Fluid Loss Calculator

Enter Fluid Loss Data

Pre-activity weight (kg)

Body weight before activity or illness period.

Post-activity weight (kg)

Use similar clothing and scale conditions.

Drinks consumed (L)

Water, sports drink, oral rehydration drink, or similar.

Fluid from food or ice (L)

Optional. Count soups, fruit, ice chips, or watery foods.

Urine output (L)

Measured during the same tracking period.

Stool loss (L)

Useful during diarrhea or GI recovery tracking.

Vomiting loss (L)

Enter total measured or estimated loss.

Duration (minutes)

Used to calculate hourly sweat rate.

Sweat sodium concentration (mg/L)

Use your test value or a practical estimate.

Replacement factor (%)

Common planning range is 125% to 150%.

Ambient temperature (°C)

Used for interpretation and planning notes.

Relative humidity (%)

Humidity can raise perceived strain and sweat burden.

Formula Used

This calculator treats 1 kilogram of short-term body mass change as approximately 1 liter of fluid change. It combines weight change with recorded intake and measured outputs.

1) Raw body mass change

Raw Body Mass Change (L) = Pre-activity Weight - Post-activity Weight

2) Estimated sweat loss

Estimated Sweat Loss (L) = Raw Body Mass Change + Drinks + Food Fluid - Urine - Stool - Vomiting

3) Total recorded fluid loss

Total Recorded Fluid Loss (L) = Estimated Sweat Loss + Urine + Stool + Vomiting

4) Body mass loss percentage

Body Mass Loss (%) = ((Pre-activity Weight - Post-activity Weight) / Pre-activity Weight) × 100

5) Hourly sweat rate

Hourly Sweat Rate (L/hour) = Estimated Sweat Loss / Duration in Hours

6) Replacement target

Replacement Target (L) = Total Recorded Fluid Loss × (Replacement Factor / 100)

7) Estimated sodium loss

Estimated Sodium Loss (mg) = Estimated Sweat Loss × Sweat Sodium Concentration

Temperature and humidity are used only for interpretive advice, not the core fluid-loss equation.

How to Use This Calculator

Measure body weight before the activity, practice, shift, or illness period.
Track drinks, watery foods, urine, stool loss, and vomiting during the same period.
Measure body weight again under similar conditions after the session ends.
Enter the duration so the tool can estimate hourly sweat rate.
Add sweat sodium concentration if you have a lab result or trusted estimate.
Choose a replacement factor, commonly 125% to 150%, for practical rehydration planning.
Review the result summary, hourly rate, sodium loss, and planning notes.
Use the CSV or PDF buttons to save your results for records or coaching review.

Example Data Table

Pre Weight (kg)	Post Weight (kg)	Drinks (L)	Food Fluid (L)	Urine (L)	Stool (L)	Vomit (L)	Duration (min)	Sodium (mg/L)	Replacement (%)	Estimated Sweat Loss (L)	Replacement Target (L)
72.4	71.6	1.00	0.20	0.15	0.00	0.00	90	850	150	1.85	3.00
65.0	64.4	0.60	0.10	0.10	0.05	0.00	60	700	130	1.15	1.69

FAQs

1) What does this calculator estimate?

It estimates fluid leaving the body through sweat, urine, stool, vomiting, and body mass change. That helps you plan rehydration after exercise, heat exposure, or stomach upset.

2) Why use body weight change?

During short tracking periods, one kilogram of body mass change is roughly one liter of fluid. It is a practical field method for hydration planning.

3) Is 1 kilogram always equal to 1 liter?

Yes, as a working estimate for short sessions. Glycogen, food mass, and clothing can slightly distort results, so consistent weighing conditions improve accuracy.

4) Why is the replacement target higher than measured loss?

Many athletes replace 125% to 150% of measured losses over several hours, especially after heavy sweating. Smaller, spaced drinks are often easier than one large bolus.

5) Can this diagnose dehydration?

No. It is an educational planning tool, not a diagnosis. Severe dehydration, persistent vomiting, fainting, or confusion needs urgent medical care.

6) Can I use it during illness?

Yes, for tracking vomiting or stool losses, but illness can change electrolyte needs quickly. Use extra caution with children, older adults, or kidney disease.

7) Why enter sweat sodium concentration?

Sodium loss varies widely between people. Entering your sweat sodium estimate helps match drinks or foods more closely to your likely electrolyte loss.

8) When are results less reliable?

Results become less reliable when weights are inconsistent, clothing is wet, bowel contents change, or the tracking period is very long. Extreme heat illness also needs clinical assessment.