How the Race Time Predictor Works
This tool uses the Riegel formula, published in American Scientist in 1981 by Peter Riegel. The formula is: T2 = T1 × (D2 ÷ D1)^1.06, where T1 is your known race time, D1 is the distance you ran, and T2 is the predicted time for distance D2.
The exponent 1.06 reflects the fact that longer races are proportionally harder — your pace slows as distance increases because of glycogen depletion, fatigue accumulation, and muscle breakdown. The formula works remarkably well for distances from 800m to ultramarathons when your input was run at genuine race effort.
Which Input Race Gives the Best Prediction?
The closer the input race is to the target distance, the more accurate the prediction:
- Best for marathon prediction: Half marathon time trial run 4–6 weeks before race day. This is the gold standard.
- Good for 10K prediction: 5K parkrun or time trial.
- Use with caution: Predicting a marathon from a 5K — the formula will give an optimistic result because the marathon's unique demands (nutrition, wall avoidance, mental stamina) aren't expressed in a 5K effort.
Why Your Predicted Marathon Time Might Be Faster Than You Run
The Riegel formula assumes your fitness is proportional across distances. But marathon performance depends on factors that don't scale linearly:
- Glycogen stores: You carry roughly 90 minutes of glycogen. Beyond that, you rely on fat — and fat oxidation is much slower, forcing your pace down.
- Training specificity: If you've run lots of 5Ks but no long runs, your short-distance speed won't translate to marathon fitness.
- Nutrition strategy: Runners who miss their gel timing can lose 10–20 minutes in the final 10 km.
A safe rule: for marathon goal-setting, take your Riegel-predicted time and add 3–8 minutes as a buffer, especially for your first marathon.
Using This Predictor in Training
Race time predictors aren't just for race-day goals — they're useful throughout your training:
- Run a 10K time trial mid-training to check whether your marathon goal is still realistic
- Use a 5K effort to benchmark fitness improvements across a training block
- Compare predictions before and after a 16-week marathon build to quantify improvement
- Set realistic "B" and "C" goals for race day using different input distances