Marathon Time Predictor

The Riegel Formula, published by researcher Peter Riegel in 1977, is the most widely used race time prediction model in running. It accounts for the fact that you cannot maintain the same pace over longer distances due to physiological fatigue.

T₂ = T₁ × (D₂ / D₁)^1.06

Where:

• T₁ = your known race time (in any unit — seconds, minutes, etc.)
• D₁ = the distance of the race you already ran
• D₂ = the target distance you want to predict
• 1.06 = the fatigue exponent. This value accounts for the fact that average pace slows as race distance increases. A value of 1.0 would mean no slowdown at all.

Standard race distances:

Practical Example: You ran a 10K in 50 minutes and want to predict your marathon time: T₂ = 50 x (42.195 / 10)^1.06 = 50 x 4.2195^1.06 = 50 x 4.542 = 227 minutes (3:47:08) Your predicted pace slows from 5:00/km (10K) to about 5:23/km (marathon), which reflects the natural fatigue over the longer distance.

How accurate is the Riegel Formula? The formula works best when predicting a distance that is 2-4 times your known distance. Predicting a marathon from a 5K is less reliable than predicting it from a half marathon, because training specificity matters more over larger jumps. Experienced runners who train consistently for the target distance will find the predictions most accurate.

Factors that affect real-world performance:

• Terrain and elevation gain
• Weather conditions (heat, wind, humidity)
• Nutrition and hydration strategy
• Race-day pacing discipline
• Specific training for the target distance

Tips:

• Use your most recent race result for the most accurate prediction
• If you have both a 10K and half marathon time, the half marathon will give a more reliable marathon prediction
• Add 5-10% to the predicted time if it is your first race at the target distance
• Elite runners may use a fatigue factor closer to 1.05, while recreational runners may experience closer to 1.08