Electrolyte Replacement Calculator - Sweat Rate + Duration

This electrolyte replacement calculator uses sweat rate and session duration to estimate practical fluid and sodium targets for training days. It also compares your current drink plan against your estimated sodium needs.

Use it before long, hot, or high-sweat sessions to avoid under-fueling hydration. You can quickly choose your sweat sodium profile, replacement strategy, and bottle size to get actionable targets.

Creator

Kody Abberton

Fitness coach focused on practical hydration and recovery protocols for training.

Last updated February 10, 2026

Quick summary

The calculator combines sweat rate, duration, sodium profile, and replacement goals to output target fluid and sodium intake during exercise, plus a post-session top-up estimate.

Table of contents

Calculator Formula Example FAQ Resources Recommended calculators

Electrolyte Replacement Calculator

Calculator

Enter sweat rate and session duration, then set sodium profile and replacement strategy. The calculator estimates your fluid and sodium targets with bottle planning.

Sweat rate (L/hour)Session duration (minutes)Sweat sodium profileReplacement goalPlanned fluid intake (ml/hour)Drink sodium concentration (mg/L)Bottle size (ml)

Total sweat loss1.50 L

Target fluid during workout850 ml/hour (1275 ml total)

Sodium target during workout680 mg/hour (1020 mg total)

Planned sodium intake683 mg total

Sodium gap vs plan+337 mg

Bottles needed2.6 bottles

Post-workout top-up656 ml

Hydration note: Replacing 70-100% of sweat losses during exercise is a practical range for many sessions.

Sodium plan note: Your plan under-replaces sodium. Increase drink sodium or add sodium capsules.

Formula

This tool uses sweat rate and duration first, then applies replacement percentage and sodium concentration to estimate intake targets.

Sweat loss (L) = sweat rate (L/h) x duration (hours)

Target fluid (L) = sweat loss x replacement goal

Sodium target (mg) = target fluid (L) x sodium profile (mg/L)

Post-workout top-up (ml) = max(0, sweat deficit in L x 1000 x 1.25)

Most athletes can start with a balanced replacement goal and then adjust based on body mass changes, thirst, and tolerance.

Example calculation

Example: sweat rate 1.2 L/hour, duration 90 minutes, balanced replacement (85%), moderate sodium profile (800 mg/L). Sweat loss is 1.8 L and target fluid is 1.53 L.

Sodium target becomes 1.53 x 800 = 1224 mg total. If planned fluid is 650 ml/hour for 1.5 hours (975 ml total) at 700 mg/L, planned sodium is 683 mg, leaving about a 541 mg gap to close.

FAQ

How do I estimate my sweat rate quickly?

Weigh before and after a session, adjust for fluid consumed, then divide net fluid loss by session hours.

Should I replace 100% of sweat loss during exercise?

Not always. Many sessions feel best at 70-100% replacement depending on duration, heat, and gut comfort.

Why does sodium matter for hydration?

Sodium helps retain fluid and replace sweat sodium losses, especially in long or hot sessions.

What if my sodium gap is high?

Increase drink sodium concentration, increase intake volume, or add sodium capsules if needed and tolerated.

Resources

Practical hydration references: CDC/NIOSH Heat Stress Recommendations, GSSI: Fluid and Electrolyte Needs, USADA: Electrolyte Replacement.

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