Electrolyte
calculator

How much sodium you sweat out in a training session, plus the daily sodium and potassium baseline — from the National Academies 2019 intakes and Baker's sweat-sodium data. It shows a range, not a false-precise single number, and keeps the daily ceiling separate from the loss you replace.

How much sodium and potassium you actually need

Two different questions hide inside "electrolytes," and merging them is where most calculators go wrong. The daily baseline is a resting target: sodium's adequate intake is 1,500 mg with a ceiling to stay under at 2,300 mg, and potassium sits at 3,400 mg for men and 2,600 mg for women (National Academies, 2019). The exercise loss is separate: a debt you refill. It is the sweat you lose times the sodium in that sweat — for a one-hour session at a moderate 1.0 L/h, about 825 mg of sodium, though anywhere from 418 to 1,628 mg depending on how salty a sweater you are (Baker et al. 2016).

Most electrolyte tools print one sodium-per-hour figure with no source and treat sweat as a fixed recipe. It isn't. Sweat sodium concentration varies roughly four-fold between people, so the useful answer is a band — and the resting sodium number is a limit, not something to top up toward with tablets on a rest day.

How the math works

The daily baseline is read straight from the National Academies 2019 Dietary Reference Intakes. The session sodium loss is the sweat volume — duration times sweat rate — multiplied by the sodium concentration of sweat, converted from millimoles to milligrams with sodium's molar mass.

The concentration line is the one generic tools hard-code. At the mean, 35.9 mmol/L works out to 825 mg of sodium per litre of sweat; at the low end (18.2 mmol/L) it's 418 mg, and at the high end (70.8 mmol/L) it's 1,628 mg. Same litre of sweat, four-fold difference in sodium — which is why a single number is a guess.

Measure your sweat volume, don't guess it

The sweat presets are a starting point; the real volume is easy to measure, and it beats any formula. One litre of sweat is one kilogram of body mass, so weigh in before and after a session:

An athlete who drops from 70.0 kg (154 lb) to 69.0 kg (152 lb) over an hour, having drunk 0.5 L, sweated 1.5 L. Feed that volume to the sodium math instead of a preset, and the estimate is yours rather than the population's.

Worked example

A man doing a 60-minute moderate session (about 1.0 L/h of sweat):

• Sweat volume: 60 min × 1.0 L/h = 1.0 L (33.8 fl oz)
• Sweat sodium (mean): 35.9 mmol/L × 22.99 = 825 mg/L
• Sodium to replace: 1.0 L × 825 mg/L = 825 mg (band 418–1,628 mg)
• Daily baseline (male): sodium AI 1,500 mg, ceiling 2,300 mg, potassium 3,400 mg

Now stretch it to a 3-hour hot session at a heavy 1.5 L/h. Sweat volume is 4.5 L (152.2 fl oz), so the mean sodium loss is 4.5 × 825 = 3,714 mg (band 1,883–7,325 mg). That one session sheds more sodium than the whole day's 2,300 mg ceiling — so the honest instruction that day is to replace what you sweat, not cut salt.

For contrast, a woman on a rest day: no session to replace, and the numbers are a daily target — potassium 2,600 mg, sodium under the 2,300 mg ceiling. Nothing to top up.

When this calculator is wrong

A single "sodium per hour" number is false precision. Whole-body sweat sodium concentration varies about four-fold between people — Baker et al. (2016) measured 18.2 to 70.8 mmol/L (mean 35.9) across 506 athletes — so any tool that prints one exact figure is guessing the input that matters most. The honest output is a band. And the two sodium numbers point opposite ways: the resting DRI is a ceiling, not a floor — NASEM (2019) set a Chronic Disease Risk Reduction intake of 2,300 mg/day, the level to stay under — while the exercise figure is a loss to refill. Loading electrolyte tablets onto an already-salty rest-day diet optimises the wrong direction.

• The counter-case is the long, hot session. There, under-replacing sodium is the real risk and the numbers flip: a 3-hour effort at a heavy 1.5 L/h sheds roughly 3,714 mg at the mean — past the day's 2,300 mg ceiling — and replacing that loss with plain water is what drives exercise-associated hyponatremia (Sawka et al. 2007). On those efforts the high end of the band and a real sweat test earn their keep.
• Potassium isn't a sweat problem. Sodium is the electrolyte lost in quantity; potassium losses in sweat are modest, and the daily AI from food covers them (Thomas, Erdman & Burke 2016). This page treats potassium as a daily target, not a per-session replacement.
• Sweat sodium isn't fixed by sport or supplement. It's an individual trait that shifts a little with heat acclimatisation and diet but stays roughly four-fold spread across people. If you sweat visibly salty (white marks, stinging eyes), assume the high end until a sweat test says otherwise.

What to do with the result

On sessions under about an hour, a normal diet covers the sodium you lose — the replacement number matters most past that, and on hot or heavy-sweat efforts. Take the mean as a first pass and adjust from what you notice: cramping, salt crust on your kit, or feeling wrung out after long sessions all argue for the high end. The one number worth nailing is your sweat volume on the efforts that count — weigh in before and after once or twice and use that, not a preset.

On rest days, treat the sodium figures as a ceiling and leave the tablets in the drawer; there's no session to replace. Pair this with the water intake calculator for the fluid side of the same session — sodium without enough fluid, or fluid without enough sodium, both miss — and if you're building a full plan, the macro calculator handles the food that supplies most of your potassium.

Common questions

How much sodium do I need a day?

For a healthy adult, the adequate intake is 1,500 mg/day and the level to stay under is 2,300 mg/day — the Chronic Disease Risk Reduction intake (National Academies, 2019). It's framed as a ceiling, not a floor. Exercise sodium losses are separate: those are a debt you replace, not part of this daily target.

How much sodium do you lose in sweat?

Sweat volume times the sodium in your sweat. At the whole-body mean of 35.9 mmol/L that's about 825 mg per litre, but the population range is 418 to 1,628 mg per litre (Baker et al. 2016). A one-hour session at 1.0 L/h loses roughly 825 mg at the mean — anywhere from 418 to 1,628 mg depending on how salty you sweat.

Do I need electrolytes for a one-hour workout?

Usually not beyond a normal diet. Sodium replacement earns its place on longer, hotter, heavy-sweat sessions — where losses stack toward and past the daily ceiling — and for people who sweat visibly salty. For a short session, food covers it.

Sodium or potassium — which matters more for endurance?

Sodium. It's the electrolyte lost in quantity in sweat and the one whose dilution by over-drinking causes hyponatremia. Potassium losses in sweat are modest and the daily intake from food covers them (Thomas, Erdman & Burke 2016), which is why this calculator replaces sodium per session and treats potassium as a daily target.

Can you get too many electrolytes?

On rest days, yes in the sense that more sodium isn't better — the DRI is a ceiling (2,300 mg/day), and tablets on top of a salty diet add nothing useful. During long exercise the risk runs the other way: under-replacing sodium while drinking plain water is what causes exercise-associated hyponatremia (Sawka et al. 2007). Match the context.

How do I know my own sweat sodium concentration?

It varies about four-fold between people and isn't set by your sport. A lab or patch sweat test measures it directly; short of that, use the band this page shows and lean high if you sweat visibly salty, then calibrate from how you feel and perform on long sessions.