Race Time Predictor

What Is the Riegel Formula?

The Riegel formula is the most widely used method for predicting race times across different distances. Published by Peter Riegel in 1977, it uses a simple power-law relationship to estimate how your performance scales with distance:

T2 = T1 × (D2 / D1)^1.06

Where T1 is your known time, D1 is the known distance, D2 is the target distance, and T2 is the predicted time. The exponent 1.06 accounts for the fact that you cannot maintain the same pace over longer distances — fatigue causes a predictable slowdown.

How Accurate Is It?

For well-trained runners competing at distances between 1,500 meters and the marathon, the Riegel formula is remarkably accurate — typically within 1–3% of actual race times. It works best when:

• Your known time is from a recent, all-out effort (not a training run)
• The distances are within a reasonable range of each other (e.g., 5K to half marathon)
• You have trained specifically for the target distance

The formula becomes less reliable for extreme jumps (e.g., predicting a marathon from a single mile time) because it assumes a consistent training base for the target distance.

Limitations to Keep in Mind

The Riegel formula assumes that your aerobic fitness and endurance are evenly developed across distances. In practice, this is rarely true:

• Speed-biased runners (those who train mostly short intervals) will find their actual long-distance times slower than predicted.
• Endurance-biased runners (those with a high weekly mileage base) may actually beat the predictions at longer distances.
• Beginners tend to slow down more than the formula predicts over the marathon, because fuelling strategy and mental endurance become major factors beyond the half marathon.

Use the predictions as a guide, not gospel. They give you a realistic starting point for setting goal paces and planning race-day strategy.

How Helm Predicts Your Race Times

Helm goes beyond the basic Riegel formula by using your actual GPS running data. Instead of relying on a single race result, Helm analyses your recent training runs — including pace, distance, elevation, and heart rate — to generate personalised race predictions that update automatically as your fitness changes.

As you build your training log, Helm's predictions become more accurate because they are based on your real performance data, not just a theoretical formula. The race predictor appears right in the app's cardio section, always up to date.