Calculator

Weight impact calculator.

Estimate how race day time might shift when your body weight changes. Tune the sensitivity slider to reflect how strongly weight impacts your pace.

Project finish time changes

Current weight (kg)

Use your typical training weight.

Target race weight (kg)

A higher number will project a slower finish.

Baseline finish time (hh:mm:ss)

You can enter mm:ss (e.g., 45:00) or seconds (e.g., 2250).

Race distance (optional)

Weight impact factor

Each 1% change in body weight is assumed to shift finish time by 0.75%. Adjust to 0.5–1.0% to reflect course, heat, and fitness.

Why weight shifts pace

Carrying less mass means each stride requires slightly less force, so runners can usually move faster at the same effort. Research and coaching heuristics suggest that every 1% change in body weight translates to roughly 0.5–1.0% change in finish time for steady-state road races. Hills, heat, injury risk, and fueling can all magnify or mute that effect.

The slider lets you tune the sensitivity to match your experience: use the lower end when terrain is flat or you are already lean, and the higher end when racing hilly routes or carrying extra gear. Combine the estimate with objective markers like heart-rate, RPE, and recent workouts to decide whether a target weight is realistic and healthy.

How to use this calculator

Enter your current weight, target race weight, and a recent finish time.
Add the race distance to see how pace per kilometre and mile may change.
Adjust the impact factor until the projection matches past experiences, then use that factor for future planning.

Interpreting your result

Add a current weight, target weight and finish time to see a personalised explanation. We’ll calculate the theoretical time savings for every kilogram or pound lost, helping you weigh up the benefits against the training cost.

Remember that "lighter is faster" has a limit. Use this tool to model small, healthy adjustments (e.g., tapering off bulk phases) rather than drastic cuts which harm performance. The 0.75% default factor is a widely accepted middle-ground for road running.