Marathon Time Predictor

Marathon Time Predictor

Use this marathon time predictor based on the Riegel performance model to estimate your marathon finish time from a recent race result. Adjust the fatigue exponent, course profile, and temperature to refine your prediction, then review target pace and even splits.

Accepts H:MM:SS or MM:SS
Advanced adjustments
Typical range 1.03–1.10 (lower = better endurance)
Adjust for heat; neutral ~10–12 °C
Rough additive cost factor

Marathon Time Predictor – Estimate Finish Time from Your Recent Race

This marathon time predictor estimates your marathon finish time using the Riegel model, an empirically validated relationship between performance at two different distances. Enter a recent race (distance and time), keep the default fatigue exponent or fine-tune it, and optionally adjust for temperature and course profile. You’ll also get an even-pace target (per km and per mile) and a split table to plan your checkpoints.

How the Marathon Time Predictor Works

Jack Riegel proposed a simple formula linking performances at different distances: T₂ = T₁ × (D₂/D₁)k, where T is time, D is distance, and k is an exponent that reflects endurance or fatigue. For many trained runners, k ≈ 1.06 is a reasonable default: shorter-distance speed does not translate linearly to marathon performance; the exponent introduces the realistic slowdown over longer distances. (Wikipedia – Riegel formula)

Inputs Explained

  • Recent race distance & time: choose a standard preset (5K, 10K, Half) or enter a custom distance. Make sure the result is from the last 8–12 weeks to reflect current fitness.
  • Target distance: default is the marathon (42.195 km), but you can choose Half, 10K, or any custom distance for general race-time prediction.
  • Fatigue exponent (k): the lower the value, the better your endurance relative to speed. Strong endurance athletes may sit around 1.03–1.05; newer runners or speed-biased athletes may need ~1.07–1.10.
  • Temperature: performance typically degrades above ~12 °C; heat adds cardiovascular and hydration stress. The predictor includes a simple penalty factor to remind you to pace conservatively in warm conditions. See research on temperature effects in endurance performance via NIH/PMC.
  • Course profile: even small rolling hills change energy cost. A gentle percentage factor can nudge the estimate up or down to reflect the course’s nature.

Interpreting the Prediction

Race performance is probabilistic. This marathon time predictor delivers a central estimate assuming even pacing, normal conditions, and intact fueling. A single recent race doesn’t capture every variable (terrain, taper, wind). Treat the output as a guidance point and adjust with your coach and training logs.

Target Pace and Splits

Once you have the predicted finish time, the tool converts it to target pace per kilometer and per mile, then creates an even-pace split table. Even pacing is often optimal when conditions are steady. Small negative splits (slightly faster second half) may work for experienced runners on flat courses. If your marathon is hilly, plan modest positive splits on uphill sections and even/negative splits on downhills to normalize effort.

Choosing the Fatigue Exponent

  • 1.03–1.05: excellent endurance relative to speed; results may come from higher mileage, long tempo runs, and consistent fueling.
  • 1.06 (default): typical trained runner; a reliable starting point if you aren’t sure.
  • 1.07–1.10: speed-skewed profile or insufficient long-run endurance; consider conservative pacing and more frequent fueling.

Use multiple recent races to triangulate your personal exponent: run the predictor with a 10K and a Half Marathon and see which k makes both predictions align with actuals.

Temperature and Course Adjustments

Scientific and coaching practice agree that heat hurts endurance. Warm races (≥ 18–20 °C) require slower targets and aggressive hydration/cooling strategies. This marathon time predictor includes a pragmatic penalty model (slight increases per degree away from the “neutral” range) to remind you to adjust. For courses, gentle rollers add drift; net-downhill courses, while faster, can tax quads late — the model allows a modest reduction but strategy still matters. For pacing insights, see Runner’s World and Strava Blog.

Training Implications

  • Long runs: cornerstone of marathon prep; simulate fueling and late-race fatigue.
  • Marathon-pace segments: rehearsal for your predicted pace; confirm comfort under realistic fatigue.
  • Threshold work: improves sustainable speed; narrows the gap between 10K/Half pace and marathon pace.
  • Recovery: honest easy days support consistent training and reduce injury risk.

Limitations and Good Practices

  • Use a recent race similar to your marathon conditions when possible (terrain, weather).
  • Beware of outlier performances (illness, extreme heat/cold, heavy wind) — they skew predictions.
  • Re-run the marathon time predictor as fitness changes (every few weeks).
  • Remember fueling: even the perfect pace fails without adequate carbohydrates, fluids and sodium.

Related Tools

Further Reading

Disclaimer: This marathon time predictor provides estimates for planning and educational purposes. Race day outcomes vary with training, health, weather, elevation, fueling and execution. Listen to your body and professional guidance.