Question 1

How accurate is the Riegel formula?

Accepted Answer

For predictions between similar distances (e.g. 5K to 10K, or 10K to half marathon), the Riegel formula is typically accurate within 2–5% for well-trained runners. Accuracy decreases significantly when predicting across very different distances — e.g. 5K to marathon — because the physiological demands are fundamentally different. A 5K relies heavily on VO2max, while a marathon is primarily a fat metabolism and pacing challenge.

Question 2

Why does the formula use 1.06 as the exponent?

Accepted Answer

The 1.06 exponent was derived by Peter Riegel from analysis of world record performances across distances, published in American Scientist (1981). It represents the average rate at which human performance degrades with increasing distance. Some researchers have proposed alternative exponents (Cameron 1985 used distance-specific exponents), but 1.06 remains the standard for its simplicity and reasonable accuracy.

Question 3

Should I use a 5K or 10K time to predict my marathon?

Accepted Answer

A 10K or half marathon time gives more accurate marathon predictions than a 5K, because the physiological systems (aerobic capacity, fat metabolism, muscular endurance) involved in a 10K or half marathon are more similar to marathon demands. Using a 5K time will typically give an optimistic marathon prediction unless you have specifically trained for marathon endurance. A recent half marathon is the gold standard input for marathon prediction.

Question 4

What is the 10% rule for increasing training volume?

Accepted Answer

The 10% rule — not increasing weekly mileage by more than 10% per week — is a widely used guideline to reduce injury risk. It was popularised by running coaches in the 1980s and is referenced in most recreational running programmes. Research from the British Journal of Sports Medicine (2014) supports gradual progression, though the exact percentage is debated. Most coaches recommend 2–3 easy weeks for every 3–4 weeks of progression.

Question 5

Why do two runners with the same 10K time get different marathon predictions from different apps?

Accepted Answer

Different running apps and coaches use different prediction models — some use the Riegel formula with the standard 1.06 exponent, others use McMillan's tables, VDOT-based tables (Jack Daniels), or Cameron's distance-specific exponents. Each was built from a slightly different dataset and mathematical approach, so predictions for the same input time can reasonably differ by several minutes at marathon distance. None is definitively "correct" — they're all statistical approximations from different populations.

Question 6

Does the Riegel formula account for hills or heat?

Accepted Answer

No. The formula uses only your input time and target distance — it assumes similar course conditions (flat, moderate temperature) for both the reference performance and the predicted race. A hilly or hot target race will realistically run slower than the flat-course prediction suggests, and a downhill or ideal-weather race can run faster. Treat the raw prediction as a flat-course, ideal-conditions baseline and adjust for known course and weather differences.

Question 7

Can I use a treadmill time for race prediction?

Accepted Answer

Treadmill times are generally 1-3% faster than equivalent outdoor road times at the same effort, since there's no wind resistance and the belt provides some assistance to leg turnover. If using a treadmill time as your known input, expect the prediction to be slightly optimistic for an outdoor race — a treadmill time run at 1% incline is often recommended as a closer proxy for true outdoor effort.

Race Time Predictor

Race Time Predictor

Recent Race Performance

Predicted Race Times

What is a Race Time Predictor?

How to Use This Calculator

Typical Race Equivalency (Average Club Runner)

Why Race Predictions Are a Planning Tool, Not a Promise

?Frequently Asked Questions