Race time predictor

Predicting race times with the Riegel formula

The race time predictor uses the Riegel formula, first published by Peter Riegel in 1977. It models how running performance degrades as distance increases — because fatigue is not linear, doubling the distance takes more than twice as long:

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 is the key — it captures the fatigue factor that makes longer races disproportionately harder. Once you have a predicted finish time, use the target pace calculator to find the exact pace you need to run to achieve it.

Three variants are available to reflect different levels of fitness and race conditions:

• Realistic — the standard Riegel formula (exponent 1.06), best for most runners
• Conservative — uses a slightly higher exponent (1.07), recommended for hilly courses, hot conditions, or lower weekly mileage
• Optimistic — uses a lower exponent (1.05), suitable for highly trained runners peaking in form

Frequently asked questions

How does the race time predictor work?

The predictor uses the Riegel formula, developed by Peter Riegel and published in 1977: T₂ = T₁ × (D₂ / D₁)^1.06. It models the fact that performance degrades non-linearly as distance increases — doubling the distance takes more than twice as long, because fatigue compounds.

Which formula should I choose — conservative, realistic or optimistic?

The realistic (standard Riegel) formula is the most widely validated and is a good starting point for most runners. Choose conservative if you are newer to racing, training on lower mileage, or racing in hot or hilly conditions. Choose optimistic if you are peaking in training, highly experienced at the longer distance, and racing in ideal conditions.

How accurate is the prediction?

The Riegel formula is well-validated for distances between 5 km and the marathon, and typically accurate to within a few minutes for consistent, well-trained runners. Accuracy drops for: very short distances (under 1500 m), ultras (over 50 km), runners who are not yet aerobically developed, and large jumps in distance (e.g. predicting a marathon from a 5 km time).

Can I predict a marathon from a 5K time?

Technically yes, but the further apart the two distances are, the less reliable the prediction becomes. A 5K result reflects speed and short-duration fitness, while marathon performance depends heavily on endurance adaptations that take months to build. A 10K or half marathon is a much better predictor for marathon time.