Which race should I use as input?

Use your most recent race where you ran at maximum sustainable effort and paced reasonably well. Ideally, the race should be within the last 4–8 weeks and not dramatically shorter or longer than your target distance. For marathon predictions, a recent 10K or half marathon usually works better than a 5K.

What is the Riegel formula used in this calculator?

The Riegel formula estimates performance at a new distance based on a known performance: T2 = T1 × (D2 / D1)^1.06, where T1 and D1 are your known time and distance, and T2 and D2 are the predicted time and distance. The exponent 1.06 reflects how pace typically slows as distance increases.

Can this predictor adjust for hills, heat, or fitness changes?

Yes. The advanced options let you apply simple adjustment factors for course difficulty, temperature, and training status. These are based on common coaching rules of thumb and give you a more realistic range when conditions differ from your input race.

Is this calculator suitable for beginners?

It works for beginners, but predictions are usually less stable when you are new to running because your fitness can improve quickly between races. Use the results as a rough guide and err on the conservative side when setting race goals or pacing plans.

Race Time Predictor – Predict Your Finish Time for Any Distance

Q: How accurate is this race time predictor?

For most runners, predictions based on a recent race of similar distance and conditions are often within a few percent of actual performance. However, accuracy depends on factors like training, pacing, weather, terrain, and how well the input race reflects your current fitness. Treat the result as a realistic range, not a guarantee.

Split	Cumulative time
Run a prediction to see detailed splits.

How the race time predictor works

This race time predictor estimates your finish time for a new distance based on a recent race. It combines two well-known approaches:

Riegel model – models how performance changes with distance.
Pace-based model – scales your current pace up or down with distance.

The tool also lets you adjust for course difficulty, temperature, and fitness changes to get a realistic range instead of a single number.

1. Riegel formula

The Riegel formula is a classic way to predict race times:

Riegel formula

\( T_2 = T_1 \times \left(\dfrac{D_2}{D_1}\right)^k \)

\(T_1\) – time of your known race
\(D_1\) – distance of your known race
\(T_2\) – predicted time for the new race
\(D_2\) – distance of the new race
\(k\) – fatigue exponent (typically around 1.06 for distance running)

In this calculator we use \(k = 1.06\) by default, which works well for most runners from 3K to marathon. Short sprints or ultra distances may deviate more.

2. Pace-based model

The pace-based model starts from your average pace in the input race and applies a small slowdown as distance increases:

Pace-based model

1. Compute your current pace: \( P_1 = \dfrac{T_1}{D_1} \)

2. Apply a distance factor: \( P_2 = P_1 \times \left(\dfrac{D_2}{D_1}\right)^c \)

3. Predicted time: \( T_2 = P_2 \times D_2 \)

\(c\) – smaller exponent than Riegel (e.g. 0.03–0.05) to reflect a gentler slowdown in pace.

This gives a slightly more conservative prediction for big jumps in distance and can be more realistic for newer runners.

3. Combined prediction and adjustments

The “Combined (recommended)” result blends both models and then applies simple adjustment factors:

Course difficulty – hilly or technical courses add time; fast/flat courses subtract a little.
Temperature – above ~18°C (65°F) we apply a small slowdown that grows with heat.
Fitness change – if you’ve improved or detrained since the input race, we scale the time accordingly.

The “likely range” you see in the results is ±2–4% around the combined prediction, depending on how extreme the adjustments are.

Tips for getting realistic predictions

Use a race from the last 4–8 weeks that reflects your current fitness.
Avoid using training runs with lots of stops, surges, or very uneven pacing.
For marathon predictions, a recent 10K or half marathon is usually more reliable than a 5K.
Be conservative if the target race is much hotter, hillier, or at higher altitude than your input race.

Example

Suppose you recently ran a 5K in 25:00 (5:00/km) on a flat course at 15°C, and you want to predict a 10K:

Input race: 5 km, 0:25:00 → pace = 5:00/km.
Target race: 10 km, similar conditions.
Riegel model predicts roughly 52–53 minutes.
Pace-based model predicts slightly faster or slower depending on the exponent.

The combined prediction might be around 52:30 with a likely range of ~51:30–53:30, assuming similar training and conditions.

Frequently asked questions

How accurate is this race time predictor?

For experienced runners using a recent race of similar distance and conditions, predictions are often within a few percent of actual performance. For beginners, very long races, or big changes in conditions, the error can be larger. Treat the result as a planning tool, not a guarantee.

Can I use a training run instead of a race?

You can, but results will usually be less accurate. Training runs are often paced more conservatively and may include stops or terrain changes. If you use a training run, choose one where you ran hard and steadily, and consider the prediction a bit conservative.

How should I use the prediction for pacing?

Many runners aim to start slightly slower than the predicted average pace for the first 10–20% of the race, then settle into target pace, and only speed up in the final third if they feel strong. Use the splits table to plan your checkpoints (e.g., every km or mile).

Race Time Predictor

Race Time Predictor Tool

1. Recent race performance

2. Target race

3. Advanced options (optional)

Predicted finish time

Pacing splits