Question 1

How accurate is the race time predictor?

Accepted Answer

The Riegel formula is a well-established statistical model that is reasonably accurate when predicting from a race of similar or shorter distance. Predictions get less reliable for large jumps (e.g. predicting a marathon from a 1-mile time) because endurance becomes more important than speed.

Question 2

What is the Riegel formula?

Accepted Answer

T2 = T1 × (D2 / D1)^1.06. It was published by Pete Riegel in 1977 and states that as race distance increases, your pace slows by a predictable power-law factor.

Question 3

What is the best race to use as input?

Accepted Answer

Use a recent race of similar distance to your target, run on a flat course with good conditions. A race run within the last 2-3 months at 5K-to-half-marathon distance gives the most reliable predictions.

Question 4

Is my running data stored?

Accepted Answer

No. All calculations happen entirely in your browser. No race times or personal data are sent to any server.