Marathon Finish Time Predictor

How This Marathon Predictor Works

This tool uses the Riegel formula to predict your race finish time based on a recent performance at a different distance. The formula accounts for the fact that pace slows as distance increases, using an empirically-derived fatigue factor.

The Prediction Formula

The tool calculates your predicted time using:

Where the exponent 1.06 represents the typical performance decline as distance increases. This formula has been validated across thousands of race performances.

Example Prediction

Using Your Prediction Effectively

Best Input Distances for Accuracy

• Half Marathon: Most accurate predictor for full marathon performance
• 15K-20K: Good indicator if run at race effort
• 10K: Useful but may overestimate marathon performance
• 5K: Less reliable for marathon prediction due to different energy systems

Factors That Affect Accuracy

• Training base: More weekly mileage improves endurance and accuracy
• Long run experience: Regular 20+ km runs help maintain predicted pace
• Race effort: Input time should be from a hard effort, not easy training run
• Time gap: Recent performances (within 4-8 weeks) predict better than old ones
• Course similarity: Flat to flat, hilly to hilly = better predictions

Common Mistakes to Avoid

• Using training run times instead of race efforts
• Predicting from races run in significantly different conditions
• Not accounting for marathon-specific training (long runs, nutrition)
• Assuming prediction accuracy without proper taper and recovery
• Starting too fast on race day based solely on predicted pace

Frequently Asked Questions

How accurate are marathon time predictions?

The Riegel formula is accurate within 2-5% for most runners when using half marathon or 15K times with proper marathon-specific training. Accuracy decreases when predicting from shorter distances like 5K or 10K, or when training volume is insufficient.

Why is my marathon prediction slower than double my half marathon time?

The "double half marathon plus 10-20 minutes" rule is common because fatigue compounds over distance. The Riegel formula's 1.06 exponent captures this mathematically. Even well-trained runners slow down in the second half of a marathon.

Should I use a race time or a training run time?

Always use a race time or hard effort time trial. Training runs are typically 1-2 minutes per km slower than race pace and will give you an unrealistic prediction.

Can I predict ultra-marathon times with this tool?

This formula is optimized for distances up to marathon. For ultra-marathons (50K+), different fatigue factors apply, and predictions become less reliable. Use marathon-specific predictors for best results.

What if my predicted time seems too fast or too slow?

If it seems too fast, you may need more marathon-specific training (long runs, back-to-back long runs). If it seems too slow, verify your input time was from a genuine hard effort and check that you selected the correct distances.

How should I adjust my prediction for hills or heat?

Add 3-5% time for hilly courses and 5-10% for hot/humid conditions (above 20°C / 68°F). This tool assumes ideal conditions and flat terrain. Conservative pacing is smart on challenging courses.