Precision Calculators

Why 20%?

The 20% rule balances endurance adaptation with injury prevention. Running one long run at 20% of your weekly volume builds aerobic capacity without overloading connective tissue. This ratio is backed by research showing runners who exceed 30% of weekly mileage in a single run face significantly higher injury rates.

Long Run Benefits

• Aerobic base: Increases mitochondrial density and capillary development
• Mental toughness: Builds confidence to sustain effort
• Fat adaptation: Teaches body to use fat as fuel
• Running economy: Improves efficiency at target paces

Progression Tips

• Increase weekly mileage by no more than 10% per week
• Allow 3-4 weeks of adaptation before increasing long run distance
• Every 4th week, reduce long run by 20-30% for recovery
• If weekly mileage changes, recalculate long run distance accordingly

The Science of Sweat Loss

During exercise, your body loses fluid through sweat to regulate core temperature. For every 1 pound (0.45 kg) of body weight lost during a run, you've lost approximately 16 ounces (473 ml) of fluid. Research shows that losing just 2% of body weight through dehydration can impair performance by 10-20% in endurance events like the 5K.

Citation: American College of Sports Medicine (ACSM) Position Stand on Exercise and Fluid Replacement, Medicine & Science in Sports & Exercise, 2007

Why 150% Rehydration?

The standard sports science recommendation is to consume 150% of fluid loss over 2-4 hours post-exercise. This accounts for ongoing fluid losses through urine and respiration during the recovery period. Drinking the full amount at once isn't effective—your body can only absorb about 800-1000ml per hour.

• First hour: Consume 50% of rehydration target
• Second hour: Consume remaining 50%
• Include sodium: Add electrolytes or salt to enhance absorption

Citation: Shirreffs et al., "Post-exercise rehydration in man: effects of volume consumed and drink sodium content," Medicine & Science in Sports & Exercise, 1996

Sweat Rate & 5K Performance

Individual sweat rates vary dramatically—from 0.5 to 2.5 liters per hour—based on genetics, fitness level, and environmental conditions. For a 25-minute 5K race, most runners lose 300-800ml of fluid. While this may not significantly impact a single race, chronic underhydration from accumulated training runs degrades adaptation and recovery.

Key insight: Use this calculator after typical training runs (not just races) to establish your baseline sweat rate. This informs your pre-race hydration strategy and during-race fluid needs for longer events.

Practical Rehydration Tips

• Pre-race hydration: Drink 16-20 oz (475-600 ml) 2-3 hours before your 5K, then 8-10 oz (240-300 ml) 15 minutes before the start
• Monitor urine color: Pale yellow indicates proper hydration; dark yellow signals need for more fluids
• Temperature matters: Sweat rates can double in heat (80°F+/27°C+) compared to cool conditions (50°F/10°C)
• Don't overhydrate: Drinking excessive water without electrolytes can lead to hyponatremia (dangerously low sodium levels)
• Test your strategy: Never experiment with hydration on race day— practice your rehydration routine during training

Warning Signs

Seek immediate medical attention if you experience:

• Body weight loss exceeding 3% during exercise
• Dizziness, confusion, or nausea post-exercise
• Dark urine persisting more than 2 hours after rehydration
• Inability to urinate within 6 hours post-exercise

See our Medical Disclaimer for important safety information.