Race Time Predictor
Predict your finish times for 5K, 10K, half marathon, and marathon from a recent race result. Uses the Riegel formula (and optional Cameron model) with pace and split details — all for free.
What Is a Race Time Predictor?
A race time predictor estimates how long it will take you to finish a race at a different distance, based on a recent performance. If you've run a 10K in 50 minutes, for example, a race time predictor can estimate your likely 5K, half marathon, and marathon times.
These predictions are not guarantees — they assume consistent training and similar race conditions. But they are extremely useful for:
- Setting realistic race goals before you toe the line.
- Choosing your next race distance based on current fitness.
- Planning race-pace training for an upcoming event.
- Tracking fitness changes over a training block.
The Riegel Formula (Primary Model)
The calculator uses the Riegel formula as its primary prediction model. Developed by engineer Pete Riegel and published in Runner's World in 1977, it is the most widely used race time prediction formula:
Riegel formula:
T₂ = T₁ × (D₂ ÷ D₁)1.06
Where T₁ is your known time, D₁ is the known distance, T₂ is the predicted time, and D₂ is the target distance. The exponent 1.06 is the "fatigue factor" — it assumes your pace slows by roughly 6% each time the distance doubles.
The Riegel formula works best when the target distance is within about 2× or 0.5× of the known distance. Predicting a marathon from a 5K is possible, but less accurate than predicting a half marathon from a 10K.
The Cameron Formula (Optional Model)
For advanced users, the calculator also offers the Cameron model, which uses a distance-weighted correction factor:
Cameron model:
T₂ = T₁ × (D₂ ÷ D₁) × f(D₁) ÷ f(D₂)
Where f(x) = 13.49681 − 0.000030363 × x + 835.7114 ÷ x0.7905 (x in metres). This model tends to produce more conservative (slower) predictions than Riegel, especially when predicting long distances from short ones.
Cameron's model is particularly useful if you're predicting a marathon from a shorter race — it accounts for the disproportionate difficulty of the marathon distance.
Why Use This Race Time Predictor?
- Four standard distances: 5K, 10K, half marathon, and marathon predictions in one click.
- Custom distance option: Predict any distance you want (e.g., 15K, 30K, or even 1 mile).
- Two formulas: Compare Riegel and Cameron side by side if you want a range.
- Pace details: Every predicted time comes with pace in min/km and min/mi.
- Visual chart: See predicted times across distances at a glance.
- Free & Private: No registration, no data storage.
Understanding Your Predictions
- Short-to-long predictions (e.g., 5K → marathon) tend to be less accurate than predictions between similar distances. Use them as a guide, not a certainty.
- If you're new to a distance, your actual time may be slower than predicted — endurance takes time to build.
- If you're highly trained for a specific distance, your actual time may be faster than predicted, because the Riegel formula assumes general fitness, not event-specific preparation.
- Course conditions matter: Hills, heat, wind, and altitude can all shift your finish time by several minutes.
Note: These predictions assume even pacing and similar conditions to your known race. Use them as a training and goal-setting tool, not as a guarantee of race-day performance.
How to Use the Predictions in Training
- Race-pace workouts: Use your predicted time to calculate target pace for tempo runs and interval sessions.
- Goal setting: Pick a realistic target time based on a recent race — not a wish.
- Progress tracking: Re-run the predictor after a training block and compare predictions to see fitness gains.
- Distance selection: If your 10K predicts a marathon time that's much slower than you hoped, consider a half marathon first.
❓ Race Time Predictor FAQ
What is the Riegel formula?
The Riegel formula is T₂ = T₁ × (D₂/D₁)^1.06, where T₁ is your known time, D₁ is the known distance, T₂ is the predicted time, and D₂ is the target distance. The exponent 1.06 is the fatigue factor — it assumes your pace slows by about 6% every time the distance doubles.
How accurate is the Riegel formula?
It's most accurate when the target distance is within about 2× or 0.5× of the known distance (e.g., 10K → half marathon). Predictions between very different distances (5K → marathon) are less reliable. It also assumes consistent training and similar race conditions.
What is the Cameron formula?
The Cameron model uses a distance-weighted correction factor: T₂ = T₁ × (D₂/D₁) × f(D₁)/f(D₂), where f(x) = 13.49681 − 0.000030363x + 835.7114/x^0.7905 (x in metres). It produces more conservative predictions than Riegel, especially for long distances predicted from short ones.
Which formula should I use?
Riegel is the standard and works well for most runners. Use Cameron if you're predicting a marathon from a shorter race — it accounts for the disproportionate difficulty of the marathon. The "Show both" option lets you compare.
Why are my predictions different from my actual race times?
Predictions assume even pacing, similar conditions, and consistent training. Factors like hills, heat, wind, altitude, race-day nutrition, and event-specific preparation can all shift your actual finish time. Use predictions as a guide, not a guarantee.
Can I predict a marathon from a 5K?
Yes, but accuracy drops significantly. The Riegel formula works best when distances are within 2× of each other. For a marathon prediction, a half marathon result is much more reliable than a 5K or 10K.
Does gender affect race time prediction?
The Riegel and Cameron formulas are gender-neutral — they only use distance and time. Gender is included in this calculator for reference, but it does not change the predicted times. Age-grading calculators can adjust predictions for age and gender separately.
What is a good 5K time?
It depends on age, gender, and experience. For a recreational runner, 25–35 minutes is common. For competitive runners, sub-20 minutes is a strong benchmark. Use the predictor above to see how your 5K compares to predictions from other distances.
Can I use this predictor for trail races?
The formulas assume road racing with relatively even pacing. Trail races involve elevation, technical terrain, and often much slower paces. Predictions from road races to trail races will be overly optimistic. Use trail-specific predictors or add a significant adjustment factor.
How often should I re-predict my race times?
Re-run the predictor after each race or time trial, or every 4–8 weeks during a training block. As your fitness changes, your predicted times will update accordingly.
What is a custom distance?
The custom distance option lets you predict a time for any distance — for example, 15K, 30K, 1 mile, or 50K. Enter the distance in kilometres, and the calculator will apply the chosen formula.
How do I convert pace between min/km and min/mi?
1 mile = 1.60934 km. To convert min/km to min/mi, multiply by 1.60934. To convert min/mi to min/km, divide by 1.60934. This calculator shows both automatically.