Overview

What this tool is, and who it is for

Hydration advice in the outdoor press is dominated by marketing-friendly round numbers, and most of them are wrong for you specifically. This planner treats sweat rate as something you measure rather than something you guess. It accepts a one-hour test session as input, computes your litres per hour at the conditions of that test, then scales the figure for the temperature and altitude of the planned activity. The output is a per-hour figure you can take to the trailhead and a carrying plan that includes sodium guidance.

Inputs

Change any value to recalculate instantly.

Body mass before

Body mass after

Fluid drunk during the test

Count everything, including tea or soup.

Urine output during the test

Exercise duration

minutes

Weigh yourself immediately before and after to keep the error small.

Temperature

°C

Applied as an adjustment to your measured rate.

Altitude

Above 1,500 m, respiratory water loss rises measurably.

Duration to plan for

1614

Results

Computed from the method described below.

Your sweat rate

0.96L/hour

Measured rate with temperature and altitude adjustments applied.

Water to carry for this plan

5.8litres

Over 6 hours, excluding what you drink at camp and in meals.

Body mass deficit during test1.89%

Around 1% body mass deficit, which is close to the practical target. Aim to keep it here rather than going lower.

Sodium replacement guidance for this rate: 481–1,443 mg/hour. Sweat sodium concentration varies by an order of magnitude between individuals, so begin at the low end and increase only if you see salt crusting on clothing or dark-coloured clothing staining.

Why the adjustment factors differ

Temperature factor: ×1.04

Altitude factor: ×1.00

Measured rate before adjustment: 0.93 L/h

What you get

Your personal sweat rate in litres per hour, plus a water carry plan with sodium guidance.

•Calculates sweat rate from body mass change, fluid intake and urine output
•Projects total fluid requirement for a planned duration
•Adjusts for temperature and altitude, both of which increase losses
•Provides indicative sodium replacement guidance
•Flags when your rate suggests you should reconsider your electrolyte strategy

Honest limits

The weight-based method requires a reasonably accurate scale and control of urine output, so field measurements carry error of perhaps 10 to 15 percent.
Sweat sodium concentration varies by an order of magnitude between individuals; a single measurement cannot establish your personal concentration.
No hydration plan should be followed rigidly in conditions that change rapidly. Thirst and urine colour remain useful ongoing checks.

How to use

Step-by-step user guide

How to operate the calculator in your browser, what to enter, and what to do with the result.

01
Weigh yourself nude or in dry clothes, then walk, run or hike for a measured hour at the intensity you plan to do outdoors.
02
During that hour, record exactly how much fluid you drink (in millilitres) and any urine you pass (in millilitres).
03
Weigh yourself again immediately afterwards, in the same clothes (or nude).
04
Enter the four numbers into the planner — start weight, end weight, fluid in, urine out — and your sweat rate appears.
05
Re-run for the temperature and altitude of the planned trip, then read the carry plan from the result card.

The method

What the calculator actually does with the numbers you enter

01
Sweat rate is computed as body mass change in grams, plus fluid consumed in millilitres, minus urine output, divided by exercise duration in hours — the standard field method.
02
A one percent body mass deficit is treated as the practical target, since losses beyond two percent measurably degrade aerobic performance and cognitive function.
03
Temperature and altitude adjustments are multiplicative factors applied to the measured rate, based on published environmental physiology findings.
04
Sodium guidance uses typical sweat sodium concentration ranges rather than a fixed figure, because individual variation is large.

References

Published sources behind the formulas

[1]
Exercise and fluid replacement position stand
American College of Sports Medicine (ACSM) · 2024
[2]
Hydration and electrolyte guidance for outdoor activity
Wilderness Medical Society · 2024
[3]
Sweat rate and sodium loss variability in athletes
Journal of Applied Physiology · 2023

FAQ

Frequently asked questions

Short, sourced answers to the questions readers send us most often about this calculator.

Are electrolyte drinks worth the weight?

Above roughly 1.5 litres per hour, plain water intake starts to outpace sodium replacement and risks dilutional hyponatraemia on multi-hour efforts. Most healthy adults on a moderate day can drink plain water and add salt to food; if your sweat rate exceeds 1.5 litres per hour or your trip exceeds six hours at altitude, electrolyte mixes start to pay for their weight.

How accurate is the weight-based sweat-rate method?

It is accurate to within roughly 10 to 15 percent when the test is done carefully. The biggest source of error is unrecorded fluid intake — drinking from a stream, eating water-rich food, or losing track of urine output. Repeat the test two or three times and average.

What about cold-weather water intake?

Cold suppresses thirst and reduces perceived sweat, but losses continue and often exceed what people expect. The calculator applies a smaller adjustment in cold than in heat, but plan to drink at scheduled intervals regardless of thirst.

How do I plan for filter delays at water sources?

Add roughly 30 minutes per cache to your carry plan and assume one to two litres of buffer on either side of any resupply gap. The calculator's carry figure assumes you will arrive at the next source with the result's amount left.