Fitness & Sports

Race Time Predictor

What this does

Predict finish times at new race distances from a recent result using the Riegel formula; from 5K through the marathon.

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Calculator inputs

Decimal minutes: 50:30 → 50.5.

Prediction assumes you train specifically for the target.

How to use it

Using the race time predictor

  1. 01

    Choose your recent race

    The fresher and harder the effort, the better; ideally within the last six weeks.

  2. 02

    Enter the finish time

    Decimal minutes: 1:52:30 for a half equals 112.5.

  3. 03

    Pick a target distance

    Read the projected clock time plus the required average pace per kilometre.

Good to know

Where Riegel comes from

Analyzing world-record performances across distances, Riegel found time grew almost with distance^1.07; he published 1.06 for trained athletes, and five decades of race-data validation have kept it in place essentially unchanged.

Short vs long predictions behave differently

Projecting down (half → 10K) tends to be optimistic unless you train speed; projecting up (10K → marathon) is optimistic unless you train endurance. The formula describes physiology, not your training log; close the gap with specific work.

Making the prediction reality

  • Match weekly long-run percentage to the target event.
  • Add race-pace tempo sessions at the predicted pace.
  • Retest with a tune-up race to recalibrate before the big day.

How it's calculated

The math behind this calculator

T₂ = T₁ × (D₂ ÷ D₁)^1.06

Peter Riegel’s 1977 empirical model scales a known performance to any other distance using the exponent 1.06; slightly worse than linear, capturing the aerobic endurance premium of longer races. It remains the standard used by Runner’s World, Strava and most race predictors.

The prediction assumes equivalent training commitment for the new distance; entering a fresh all-out 10K and reading a marathon time presumes you actually put in marathon mileage.

Assumptions & limitations

  • Recent race was a genuine hard effort on flat terrain.
  • Comparable training volume for the predicted distance.
  • Exponent 1.06 fits middle-distance-to-marathon spans best.

Worked example

A 50-minute 10K projects to roughly 1:50 for a half marathon; provided long-run training backs up the ambition.

FAQ

Frequently asked questions

Can I predict a marathon from a 5K?
Technically yes, practically poorly; the training-specificity gap grows with distance ratio. Use the closest available distance, ideally no more than 2× apart.
Why does everyone quote 1.06?
It fits elite and recreational data reasonably across road distances; alternatives (1.04–1.08) tweak hot-weather or ultra cases but rarely change decisions.
My predicted marathon seems too fast
Almost always a fueling or mileage gap rather than a formula problem. Compare your longest long run and weekly volume against marathon norms.

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