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CalcChef

Hydration Loss Replacement

Fluid, sodium, potassium and carbs to take in during a session, from your sweat rate, sweat sodium, duration, temperature and body weight. How it works

Use the Hydration Loss Replacement

Fluid to drink
1000 mL
Sweat loss 1 L · Sodium 920 mg

Sweat loss: 1 L (1.4% of body weight)
Beverage plan: 2 × 500 mL
920 mg
Sodium
20 mg
Potassium (floor)
60 g
Carbohydrates
1:5
Mix ratio (servings of 200 mg sodium)

How It Works

  • Fluid: Sweat rate (L/hr) × duration (hr) × 1000, cut by 10% below 10°C and never above your sweat loss
  • Sodium: Sweat rate × sweat Na (mEq/L) × duration (hr) × 23 (mg of sodium per mmol; mEq/L and mmol/L are the same for sodium)
  • Potassium: Sweat rate × duration (hr) × 20, a minimum: sweat usually carries more
  • Carbs: a fixed 60 g per hour of exercise; your other inputs do not change it
  • Body weight: Used to show the sweat loss as a percentage of body mass.
  • Beverage plan: Total fluid is divided into your preferred beverage volume.
Related: Calorie Deficit · BMI Calculator · TDEE Calculator

About the Hydration Loss Replacement

Sweating 1 L an hour for 1 hour at 20°C, the calculator plans 1,000 mL of fluid (2 × 500 mL bottles), 920 mg of sodium at 40 mEq/L sweat sodium, 20 mg of potassium and 60 g of carbohydrate. The fluid figure is the sweat lost, cut by 10% below 10°C and not raised in hot weather, because drinking more than you sweat risks low blood sodium.

Fluid to drink by sweat rate and session length

Sweat rate1 hour2 hours3 hours
0.5 L/h500 mL (0.7%)1000 mL (1.4%)1500 mL (2.1%)
1.0 L/h1000 mL (1.4%)2000 mL (2.9%)3000 mL (4.3%)
1.5 L/h1500 mL (2.1%)3000 mL (4.3%)4500 mL (6.4%)
2.0 L/h2000 mL (2.9%)4000 mL (5.7%)6000 mL (8.6%)

At 20°C. The percentage is the sweat loss as a share of a 70 kg body weight; above 2% the tool adds a note. Below 10°C each fluid figure is 10% lower: 900 mL for 1 L/h over 1 hour.

How to use this tool

  1. Enter your sweat rate, or open the weigh-in helper: give your nude weight before and after training, the fluid you drank and any urine passed, and it works out (before − (after − fluid + urine)) ÷ hours over the session length and can copy the result into the field.
  2. Enter body weight, session length in minutes, sweat rate, sweat sodium (40 mEq/L if you have no sweat test), the size of your bottle and the air temperature in °C or °F.
  3. Read the fluid, sodium, potassium and carbohydrate totals for the whole session and the number of bottles.

Frequently asked questions

How much water should I drink during exercise?
About what you sweat, and no more. The ACSM position stand sets the goal as keeping the water deficit under 2% of body weight. At 1 L/h for 2 hours that is 2000 mL, and a 70 kg person who drinks nothing loses 2.9% of body weight.
How do I calculate my sweat rate?
Weigh yourself nude before and after a session, add what you drank and subtract urine. Baker's 2017 review calls this change in body mass the simplest and most accurate field method, and reports typical rates of about 0.5 to 2.0 L/h. The weigh-in helper does the sum: 70 kg before and 69 kg after a 60-minute session with 500 mL drunk is 1.5 L/h, and with 100 mL of urine 1.4 L/h. It does not correct for metabolic and respiratory loss, which Baker puts at about 5 to 15% of the weight lost and advises correcting for in sessions over 2 to 3 hours.
How much sodium do I lose in sweat?
Sweat rate × sweat sodium × hours × 23 mg. At 1 L/h and 40 mEq/L that is 920 mg an hour. Baker reports whole-body sweat sodium of roughly 10 to 70 mmol/L, so the same hour can cost far less or far more; a sweat-patch test gives your own figure. The tool flags values above 80 mEq/L.
Why is the potassium figure so low?
It assumes 20 mg per litre of sweat and is labelled a floor. Baker puts sweat potassium at about 2 to 8 mmol/L, so the real loss is usually higher.
Do I need electrolytes for a short workout?
For sessions under 30 minutes the tool notes that water is usually enough. For longer sessions the ACSM stand says drinks with electrolytes and carbohydrate can help over water alone.

Planning fluid and electrolytes for a long session

The results are totals for the whole session, not an hourly schedule. No sweat rate? The weigh-in helper works it out from your nude weight before and after, the fluid drunk and any urine, and copies it into the field: 70 kg to 69 kg in 60 minutes with 500 mL drunk is 1.5 L/h, or 1.4 L/h with 100 mL of urine. A sweat-patch test gives the sodium figure; without one, the default is 40 mEq/L.

A 3-hour ride sweating 1.2 L/h at 50 mmol/L sodium and 30°C returns 3,600 mL of fluid, 4,140 mg of sodium, 180 g of carbohydrate and a mix ratio of 1:21. For a 70 kg rider that sweat loss is 3.6 L, or 5.1% of body weight.

The fluid figure never exceeds your sweat loss, because drinking more than you sweat risks hyponatraemia. The ACSM's 2007 position stand sets the goal as a deficit under 2% of body weight, so a larger loss gets a note.

Formulas behind the replacement plan

sweat rate (L/h) = (weight before − (weight after − fluid drunk + urine)) ÷ hours (Baker 2017, Table 3), with no correction for metabolic and respiratory loss.

fluid (mL) = sweat rate × hours × 1000, × 0.9 below 10°C and never raised in the heat

sodium (mg) = sweat rate × sweat Na × hours × 23

potassium (mg) = sweat rate × hours × 20 and carbohydrate (g) = hours × 60

Sweat sodium in mEq/L equals mmol/L and 23 is sodium's mg per mmol. The bottle count is fluid ÷ bottle size, rounded up; the mix ratio is sodium ÷ 200 mg, rounded. Body weight gives one figure, the share of body mass lost: sweat loss (L) ÷ body weight (kg) × 100. Sessions under 30 minutes get a note that water usually suffices.

The carbohydrate line is the tool's own fixed 60 g per hour: a 45-minute run gets 45 g whatever the weight, sweat rate or temperature. The potassium line assumes 20 mg per litre of sweat and is labelled a floor: Baker's 2017 review reports about 2 to 8 mmol/L, or 80 to 310 mg per litre.

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