Health & Medicine · Dietetics · Hydration & Sports Nutrition
Electrolyte Replenishment Calculator
Estimates sodium, potassium, and fluid replenishment needs based on body weight, sweat rate, exercise duration, and sweat sodium concentration.
Calculator
Formula
Sweat Rate is the volume of sweat produced per hour (L/h). Duration is the exercise time in hours. Sodium concentration in sweat typically ranges 230-1150 mg/L (mean ~920 mg/L). Potassium concentration in sweat is approximately 150-200 mg/L (mean ~195 mg/L). Target Fluid Intake includes a 1.5x multiplier to account for ongoing urine losses during recovery, per ACSM guidelines.
Source: American College of Sports Medicine (ACSM) Position Stand: Exercise and Fluid Replacement, Medicine & Science in Sports & Exercise, 2007; Casa et al., NATA Position Statement 2000.
How it works
The calculator uses your sweat rate (L/h) multiplied by exercise duration to determine total fluid lost as sweat. Electrolyte losses are then calculated by multiplying this fluid volume by the concentration of sodium and potassium in your sweat. Sweat sodium typically ranges from 230 to 1,150 mg/L, with a commonly cited mean of around 920 mg/L; sweat potassium averages roughly 195 mg/L. These values vary substantially between individuals and conditions, so entering your own measured values (from sweat patch testing) improves accuracy.
Target fluid intake for recovery is set at 1.5 times the fluid lost. This multiplier, recommended by the American College of Sports Medicine (ACSM), accounts for the continued urine production that occurs even when you are still in deficit — meaning simply replacing fluid volume 1:1 is insufficient to restore euhydration. A pre-exercise hydration status modifier further adjusts the target upward if you started the session already dehydrated.
These estimates are used by sports dietitians, exercise physiologists, and endurance athletes to design personalised sports drink formulations, choose appropriate electrolyte supplements, and plan race-day nutrition strategies. The body weight loss percentage output is a key clinical indicator: losses above 2% are associated with measurable impairment in aerobic performance, and losses above 3–4% increase heat illness risk.
Worked example
Example: A 75 kg runner completing a 2-hour moderate-intensity run in warm conditions.
Inputs: Body weight = 75 kg, Sweat rate = 1.4 L/h, Duration = 2 h, Sodium concentration = 920 mg/L, Potassium concentration = 195 mg/L, Pre-exercise hydration = well-hydrated (×1.0).
Step 1 — Fluid Loss: 1.4 L/h × 2 h = 2.8 L of sweat fluid lost.
Step 2 — Body Weight Loss %: (2.8 L / 75 kg) × 100 = 3.7% — this is above the 2% performance-impairment threshold, indicating significant dehydration.
Step 3 — Sodium Loss: 2.8 L × 920 mg/L = 2,576 mg of sodium (approximately 2.58 g), equivalent to about 6.5 g of table salt.
Step 4 — Potassium Loss: 2.8 L × 195 mg/L = 546 mg of potassium.
Step 5 — Target Fluid Intake: 2.8 L × 1,000 × 1.5 = 4,200 mL (4.2 L) to be consumed over the recovery period (typically within 2–4 hours post-exercise).
This athlete would benefit from a sodium-containing sports drink or salty post-exercise meal alongside their fluid intake to support effective rehydration and prevent hyponatraemia.
Limitations & notes
Sweat rate and sweat electrolyte concentration vary enormously between individuals, across environmental conditions (heat, humidity, altitude), and even between sessions for the same individual. The default sodium and potassium values used here are population averages; laboratory sweat patch testing (e.g., via Macroduct sweat collection system) is needed for precise personalisation. The 1.5x recovery fluid multiplier assumes normal kidney function and is not appropriate for individuals with renal disease, heart failure, or other conditions affecting fluid regulation. This calculator does not account for chloride, magnesium, or calcium losses, which may also be clinically relevant in ultra-endurance events. Medical supervision is strongly recommended for any athlete showing signs of hyponatraemia (nausea, headache, confusion) or severe dehydration.
Frequently asked questions
What is a normal sweat rate, and how do I measure mine?
Average sweat rates range from 0.5 to 2.5 L/h depending on body size, fitness, heat acclimatisation, and exercise intensity. You can estimate your own by weighing yourself (in minimal clothing) immediately before and after a one-hour exercise bout without drinking: each 1 kg of weight lost equals approximately 1 litre of sweat. For example, if you lose 1.3 kg in 60 minutes, your sweat rate is approximately 1.3 L/h.
Why is the target recovery fluid intake 1.5 times the sweat loss?
When you drink plain fluid to replace losses, your kidneys respond to the dilution of blood by excreting urine — even if you are still dehydrated. Research by Shirreffs et al. (1996) and the ACSM (2007) established that drinking approximately 1.5 L of fluid per 1 L of sweat lost is needed to achieve full rehydration over a 4–6 hour recovery window. Consuming sodium alongside fluid reduces urine output and improves retention.
How much sodium should I consume to replace my sweat losses?
Sodium replacement needs equal your total sodium loss in milligrams. For most recreational athletes this falls between 500 mg and 2,500 mg per hour of intense exercise in the heat. Many commercial sports drinks contain 400–700 mg of sodium per litre, which is often insufficient for heavy sweaters. High-sodium athletes (those who notice white salt crust on skin or clothing) may need dedicated sodium supplements such as salt capsules or electrolyte tablets in addition to sports drinks.
What is the risk of drinking too much water during exercise?
Drinking excessive plain water without electrolytes can cause exercise-associated hyponatraemia (EAH), a potentially life-threatening drop in blood sodium levels. EAH is most common in slower endurance athletes (e.g., marathon finishers, triathletes) who drink beyond thirst and do not replace sodium. The NATA and ACSM recommend drinking to thirst, consuming sodium during events lasting more than 2 hours, and never drinking more than sweat and urine losses combined.
Does potassium replacement matter as much as sodium after exercise?
Potassium is lost in much smaller amounts than sodium (typically 150–200 mg/L of sweat vs. 500–1,150 mg/L for sodium). For most exercise sessions under 2 hours, potassium losses are readily replaced by normal dietary intake from foods like bananas, potatoes, and dairy. However, in ultra-endurance events or where dietary intake is poor, targeted potassium replacement via sports drinks or food (a medium banana contains ~422 mg K) is advisable. Potassium is primarily an intracellular ion, and serum potassium can actually rise during very intense exercise due to cellular redistribution.
At what level of body weight loss does performance start to decline?
Research consistently shows that aerobic exercise performance begins to deteriorate at body mass deficits of approximately 2% (e.g., 1.4 kg loss in a 70 kg athlete). At 3–4% deficit, both physical and cognitive performance are significantly impaired, and heat illness risk increases substantially. At losses above 5–6%, risk of exertional heat stroke and clinical dehydration requiring medical intervention rises markedly. This calculator flags your estimated body weight loss percentage so you can assess where you fall on this continuum.
Do I need electrolyte drinks for short workouts?
For exercise sessions lasting less than 60 minutes at moderate intensity in temperate conditions, plain water is generally sufficient for most people. The ACSM advises that electrolyte replacement becomes important during exercise lasting longer than 1–2 hours, in hot or humid environments, or for individuals with particularly high sweat rates or salty sweat. For everyday gym sessions under an hour, a balanced meal with sodium post-workout is typically adequate without requiring specialised electrolyte products.
Last updated: 2025-01-30 · Formula verified against primary sources.