Race Time Predictor

Predicting a Finish Time From a Race You’ve Already Run

A race time predictor estimates your likely finish time at one race distance from a real finish time you’ve already run at a different distance, using Pete Riegel’s endurance formula. Riegel first published the formula in Runner’s World in 1977 and refined it in American Scientist in 1981, based on race data spanning sprints to ultramarathons. Enter the distance and time from a race you’ve completed, pick the distance you want a prediction for, and this calculator projects your likely finish time and pace.

The Formula

$$\vA{T_2} = \vB{T_1} \times \left(\frac{\vC{D_2}}{\vD{D_1}}\right)^{1.06}$$

Where $\vB{T_1}$ is your known finish time at distance $\vD{D_1}$, and $\vA{T_2}$ is the predicted finish time at your target distance $\vC{D_2}$.

The exponent, 1.06, is slightly above 1 — it’s a fatigue factor accounting for the fact that pace naturally slows a little as distance increases. Runners can’t hold their shorter-race pace over a longer distance indefinitely, so a straight linear scale-up (multiplying by the plain distance ratio) would understate how much slower a longer race actually runs, and overstate how much faster a shorter one runs. The same formula works in either direction — predicting a shorter race from a longer known result is just as valid as the more common longer-from-shorter case.

Worked Example

A runner finishes a 5K in 25:00 and wants to know their likely 10K time:

1. Distance ratio: $\vC{10} \div \vD{5} = 2$.
2. Apply the exponent: $2^{1.06} \approx 2.0847$.
3. Predicted time: $\vB{\text{25:00}} \times 2.0847 \approx \vA{\text{52:07}}$.
4. Predicted pace: $\vA{\text{52:07}} \div \vC{10} \approx \text{5:13/km}$.

Common Mistakes

A prediction is a realistic training target, not a guarantee — actual race-day results depend on your training between races, the course’s terrain and elevation, weather, and how well you pace yourself. Predictions also tend to be more reliable the closer the known and target distances are to each other; projecting a 5K time out to a full marathon carries more uncertainty than projecting a 10K to a half marathon.

Key Factors to Consider

• The Riegel formula assumes proportionally similar training and pacing strategy between the known and target distances. It works best as a rough guide for a well-trained runner transitioning between similar race types — someone who trains specifically for short, fast races but has done little endurance-building work may find a marathon prediction from a 5K time overly optimistic, since the formula doesn’t account for endurance-specific training gaps.
• Marathon predictions in particular tend to run optimistic for less-experienced marathoners. The marathon distance involves a distinct physiological challenge (glycogen depletion, commonly called “hitting the wall”) that shorter races don’t really test — Riegel’s own formula was calibrated across many distances and runners, but an individual’s specific marathon experience can diverge from the prediction more than for closer-distance comparisons.
• Course difficulty (hills, terrain, heat) at the target race isn’t captured by this calculator at all. A prediction assumes similarly favorable conditions to the reference race — a hilly or hot target race will likely run slower than this formula predicts, regardless of fitness.
• Using a recent, well-paced race result as the known input produces a more reliable prediction than an older or poorly-paced one. A race run months ago (before further training changes) or one where pacing broke down significantly (like an “all-out then fade” effort) is a less reliable reference point than a recent, evenly-paced race result.

Useful to Know

• Want to convert your predicted finish time into a per-mile or per-kilometer training pace? Running Pace Calculator converts between time, distance, and pace directly.
• Training toward this predicted time and want to plan your effort by heart rate zone? Target Heart Rate Calculator calculates target zones from your age and resting heart rate.
• Want to estimate the calorie cost of the race itself? Calories Burned Calculator estimates energy burned from activity, weight, and duration.