Race Weight Calculator — Range, Not a Single Target

Race weight is a training-culture term for the body mass a runner carries into competition, popularised by Matt Fitzgerald's Racing Weight. The common method estimates it from fat-free mass and a chosen target body-fat percentage. It is a practical heuristic — a literature search returns no peer-reviewed validation of that formula, and no authoritative optimal body-fat target for distance runners.

Two reasons, both about honesty rather than hedging. First, there is no evidence base for an optimal body-fat target, so a single number would be invented precision. Second, the body-fat figure you enter almost always comes from a bioimpedance scale, and those carry meaningful individual variability on both single and repeated measurements (Moon 2013).

Not reliably, and under-fuelling reverses it. The evidence linking lower body fat to faster times is cross-sectional — body-fat percentage and half-marathon times are related in recreational runners, but that is correlation, not a causal dose-response you can plan around. Meanwhile low energy availability is associated with decreased run performance, endurance, coordination and explosive power.

Energy availability is dietary energy intake minus exercise energy expenditure, expressed per kilogram of fat-free mass per day. A controlled trial found luteinising-hormone pulsatility was unaffected at 30 kcal/kg lean body mass per day but disrupted below that threshold (Loucks and Thuma 2003). The ACSM position stand adopted the same figure; the 2023 IOC consensus dropped fixed cut-offs entirely, calling a single universal threshold frail and treating low energy availability as a spectrum instead — which is why this tool uses 30 kcal/kg as a reference line, not a diagnostic threshold. Note the original subjects were 29 habitually sedentary young women, not trained athletes.

More common than most runners assume. A meta-analysis of 6,118 athletes found 44.7% in low energy availability, and 63.0% of a smaller subgroup at risk of Relative Energy Deficiency in Sport. Prevalence was higher in men (49.4%) than women (44.2%), which contradicts the assumption that this is a female-athlete issue.

Use the same device, at the same time of day, in the same hydration state, and track the trend rather than any single reading. Bioimpedance devices vary meaningfully between individuals and between repeat measurements, so a one-off number is a weak input. If a decision matters, DXA or a skinfold assessment by a trained practitioner is more reliable.

Keep energy availability above the threshold and protect lean mass. Higher protein intakes of 2.3-3.1 g per kg body weight per day may be needed to maximise lean-mass retention during hypocaloric periods — though the ISSN position stand frames that for resistance-trained subjects, so applying it to endurance runners is an extrapolation. Losing weight slowly and keeping fat-free mass is also what makes the arithmetic in this tool hold — it assumes your fat-free mass stays put.