📖 Overview
This calculator turns before and after weigh ins into practical hydration and sodium replacement targets.
It helps endurance athletes rehydrate accurately after hard sessions.
🧪 Example Scenarios
Use these default and higher-pressure example inputs to explore how sensitive this calculator is before using your real numbers.
| Input | Base Case | Higher Pressure Case |
|---|---|---|
| Weight Before Session (lb) | 176 | 202.4 |
| Weight After Session (lb) | 173.8 | 199.87 |
| Fluid Consumed During Session (oz) | 24 | 27.6 |
| Session Duration (minutes) | 70 | 84 |
| Sweat Sodium Concentration (mg per L) | 850 | 977.5 |
⚙️ How It Works
Computes total fluid loss and sweat rate from weigh-ins and intake, then calculates sodium replacement needs from sweat concentration.
The Formula
Sweat Loss L = (PreWeight − PostWeight) × 0.4536 + Fluid Intake L
| Pre Weight | Body weight before session |
| Post Weight | Body weight after session |
| Fluid Intake | Fluid consumed during session |
| Duration | Total session duration in minutes |
| Sodium Concentration | Sweat sodium mg/L from testing or estimate |
💡Weigh-in delta plus fluid intake is one of the most useful field methods for hydration planning in endurance training.
Quick Reference
| Total Loss | Duration | Sweat Rate | Sodium Need @ 900 mg/L |
|---|---|---|---|
| 1.2 L | 60 min | 1.2 L/h | 1,080 mg |
| 1.8 L | 90 min | 1.2 L/h | 1,620 mg |
| 2.4 L | 120 min | 1.2 L/h | 2,160 mg |
When To Use This
- Use this tool when you need a fast decision during active planning or execution.
- Use this before committing money, time, or tradeoffs that are hard to reverse.
- Use this to compare options using the same assumptions across scenarios.
Edge Cases To Watch
- Results can be misleading if key inputs are missing, stale, or unrealistic.
- Very small or very large values may amplify rounding effects and interpretation risk.
- If assumptions change mid-decision, recalculate before acting.
Practical Tips
💡 Use similar clothing and scale conditions pre and post workout.
💡 Replace more than 100% of losses over recovery window not instantly.
💡 Re-test sweat rate in different temperatures and intensities.
Frequently Asked Questions
❓ Why include fluid consumed?
Because intake masks true sweat loss if you only look at scale weight change.
❓ Is sodium concentration fixed for everyone?
No. It varies significantly by person and should be personalized when possible.