Marathon Time Predictor
Forecast your marathon finish time and establish realistic race-day pacing splits using the benchmark Pete Riegel endurance formula. Input a recent race performance from a 5K, 10K, or Half Marathon to calculate your predicted 42.195-kilometre finish time, required per-kilometre and per-mile splits, and equivalent race benchmarks.
Choose your most recent and accurately measured road race performance
Hours component of your baseline race finish time
Minutes component of your baseline finish time
Seconds component of your baseline finish time
Estimated 42.195 km marathon finish time (hh:mm:ss) assuming adequate weekly mileage
How to Use This Calculator
- Select the distance of a recent competitive race (Half Marathon is most reliable).
- Enter your official chip-timed finish time in hours, minutes, and seconds.
- View your predicted marathon finish time and target pacing boundaries.
- Check equivalent race times for 5K and 10K to validate intermediate training blocks.
- Formulate a conservative negative-split strategy for the first 30 kilometres.
Formula & Calculation Methodology
Pete Riegel endurance model published in American Scientist (1981). The 1.06 exponent models the predictable exponential physiological decay of human sustainable pace as distance increases, assuming the runner is properly trained for the target distance.
Worked Example
A runner who completed a Half Marathon in 1:45:00
Frequently Asked Questions
Why is the Half Marathon the best predictor for a full Marathon?↓
The half marathon requires both substantial aerobic lipid metabolism and glycogen management over 70 to 120 minutes of continuous running. Shorter races like the 5K rely more heavily on anaerobic threshold and VO2 max, which can overestimate marathon endurance if weekly long-run volume is lacking.
Under what conditions does Riegel formula fail or become inaccurate?↓
Riegel formula assumes the runner has completed sufficient long runs (peaks of 28–35 km) and weekly mileage (at least 55–80 km/week). Without adequate muscular endurance and carbohydrate fuel replenishment, runners inevitably hit the wall around kilometre 32.
What is the 2× Half Marathon + 10 minutes rule of thumb?↓
A classic marathon rule of thumb is: Marathon Time = (Half Marathon Time × 2) + 10 to 15 minutes. For a 1:45:00 half-marathoner, this yields 3:40:00 to 3:45:00, which closely aligns with Riegel calculated 3:38:59.
How should weather conditions affect my marathon time goal?↓
Ideal marathon temperatures are between 7°C and 11°C (45°F–52°F). For every 5°C rise above 15°C (59°F), exercise physiologists recommend adjusting your target pace by 1.5% to 3% to counter increased cardiovascular strain from thermal regulation.
What pacing strategy is most effective for achieving the predicted time?↓
Virtually all marathon world records feature even or slight negative splits (running the second half 1 to 2 minutes faster than the first). Running the first 10K too fast expends scarce muscle glycogen prematurely.
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Open calculator →Source Citation & Disclaimer
Informational tool only; not licensed tax, legal, or financial advice.