Running Race Time Predictor: Riegel formula

This calculator predicts your race time at a target distance based on a known time at another distance, using Riegel's formula which accounts for accumulated fatigue.

Riegel's formula

The predictive formula is:

T2 = T1 × (D2 / D1)^1.06

Where T1 is the known time, D1 is the known distance, D2 is the target distance and 1.06 is the fatigue exponent. This exponent reflects that pace decreases as distance increases.

Example 1: from 5K to 10K

Problem: You run 5 km in 25 minutes. What would be your 10 km time?

1. Calculation:
   • T2 = 25 × (10/5)^1.06 = 25 × 2^1.06 ≈ 25 × 2.08 ≈ 52.0 minutes.

Answer: Estimated 10K time: 52:00.

Example 2: from half marathon to marathon

Problem: Half marathon (21.1 km) in 1h 50min (110 min). Marathon time (42.2 km)?

1. Calculation:
   • T2 = 110 × (42.2/21.1)^1.06 = 110 × 2^1.06 ≈ 110 × 2.08 ≈ 228.8 min ≈ 3h 49min.

Answer: Estimated marathon time: 3:49:00.

Common uses of the race predictor

• Setting time goals for races at different distances.
• Planning target pace for a competition.
• Evaluating whether a target time is realistic.
• Comparing relative performance across different distances.
• Planning training periodization.
• Staying motivated by seeing potential progression.

Common mistakes with time prediction

• Not considering that the formula assumes specific training for the distance.
• Using unrepresentative race times (with stops or adverse conditions).
• Predicting very large jumps (5K to ultramarathon) where the formula loses accuracy.
• Ignoring factors like course profile and weather conditions.

Pro tip

Riegel's formula is more accurate for similar distances. If predicting from 5K to marathon, add 5-10% to the result because fatigue affects longer distances more.