Project finish time changes
Use your typical training weight.
A higher number will project a slower finish.
You can enter mm:ss (e.g., 45:00) or seconds (e.g., 2250).
The model assumes each 1% change in body weight shifts finish time by 0.75%. The 0.5–1.0% range is for sensitivity testing, not a clinically validated personal range.
Scenario finish
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Why weight shifts pace
Moving body mass contributes to the energy cost of running. Experimental research using modest torso loads found that added mass can impair running economy, while observational research links body composition with running economy. Those findings do not prove that intentionally losing a given percentage of body weight produces the same percentage improvement in race time.
The slider therefore tests a range of assumptions rather than selecting a biologically correct value. Training status, body composition, where mass changes, terrain, heat, fuelling, recovery, and health can all change the real outcome.
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.
- Compare several scenario factors and treat the spread as model uncertainty, not a target.
Interpreting your result
Add a current weight, target weight and finish time to see a scenario explanation. We’ll calculate the modelled time change for every kilogram or pound, based only on the selected assumption.
Lower weight does not automatically mean better performance. Inadequate energy intake relative to training can impair health and performance in athletes of any sex. Do not use the result to set a weight-loss target; discuss body-composition changes with an appropriately qualified sports dietitian or clinician.
Formula, worked example, and limitations
The calculator applies this scenario equation:
Scenario time = baseline time × [1 + ((target weight − current weight) ÷ current weight) × factor].
Example: changing from 70 kg to 68 kg is a 2.86% reduction. With a 45:00 baseline and the 0.75 factor, the model applies a 2.14% time reduction and returns approximately 44:02—about 58 seconds faster. This is arithmetic from the selected assumption, not evidence that a runner will achieve that result.
- The model assumes fitness, power, technique, equipment, weather, course, fuelling, and recovery stay unchanged.
- It does not distinguish fat mass, lean mass, body water, or carried equipment.
- It extrapolates linearly even though real responses may be non-linear and individual.
- Short-term body-mass change during a race is often fluid change and should not be interpreted as planned tissue loss.
Evidence and editorial check
Editorial check completed by the RunCalcs Editorial Team: August 21, 2026. The calculator is an educational scenario tool and has not been medically or clinically reviewed.