Running Time Predictor

Predict your race times and paces for any distance from a recent performance. Compare Riegel and VO2-based models and fine‑tune goals with custom distances.

1. Enter your recent race

Lower = better endurance, higher = more speed‑oriented

2. Choose target distances

Standard distances

Custom target distance

3. Predicted race times & paces

Distance Predicted time Pace /km Pace /mile Speed Model
Enter a race result and click “Calculate predictions” to see your equivalent times.

Pacing planner (optional)

Turn any predicted time into a simple pacing plan.

How the running time predictor works

This tool predicts your performance at other distances from a single recent race or time trial. It combines two well‑known approaches:

  • Riegel power‑law model – widely used for race equivalence.
  • VO2-based model – similar in spirit to VDOT calculators, using a simple VO2 vs. pace curve.

1. Riegel prediction formula

The Riegel model assumes that your race time grows with distance according to a power law:

Let:

  • \(T_1\) = time for distance \(D_1\)
  • \(T_2\) = predicted time for distance \(D_2\)
  • \(E\) = endurance exponent (default 1.06)

Then:

\[ T_2 = T_1 \left(\frac{D_2}{D_1}\right)^E \]

Typical values of \(E\):

  • 1.03–1.05 – very strong endurance (marathon specialists).
  • 1.06 – average trained runner (our default).
  • 1.07–1.10 – speed‑oriented runners or those under‑trained for long distances.

2. VO2-based model (VDOT‑style)

VO2-based calculators estimate your aerobic capacity from a known performance and then find the pace that corresponds to the same effort at other distances.

A simplified version:

Estimate effective VO2 from your input race:

\[ \text{VO}_2 \approx a \cdot v + b \]

where \(v\) is speed (m/s) and \(a, b\) are empirical constants. Then invert the relationship to find the speed that matches the same VO2 at another distance and convert that speed to time.

In practice, VO2-based predictions behave similarly to Riegel but can differ slightly at very short or very long distances. Showing both helps you see a realistic range.

3. Blended estimate

The default “Blended” option simply averages the Riegel and VO2-based predictions (after converting both to seconds). This tends to smooth out extremes and gives a practical target time for most runners.

Interpreting your predictions

  • Shorter than input distance – predictions are usually quite accurate if you pace well.
  • Similar distance – expect predictions to be within a few percent of your real performance.
  • Much longer than input distance – treat as optimistic best‑case unless you have the training volume and fueling to support it.

Example

Suppose you run a 10 km race in 45:00 (4:30/km). Using the default exponent \(E = 1.06\):

\(T_1 = 45 \text{ min} = 2700 \text{ s}\), \(D_1 = 10 \text{ km}\), \(D_2 = 21.0975 \text{ km}\)

\[ T_2 = 2700 \left(\frac{21.0975}{10}\right)^{1.06} \approx 2700 \times 2.13 \approx 5751 \text{ s} \]

\(5751 \text{ s} \approx 1:35:51\) for a half marathon.

Tips for better predictions

  • Use a race or time trial run at maximum sustainable effort, not an easy long run.
  • Make sure the course was measured accurately and not excessively hilly or technical.
  • Use a performance from the last 4–8 weeks to reflect current fitness.
  • For marathons, consider adding a small buffer (e.g., 2–5%) to the predicted time unless you are well‑prepared.

Running time predictor FAQ

How does this running time predictor compare to other tools?

Many online calculators use only the Riegel formula or a single VO2-based model. This tool shows three perspectives (Riegel, VO2, blended), supports custom distances, and includes a built‑in pacing planner so you can turn predictions into practical race strategies.

Which prediction should I trust for my race goal?

For most runners:

  • Use the blended prediction as a realistic goal.
  • If you know you have strong endurance, lean toward the Riegel prediction (faster).
  • If you struggle at longer distances, lean toward the VO2 prediction (slightly slower).

Can I adjust the Riegel exponent?

Yes. Advanced users can change the exponent between 1.00 and 1.20. Lower values assume better endurance (smaller slowdown with distance), higher values assume more drop‑off. If you have several race results, you can experiment to find an exponent that best fits your personal data.

Does this account for hills, heat, or altitude?

No. The calculator assumes similar conditions between your input race and target race. If your upcoming race is significantly hotter, hillier, or at altitude, you should adjust your expectations upward manually.