Marathon Time Predictor (from 5K, 10K, or Half)

Marathon Prediction Guide

What should I know about The Riegel Formula?

Peter Riegel's 1977 formula: T2 = T1 × (D2/D1)^1.06. Where T = time, D = distance, exponent 1.06 represents the typical race performance fatigue factor. Used by virtually all major running calculators. Accuracy: very accurate from 5K to half-marathon predictions. Less accurate for marathon prediction — the 1.06 exponent works for shorter distances but marathon has additional 'wall' factor most runners experience. Standard Riegel marathon predictions are systematically optimistic by 10-30 minutes for most runners, which is why experienced coaches often apply a correction factor specifically for the marathon distance.

What should I know about Typical Predictions?

From a 22-minute 5K (4:30/mile): Riegel marathon = 3:31. Realistic for high-mileage runner: 3:35-3:45. Realistic for moderate runner: 3:45-4:00. Realistic for low-mileage: 4:00+. From a 45-minute 10K: Riegel marathon = 3:30. Note: 10K time gives more accurate marathon prediction than 5K. The aerobic component is more similar. From 1:35 half-marathon: Riegel marathon = 3:20. Closer prediction — half marathon physiology much more similar to marathon. Half-marathon time × 2 + 10-15 minutes gives a more realistic marathon estimate for most runners than the pure Riegel formula.

What should I know about Adjustments for Training Volume?

Mileage matters MORE than speed for marathon prediction. Two runners with same 10K time: 60-mile/week runner: closer to Riegel marathon prediction. 25-mile/week runner: 15-30 minutes slower marathon. The aerobic engine needed for marathon is built by total time on feet, not interval workouts. Long run distance: peak long run within 6 weeks of marathon should reach 18-22 miles. Without this: very high risk of crashing in last 6 miles. Marathon training plans: minimum 16-week build, 4-5 runs per week, is the widely recommended baseline for a first-time marathoner aiming to finish comfortably rather than simply survive.

What's the key thing to understand about Race-Day Pacing?

Negative split strategy: run the second half slightly faster than the first. Conservative first 6-10 miles at goal pace + 10-15 seconds. Settle into goal pace miles 10-20. Accelerate slightly final 10K if energy allows. Positive split (first half faster): nearly always ends in crashing. Almost every sub-3-hour marathon is negative split. Pace bands: many marathons sell pace bands showing splits for target time. Useful for staying disciplined. Pacing groups: official pacers at major marathons, such as London, run at a steady, practised pace and carry a sign showing their target finish time — a reliable way to hit a specific goal without constantly checking your own watch.