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Race Time Predictor Calculator

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Predict your finish time for any race distance from a recent race result using Riegel's formula. Compare 5K, 10K, half marathon, and marathon predictions.

Uses Riegel's formula: T2 = T1 × (D2 ÷ D1)^1.06 — an empirical model, most accurate for distances reasonably close to your known race and assuming similar training and conditions.

Predicted Finish Time

1:50:19
Predicted Pace
5:14 / km

What is a Race Time Predictor?

The Race Time Predictor Calculator estimates your likely finish time at a new race distance based on a recent performance at a different distance, using Riegel's formula — a widely used empirical model in endurance sports science. Enter your known distance and time, plus your target distance, and get a predicted finish time and pace.

This is especially useful for setting realistic goal times when moving up (or down) in race distance. For pacing out a specific marathon goal time into checkpoint splits, see the Marathon Pace Calculator.

How to use this Race Time Predictor calculator

  1. Enter your known race distance — the distance of a recent race or time trial result, in kilometers.

  2. Enter your known race time — your finish time for that race, in total minutes.

  3. Enter your target race distance — the distance you want to predict a finish time for, or pick one of the quick-select common distances (5K, 10K, half marathon, marathon).

  4. Read the predicted finish time and pace — shown in clock format for both the total time and per-km pace.

Formula & Methodology

Riegel's formula:
T2 = T1 × (D2 ÷ D1)^1.06

Variable definitions:
- T1 — known finish time at the known distance
- D1 — known race distance
- D2 — target race distance
- T2 — predicted finish time at the target distance
- 1.06 — empirically derived exponent accounting for expected endurance slowdown over longer distances

Worked example:

Known: 10K (D1 = 10 km) in 50 minutes (T1 = 50 min)

Target: Half Marathon (D2 = 21.0975 km)

Distance ratio = 21.0975 ÷ 10 = 2.10975

Ratio^1.06 ≈ 2.235

Predicted time = 50 × 2.235 = 111.7 minutes (≈ 1:51:45)

Note: Riegel's formula is a statistical model based on typical endurance performance trends — it does not account for course elevation, weather, altitude, or how specifically trained you are for the target distance. Treat predictions as a planning estimate, not a guarantee.

Frequently Asked Questions

Riegel's formula is T2 = T1 × (D2 ÷ D1)^1.06, an empirical model developed by Pete Riegel in 1977 that predicts race finish time (T2) at a new distance (D2) based on a known finish time (T1) at another distance (D1), using an exponent of 1.06 to account for endurance decay over longer distances.
If the exponent were exactly 1.0, pace would stay constant regardless of distance, but in reality, runners naturally slow down as distance increases due to glycogen depletion, fatigue, and pacing strategy. The 1.06 exponent, derived empirically from large datasets of race results, captures this expected slowdown.
Riegel's formula tends to be most accurate when predicting between distances that aren't too far apart (like 10K to half marathon) and for well-trained runners with consistent endurance across distances. It becomes less reliable for very short sprints, ultra-distances, or when the runner's training is heavily biased toward one distance type.
Yes, though predictions from a 5K to a marathon (a much larger distance jump) are generally less reliable than shorter-range predictions, since marathon performance depends heavily on endurance-specific training (like long runs and fueling strategy) that a 5K result doesn't fully capture.
The formula doesn't account for course-specific factors like elevation gain, weather conditions, altitude, race-day pacing strategy, or how well-trained the runner is specifically for the target distance — it's a statistical estimate based on typical relationships between distances, not a personalized physiological model.
Simply applying your current pace linearly would assume you could sustain the exact same per-km speed regardless of distance, which overestimates performance at longer distances. Riegel's formula instead applies a slight, distance-dependent slowdown, which better matches how real endurance performance degrades over longer races.
A common use is predicting a marathon or half marathon goal time from a recent 5K or 10K race result, to set a realistic training pace target — or the reverse, checking what 5K time would be 'equivalent' in effort to a known marathon performance.
This calculator predicts your likely finish time at a new distance based on a past performance, while the [Marathon Pace Calculator](/marathon-pace-calculator/) works in the opposite direction — starting from a target marathon finish time and calculating the pace and checkpoint splits needed to hit it.
Treat the predicted time as a reasonable estimate rather than a guarantee — actual race performance depends on training specificity, taper, weather, course profile, and pacing execution on race day, all of which can shift your actual result from the statistical prediction.
Riegel's formula was developed and validated specifically for running (and has some validated use in other endurance sports like swimming and cycling with sport-specific exponents), but the version and exponent used in this calculator (1.06) is calibrated for running performance specifically.
Enter both the known and target distances in kilometers, and the known race time in total minutes (for example, a 45-minute 10K would be entered as distance = 10, time = 45). The predicted time and pace are also shown in a clock-formatted display (h:mm:ss and min:sec) for easy reading.
Also known as
riegel formula calculatorrace prediction calculator5k to marathon time predictorrunning race time converter