Marathon Race Predictor

The Standard: Riegel's Formula

This calculator uses Peter Riegel's industry-standard formula: T2 = T1 * (D2 / D1) ^ 1.06. * It assumes that a runner slows down slightly as distance increases due to fatigue. * Critical Caveat: The formula assumes you have done the *appropriate training volume* for the longer distance. It predicts *potential*, not certainty.

⚠️ The "Endurance Gap"

A fast 5K time demonstrates a high VO2 Max** (engine size), but a Marathon requires **Metabolic Efficiency (fuel economy). * The Problem: A 5K burns ~300 calories (glycogen stores are full). A Marathon burns ~2,600 calories. * The Wall: Your body only stores ~2,000 calories of glycogen. Without fueling (gels/carbs), you *will* run out of fuel at Mile 20 (32km). Riegel's formula cannot predict "The Wall."

📉 Prediction Accuracy by Weekly Mileage

Your weekly volume is the best indicator of whether you can hit your predicted time. | Weekly Mileage | Prediction Reliability | Advice | | :--- | :--- | :--- | | < 30km (20mi)** | **Poor | Add 15-20 mins to prediction. Survival mode. | | 30-60km (40mi)** | **Moderate | Add 5-10 mins. Likely to fade late. | | 60-90km (55mi)** | **High | Prediction is accurate. | | > 100km (65mi)** | **Very High | You might even beat the prediction. |

🏁 Course Adjustments

Not all miles are equal. * Flat (Chicago, Berlin): Trust the prediction. * Hilly (Boston, NYC): Add 2-5 minutes. * Hot (> 20°C / 68°F): Add 30-90 seconds *per mile*. Heat drives heart rate up, decoupling it from pace.

Strategic Advice

* Negative Split: Aim to run the first half 2-3 minutes *slower* than your goal pace. * Fuel Early: Take a gel every 30-45 minutes starting at minute 45. Do not wait until you are hungry.