Tools
Race Time Predictor
Ran a recent race? Find out what that fitness is worth at 5K, 10K, half marathon, marathon — or any distance you pick.
Predicted Finish Time
52:07
Pace / km
5:13
Pace / mile
8:23
| Distance | Predicted Time |
|---|---|
| 5K | 25:00 |
| 10K | 52:07 |
| Half Marathon | 1:55:00 |
| Marathon | 3:59:47 |
Predictions assume similar fitness, course, and conditions — they are a starting point for goal setting, not a guarantee.
Turn that prediction into a plan
Connect your Garmin and Coach Icarus builds your training around the goal time — adjusting every week as your fitness changes.
Get Started FreeHow Race Time Prediction Works
In the late 1970s, researcher Peter Riegel noticed a pattern in world records: as distance doubles, performance drops off by a remarkably consistent amount. The relationship T₂ = T₁ × (D₂ ÷ D₁)^1.06 predicts a finish time at one distance from a known time at another — and it has held up for decades across millions of runners.
The formula's strength is also its limit: it assumes the same fitness, similar courses, and comparable conditions. It cannot know that you have never run further than 10K, or that your target marathon is hilly in August. Use the prediction as an honest starting point, then let your training — especially your long runs — tell you whether to adjust it up or down.
The equivalent-performances table is useful beyond race day: it gives you realistic target paces for workouts. A predicted half marathon time sets your tempo pace; a predicted 10K time sets your interval pace. Predictions earn their keep in training, not just on the start line.
Frequently Asked Questions
How does the race time predictor work?
It uses Riegel's formula: predicted time = recent time × (new distance ÷ recent distance)^1.06. The 1.06 exponent captures how endurance fades as distance grows — it was derived from world-record performances and holds up well for trained runners across standard race distances.
How accurate is Riegel's formula?
Surprisingly good for predictions within a reasonable range — a 10K result predicts a half marathon well. Accuracy drops when extrapolating far (a 5K predicting a marathon) or when fitness is very new, since beginners improve faster than the formula assumes. Treat it as a starting point for goal setting, then adjust with training.
Can I predict my marathon time from a 5K?
You can, but take it with salt. A 5K mostly tests speed and VO2max; a marathon mostly tests endurance and fueling. The prediction assumes you've done the long-run training a marathon requires. If you haven't, the number is optimistic — which is useful information in itself.
Do course and weather affect the prediction?
Yes. The formula assumes similar courses and conditions. A flat, cool 10K predicts a hilly, hot half marathon poorly. Use results from comparable courses, or nudge the prediction slower when the target race is harder than the source race.
What is a good predicted time to aim for in training?
Use the prediction to set target paces for workouts: threshold sessions near your predicted 10K–half marathon pace, long runs well below it. A prediction is most useful as a training input, not just a race-day number.