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Hydration For Hot Weather Calculator

Work out hydration for hot weather instantly with clear inputs, formula shown and shareable results.

About Hydration For Hot Weather Calculator

Baseline intake is about 33 ml per kilogram of body weight, and sweat loss is the hourly rate over the exposure period scaled up by 3% for every degree above 25 C. Sweat carries roughly 900 mg of sodium per litre, so replacing volume alone during long heat exposure risks dilutional hyponatraemia.

Formulas

Sweat loss

litres = sweat rate x hours x (1 + 0.03 x (temperature - 25))

Total intake

total = body weight kg x 0.033 + sweat loss

Worked example

With the default values below, Hydration For Hot Weather Calculator returns Total fluid for the day of 7.15. Change any input above and every figure here is recalculated in your browser.

Inputs used

Body weightminimum 20 kg
70 kg
Hours of exposure or activityminimum 0.1 h
4 h
Sweat rateminimum 0.05 L/h
1 L/h
Air temperatureaccepts -10–55 C
32 C

Results

Total fluid for the day
7.15 L
Drink per hour during exposure
1,210 ml
Estimated sweat loss
4.84 L
Sodium to replace
4,356 mg

Step by step

  1. 1Values used
  2. 2Sweat loss
  3. 3Total intake
  4. 4Total fluid for the day= 7.15 L
  5. 5Drink per hour during exposure= 1,210 ml
  6. 6Estimated sweat loss= 4.84 L
  7. 7Sodium to replace= 4,356 mg

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Hydration For Hot Weather figure worked out?

litres = sweat rate x hours x (1 + 0.03 x (temperature - 25)). Baseline intake is about 33 ml per kilogram of body weight, and sweat loss is the hourly rate over the exposure period scaled up by 3% for every degree above 25 C.

Why does this matter?

Sweat carries roughly 900 mg of sodium per litre, so replacing volume alone during long heat exposure risks dilutional hyponatraemia.

What do I need to enter?

Enter body weight, hours of exposure or activity, sweat rate, air temperature. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.

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