Weight vs Pace Calculator

What does this calculator do?

The weight-versus-pace calculator rescales a known race time when body mass changes, using t₂ = t₁ × (m₂ / m₁)^0.67. Default: 10K in 50:00 at 75 kg → a predicted time at 70 kg.

The method type is Endurometry approximation. The 2/3-power is an allometric rule of thumb used in popular running calculators. It is not a table from Daniels’ Running Formula, not ACSM, and not a body-composition model. Do not lose weight for a race time.

How does it work?

Keep distance and the original clock. Only mass changes. An exponent of 0.67 means a 1% mass change moves time by about 0.67% for small deltas — not 1:1. Fat-free mass, glycogen, shoes and hills are ignored.

The large figure is the predicted clock at the new mass. Rows show the delta, seconds per kilometre, and seconds per kilometre per kilogram. Lighter mass shortens the prediction; heavier lengthens it.

Formula

t2 = t1 × (m2 / m1)^0.67     // Endurometry approximation

Δt      = t1 − t2            // positive ⇒ predicted faster
Δs/km   = Δt / distance(km)
Δs/km/kg = |Δt| / |m2 − m1| / distance

Times round to a second. Per-km change is one decimal; per-kg two decimals. Mass 40–150 kg; times 1 min–8 h. Identical masses are rejected.

Worked example

How to interpret the result

Read the big number as “if nothing else changed except scale mass by 0.67.” Fitness, economy and how the mass came off usually dominate. A 2:16 “gain” on a 50:00 10K is a conversation aid, not a training plan.

Session energy at a pace is the calorie calculator. Scaling to another distance is the race time predictor.

Assumptions and limitations

Assumptions

• Mass is the only changing input. Fitness, economy and course stay put.
• The exponent 0.67 applies to total body mass, not fat mass alone.
• The original time is a race effort at the original mass.

Limitations

• Approximation, not a published race equation we implement from a paper. Allometric VO₂ literature often uses 0.67–0.75 for economy scaling; we do not claim 0.67 is the consensus for finish time.
• Losing muscle or under-fuelling can slow you. The formula cannot see composition.
• Not medical or diet advice. Do not chase a clock with weight loss.
• Distance only labels the per-km row; 5K and marathon share the exponent.
• Mass outside 40–150 kg, times outside 1 min–8 h, or unchanged mass: rejected.

Methodology and sources

Method type: Endurometry approximation

This calculator is an Endurometry approximation: t₂ = t₁ × (m₂/m₁)^0.67. It is not an implementation of a Daniels table or of ACSM. Use it as a scale sketch, not a weight-loss prescription.

• Methodology: Body mass and race time
• Methodology: Rounding and displayed precision

Sources: Endurometry weight scaling

Last reviewed: October 5, 2026