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Free online race time predictor

Enter a finish time from any race and this calculator predicts how fast you could run every other standard distance — from 1 mile to marathon — using the proven Riegel formula.

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About this calculator

How to use

  1. Select the race distance you have a time for.
  2. Enter your hours, minutes, and seconds.
  3. Click Predict Race Times to see predictions for all five standard distances.

The Riegel formula

Source: Riegel PS. Athletic Records and Human Endurance. American Scientist, 69(3):285-290, 1981.

T2 = T1 × (D2 / D1)^1.06
  • T1 — your known finish time (seconds)
  • D1 — your known race distance (meters)
  • D2 — the target race distance (meters)
  • T2 — predicted finish time (seconds)

The exponent 1.06 reflects the non-linear relationship between distance and pace: runners slow slightly per mile as races grow longer.

Race distances used

RaceDistance
1 Mile1,609.34 m
5K5,000 m
10K10,000 m
Half Marathon21,097.5 m
Marathon42,195 m

Worked example

Known: 5K in 25:00 (1,500 seconds).

Predict 10K: T2 = 1500 × (10000/5000)^1.06 = 1500 × 2^1.06 ≈ 1500 × 2.083 = 3,125 s ≈ 52:05

Predict Marathon: T2 = 1500 × (42195/5000)^1.06 ≈ 1500 × 8.439^1.06 ≈ 1500 × 9.288 ≈ 13,932 s ≈ 3:52:12

Notes

  • Results are best treated as training targets, not race-day guarantees.
  • The Riegel formula tends to over-predict marathon times for recreational runners who need more recovery pacing.

References

Frequently asked

What is the Riegel formula?
The Riegel formula predicts race times across distances: T2 = T1 × (D2/D1)^1.06. T1 is your known time for distance D1. The exponent 1.06 accounts for the physiological cost of sustaining pace over longer distances — it's slightly harder than a straight linear scale.
How accurate are these predictions?
The Riegel formula is a well-validated approximation, but individual results vary based on training, pacing strategy, terrain, and fitness. Most runners find it most accurate within a 2x distance range (e.g., predicting a half marathon from a 10K is more reliable than predicting a marathon from a 5K).
Why does marathon time scale non-linearly from 5K time?
Running efficiency drops with distance due to glycogen depletion, muscle fatigue, and heat. The exponent 1.06 in the Riegel formula captures this effect — a typical 25-minute 5K runner finishes a marathon in around 4:00-4:30, not the 3:45 a simple linear scale would predict.
Can I use a recent training run to predict race time?
Yes, but race times are typically faster than equivalent training efforts. Use race results for the most accurate predictions. A hard tempo run time will give optimistic predictions.
What distances does this calculator support?
1 Mile, 5K, 10K, Half Marathon (21.097 km), and Marathon (42.195 km). Distances use standard road-race lengths in meters.

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