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Race Weight Calculator

Work out the race weight your lean mass can actually support.

How it is calculated

The tool assumes you lose only fat and keep all lean tissue. Lean mass is weight × (1 − body fat% / 100). Race weight is that lean mass divided by the leftover share: race weight = lean / (1 − target fat% / 100). Weight loss is current weight minus race weight, and fat mass is current weight minus lean mass; all in kg. The time estimate uses a rule of thumb of 2 seconds per mile for every pound lost: seconds saved = 2 × miles × (kg lost × 2.2046), using 3.107, 6.214, 13.109 or 26.219 miles for 5K, 10K, half or marathon. Current and target body fat must each be 3 to under 60 %, and the target cannot be higher than the current value.

Worked example

75 kg, 18 % body fat, target 14 %, half marathon. Lean mass: 75 × 0.82 = 61.5 kg (fat mass 13.5 kg). Race weight: 61.5 / 0.86 = 71.5 kg, so the loss is 3.5 kg, about 7.7 lb. Time estimate: 2 × 13.109 miles × 7.7 lb = about 202 seconds, shown as 3:22 saved. This is a rough rule of thumb, not a prediction for any particular runner.

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Common mistakes

  • Entering a body-fat percentage from a different device or day for the target comparison. Scale and skinfold readings can differ by several points, and race weight moves with that number.
  • Entering the target as a weight instead of a body-fat percentage. The target field is percent fat, and 70 would be rejected as out of range.
  • Reading the race weight as a number to reach quickly. The calculation keeps your lean mass fixed, so reaching it means losing fat only; cutting muscle too would miss the model.
  • Treating the time gain as guaranteed. It is a 2 s per mile per pound rule of thumb and says nothing about how you would actually feel at the lower weight.
  • Choosing a distance label and then entering a different race. Unknown distances fall back to 5K, so the time saving is for that short distance.

About this calculator

Race weight is the body weight — usually a modest deficit from training weight — at which an endurance athlete carries the least mass without sacrificing the lean tissue that produces power, calculated by holding lean body mass constant and lowering only the fat-mass component to a target body-fat percentage. It is not the same as simply “losing weight,” because dropping muscle along with fat can lower running economy and power output even as the number on the scale improves.

Distance runners, cyclists and triathletes use race-weight calculations in the final build phase before a goal event, since carrying excess fat mass over race distance costs measurable time — a commonly cited rule of thumb is roughly 1-2 seconds per mile per pound of bodyweight, and this calculator uses the upper figure, 2 seconds per mile for every pound lost, for any course. Because it targets a specific, achievable body-fat percentage rather than an arbitrary number, it also helps flag when a “goal weight” is unrealistic for a given frame and lean mass.

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