for coaching use, not medical advice. Every figure is a starting estimate to be replaced by personal measurement.