Calculator

Electrolyte Needs Calculator

Sodium, potassium, and magnesium targets built from your climate, sweat losses, training, and diet.

Guessing is fine to start, but the sweat rate test below takes one workout and replaces the guess with a measurement.

Sweat sodium concentration varies four-fold between people and is largely genetic. Visible salt residue is the most reliable field indicator of a salty sweater.

Measure your real sweat rate

One workout replaces every guess above. Weigh yourself nude immediately before and immediately after a training session, towel-dried both times, and track what you drank during it.

How this works

Electrolyte requirements are the sum of two things: a baseline your kidneys need to run normal physiology, and replacement for whatever you lose. For most people in temperate climates the baseline dominates and no calculation is necessary. For anyone training hard, living in real heat, eating low carbohydrate, or fasting, losses dominate and the standard population guidance no longer describes the situation.

Sodium

The adequate intake for sodium is 1,500 mg/day and the chronic disease risk reduction intake is 2,300 mg/day, both set by the National Academies for a sedentary population. Average US intake is around 3,400 mg/day, which is why public health messaging is uniformly about reduction.

Sweat changes the arithmetic. Sweat sodium concentration ranges from roughly 460 to 1,840 mg per liter, a four-fold spread that is mostly genetic and only modestly trainable. At 1.2 L/hour, a common rate for hard training in heat, a salty sweater loses over 2,000 mg of sodium in a single session, more than an entire day's baseline intake. Failing to replace that produces the familiar picture: cramping late in sessions, persistent fatigue, lightheadedness on standing, headaches, and a craving for salty food that people often override.

The calculator adds sweat sodium losses on top of baseline, then flags the total against safety thresholds. It does not recommend high sodium to anyone with hypertension, kidney disease, or heart failure, where the risk calculus is entirely different.

Potassium

The adequate intake is 3,400 mg/day for men and 2,600 mg/day for women. Fewer than 3 percent of US adults reach it. Sweat potassium is low, roughly 200 mg per liter, so training barely moves the requirement. The gap is dietary: potassium lives in plants, tubers, dairy, and fish, and disappears from diets built around refined grains and processed foods.

Potassium and sodium are best understood as a ratio rather than as two separate numbers. Blood pressure and cardiovascular outcomes track the sodium-to-potassium ratio more tightly than either alone. A ratio near or below 1.0 by weight is the target most epidemiology supports. Typical Western intake sits closer to 1.5 to 2.0.

Magnesium

The RDA is 400 to 420 mg/day for men and 310 to 320 mg/day for women, and roughly half of US adults fall short. Sweat magnesium losses are trivial, around 3 mg per liter, so exercise raises the requirement mainly through increased energy metabolism rather than direct loss. The calculator adds a modest training increment and a larger one for very high training volume.

Serum magnesium is a poor test: less than 1 percent of body magnesium is in serum, and the body defends that number by pulling from bone. A normal serum magnesium does not exclude depletion.

Why low carbohydrate diets need more sodium

Insulin promotes sodium reabsorption in the proximal tubule. When carbohydrate intake falls and insulin drops, that reabsorption falls with it and the kidneys excrete sodium and water. The first week of a ketogenic diet routinely involves several pounds of water loss driven by this mechanism, and the associated symptoms (headache, fatigue, lightheadedness, cramping, constipation, poor exercise tolerance) are what people call keto flu. Most of it is sodium depletion, and most of it resolves within a day of adding 2,000 to 3,000 mg of additional sodium. Extended fasting drives the same natriuresis harder.

Desert heat specifically

Dry heat is deceptive in a way humid heat is not. In Phoenix or Tucson at 8 percent humidity, sweat evaporates before it becomes visible, so you never feel wet and consistently underestimate how much you have lost. Sweat rates in dry heat are frequently higher than in humid heat because evaporative cooling is efficient enough that the body keeps producing sweat rather than shutting it down.

Practical consequences for Arizona and similar climates: hydration status must be managed across the whole day rather than around workouts, morning body weight is the most reliable daily check, thirst lags actual need particularly over 60, and air conditioning does not eliminate the load from a 15 minute walk across a parking lot at 112 degrees. Heat acclimatization over 10 to 14 days lowers sweat sodium concentration substantially, so a newcomer to the desert needs more sodium than a long-term resident doing identical work.

More is not better. Overdrinking plain water during long endurance events causes exercise-associated hyponatremia, which has killed marathon runners. The mechanism is diluting blood sodium by taking in more fluid than you lose. Drinking to thirst, rather than to a schedule, prevents nearly all cases. If you are gaining weight during a long event, you are drinking too much.
Potassium supplements deserve real caution. Potassium is the electrolyte most capable of causing acute harm. Combined with an ACE inhibitor, ARB, or potassium-sparing diuretic such as spironolactone, or in the setting of reduced kidney function, supplemental potassium can produce dangerous hyperkalemia. US supplements are capped at 99 mg per dose for this reason. Get potassium from food unless a clinician has specifically told you otherwise.

Related calculators

Hydration Calculator sets the fluid volume these electrolytes go into.
Supplement Timing Optimizer places magnesium and other minerals across your day.
TDEE Calculator for the energy side of the same training load.

References

National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Sodium and Potassium. National Academies Press; 2019.

Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Med. 2017;47(Suppl 1):111-128.

McCubbin AJ, Allanson BA, Caldwell Odgers JN, et al. Sports Dietitians Australia position statement: nutrition for exercise in hot environments. Int J Sport Nutr Exerc Metab. 2020;30(1):83-98.

Hooper L, Martin N, Jimoh OF, et al. Reduction in saturated fat intake and sodium: umbrella considerations in cardiovascular risk. Cochrane Database Syst Rev. 2020;(8):CD011737.

Filippini T, Naska A, Kasdagli MI, et al. Potassium intake and blood pressure: a dose-response meta-analysis of randomized controlled trials. J Am Heart Assoc. 2020;9(12):e015719.

Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clin J Sport Med. 2015;25(4):303-320.