How much faster will I be if I lose weight?

Research suggests approximately 1.4 seconds per kilometre per kilogram of weight loss for trained runners. For a marathon, losing 3 kg could save around 3 minutes. However, this assumes the weight lost is excess body fat, not muscle.

Is there an ideal race weight for runners?

There is no universal ideal weight. A healthy racing weight depends on body composition, genetics and individual physiology. Focus on being healthy and well-fuelled rather than targeting a specific number on the scales.

Can losing too much weight hurt performance?

Yes. Excessive weight loss can deplete muscle glycogen, reduce muscle mass, impair immune function and increase injury risk. Losing more than 5% of body weight in the weeks before a race is generally too aggressive.

Does this apply to heavier gear too?

Yes, the same principle applies to carrying extra weight in the form of gear, hydration packs or clothing. Every extra kilogram costs approximately 1.4 seconds per kilometre.

RunCalcs

Race Weight Calculator — Weight and Performance Impact

Estimate the time impact of body weight changes on your race performance.

Current Weight

Weight measurement

Target Race Weight

Weight measurement

Race Distance

Distance measurement

Current Finish Hours

Time in hours

Current Finish Minutes

Time in minutes

How We Calculate This

Race Weight Impact Calculation

The calculator uses research-backed estimates of weight impact on running performance:

Time Saved per km = Weight Change (kg) x 1.4 seconds

Total Time Saved = Time Saved per km x Race Distance

This is based on the relationship between body mass, oxygen consumption and running economy. Lighter runners require less energy to maintain the same pace, resulting in faster times or the ability to sustain a faster pace.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always consult a coach or medical professional for personalised training advice.