Race Time Predictor
Predict your finish time at any race distance using a recent result from a different distance. Ideal for setting realistic goals when stepping up from a 5K to a half marathon or full marathon.
About this calculator
This calculator applies Pete Riegel's widely validated endurance performance formula: predictedTime = knownTime × (targetDistance / knownDistance)^1.06. The exponent 1.06 is empirically derived from thousands of race results and encodes the physiological reality that performance degrades slightly more than linearly as distance increases. A pure linear scaling (exponent = 1.0) would assume no fatigue penalty, while a higher exponent reflects the rising cost of sustaining pace over longer efforts. Known time is converted entirely to minutes before calculation—(knownTimeHours × 60 + knownTimeMinutes)—ensuring consistent units. The resulting prediction is most accurate when the two distances are within roughly three times of each other (e.g., 5K to half marathon) and assumes comparable training, conditions, and pacing.
How to use
Scenario: you ran a 10K in 48 minutes (0 hours, 48 minutes) and want to predict your half-marathon (21.1 km) time. Step 1: convert known time → 0 × 60 + 48 = 48 minutes. Step 2: compute distance ratio → 21.1 / 10 = 2.11. Step 3: raise to exponent → 2.11^1.06 ≈ 2.23. Step 4: multiply → 48 × 2.23 ≈ 107 minutes. Your predicted half-marathon time is approximately 1 hour 47 minutes. This gives a realistic goal time and helps set target pace per kilometre for race-day strategy.
Frequently asked questions
How accurate is the Riegel formula for predicting marathon times from shorter race results?
The Riegel formula is most accurate for predictions within a factor of roughly three in distance—for example, using a 10K to predict a half marathon. When extrapolating to the marathon from a 5K, accuracy decreases because the physiological demands shift significantly toward fat metabolism, mental endurance, and fueling strategy. Studies show the formula tends to underestimate marathon times by 2–5% for recreational runners who haven't done marathon-specific long runs. To get a better marathon estimate, use the longest race result you have available, ideally a half marathon, and add a small buffer of 5–10 minutes if your longest training run is under 18 miles.
What exponent should I use for ultra-marathon distance race time predictions?
The standard Riegel exponent of 1.06 begins to underestimate finishing times beyond the marathon distance because physiological and logistical factors—sleep deprivation, mandatory rest, extreme nutrition demands—become dominant. Some researchers suggest exponents of 1.10 to 1.15 for distances of 50 miles and beyond. For 100-mile events, empirical finishing-time databases show even greater deviation from the standard formula. This calculator uses the validated 1.06 exponent, making it best suited for road race distances from 1 mile through the marathon. For ultras, consult distance-specific pace calculators or race-history data from the specific event.
Why does my predicted race time differ from what I actually ran?
Several real-world factors cause deviations from formula predictions. Course elevation, weather temperature and humidity, race-day pacing strategy, and training specificity all play significant roles that no formula can capture. The Riegel formula also assumes you are equally well-trained for both distances; if you have done extensive 5K speedwork but no long runs, your actual marathon will be slower than predicted. Additionally, the formula is calibrated on average population data and individual running economy varies widely. Treat the prediction as a starting benchmark for goal-setting and pacing, not a guaranteed outcome, and adjust based on your training history and course conditions.