Electrolyte calculator

The baselines. These come from the NASEM (formerly IOM) Dietary Reference Intakes - the Adequate Intake (AI) and, for sodium, the Chronic Disease Risk Reduction (CDRR) limit:

              Sodium AI    Sodium CDRR limit   Potassium AI   Magnesium RDA
Men           ~1.5 g/day   2.3 g/day           3.4 g/day      400-420 mg/day
Women         ~1.5 g/day   2.3 g/day           2.6 g/day      310-320 mg/day

Why low-carb and fasting change sodium. Carbohydrate drives insulin, and insulin tells the kidneys to hold onto sodium. Cut carbs (low-carb, keto) or stop eating (fasting) and insulin falls, so the kidneys flush sodium and water - the classic first-week "keto flu" of headaches, cramps, and fatigue is largely sodium loss. Most low-carb and keto practitioners target roughly 3-5 g of sodium per day (about 7.5-12.5 g of salt) to offset it, well above the 1.5 g AI. Fasting removes dietary intake entirely, so deliberate supplementation matters even more.

Sweat losses. Sweat sodium concentration averages around 1 g of sodium per litre (roughly 40 mmol/L, though individuals range from ~0.5 to ~2 g/L). Light training that adds ~0.5-1 L of sweat a day costs on the order of 0.5-1 g of sodium; heavy or hot-weather training producing 2-3 L can cost 2-3 g or more. The calculator adds an estimate on top of your dietary baseline.

Potassium and magnesium. Potassium tracks the AI (3.4 g men, 2.6 g women) and most people fall short of it regardless of diet - cutting fruit and starchy veg on keto makes that worse, so the emphasis is on hitting the AI, not exceeding it. Magnesium follows the RDA (400-420 mg men, 310-320 mg women); low-carb and fasting states raise renal magnesium excretion, so we nudge the target toward the top of the range.

These are population estimates for healthy adults, not prescriptions. Sweat rate, climate, kidney function, and medications all shift the real numbers.