Marathon Time Predictor
Predict your full marathon finish time using your recent 10K result and training level. Useful for race-day pacing plans and training block goal-setting.
About this calculator
This predictor uses a scaling formula derived from race-performance models: Marathon Time = (10K Time ÷ 10) × 4.7 × Training Factor. The constant 4.7 reflects the physiological cost multiplier between 10 km and 42.195 km — a well-trained runner slows disproportionately over the marathon distance due to glycogen depletion and fatigue. The Training Factor adjusts for preparation quality: a value of 1.0 represents solid consistent training, while values above 1.0 indicate under-preparation and values below 1.0 indicate peak fitness. Inputting your best recent 10K time gives a realistic marathon projection, not just a linear extrapolation. This approach is similar in spirit to Pete Riegel's endurance formula and is widely used by coaches for goal-pace assignment.
How to use
Say your recent 10K time is 50 minutes and your training level factor is 1.0 (solid base training). Apply the formula: Marathon Time = (50 ÷ 10) × 4.7 × 1.0 = 5 × 4.7 = 23.5 minutes... wait — the output is in minutes: 235 minutes, or 3 hours 55 minutes. If your training has been inconsistent, use a factor of 1.05: (50 ÷ 10) × 4.7 × 1.05 = 24.675 minutes × 10 = 246.75 minutes ≈ 4 hours 7 minutes. This shows how even slight under-preparation meaningfully shifts your predicted finish.
Frequently asked questions
How accurate is predicting a marathon time from a 10K race result?
Pace-scaling predictions from 10K times are reasonably accurate within ±5–10% for runners with consistent aerobic bases and sound marathon training. The further your goal race is from the predictor distance, the more variables — long-run preparation, fueling strategy, and heat — introduce error. Runners who have not completed long training runs of 28–32 km should treat the prediction as optimistic. The training factor input helps account for these variables by allowing you to adjust the projection toward a more conservative estimate.
What training factor should I use in the marathon time predictor?
A training factor of 1.0 suits a runner who has completed a full 16–20 week marathon training plan with consistent weekly mileage and long runs reaching at least 30 km. Use 1.05–1.10 if you have missed several long runs or your weekly volume was below plan. Highly trained runners peaking with extra marathon-specific work can experiment with 0.95. When in doubt, use 1.0 and build in a conservative first-half pacing strategy on race day.
Why does the marathon take so much longer than 4.2 times a 10K pace?
The human body stores roughly 90 minutes of glycogen at marathon race effort, after which it must rely increasingly on fat metabolism, which generates energy more slowly. This metabolic shift, combined with cumulative muscular damage from thousands of strides, causes pace to degrade significantly in the final third of a marathon. The 4.7 multiplier (versus a theoretical 4.2195) captures this real-world slowdown. Proper carbohydrate fueling during the race can reduce but not eliminate this effect, which is why marathon pacing strategy matters enormously.