Pace Calculator · Practical guide
Race Time Predictor: 10K to Half Marathon
Your 10K time predicts your half marathon with surprising accuracy — here is the math.
At a glance
The Riegel Formula
The standard race-time predictor is T2 = T1 × (D2 ÷ D1)^1.06 — your time scales with distance raised to the 1.06 power, which builds in a realistic slowdown as races get longer. Enter any recent result above to see predicted 5K, 10K, half marathon, and marathon times.
Interactive tool
Pace Calculator
Enter your own values, then compare the result with the examples below.
Results
Example: A 50:00 10K
| Distance | Predicted Time |
|---|---|
| 10K (your result) | 50:00 |
| Half marathon | 1:50:18 |
| Marathon | 3:50:03 |
Example: A 25:00 5K
The same formula maps a 25:00 5K to a 52:07 10K, a 1:55:00 half marathon, and a 3:59:47 marathon. Notice how the pace drifts slower at each step — that is the 1.06 exponent at work.
When to Trust the Prediction
Predictions are tightest between neighboring distances — 10K to half marathon is a reliable jump for most trained runners. From 5K to marathon the formula can flatter runners who have not built long-run endurance, so treat long-range projections as best-case targets, not guarantees.
Frequently Asked Questions
How accurate is the Riegel formula?
Most accurate between similar distances, like 10K to half marathon. It assumes comparable training and conditions at both distances, which is why big jumps like 5K to marathon come with more error.
What half marathon time does a 45-minute 10K predict?
About 1:39:16. The prediction is simply 45 minutes scaled by the distance ratio raised to the 1.06 power.
Can I predict my 5K from a marathon time?
Yes — the formula works in both directions. Predictions from longer to shorter distances tend to be conservative if you have more endurance than speed.
Why is my real marathon slower than predicted?
The formula cannot account for training volume, fueling, heat, or hills. Marathon performance depends heavily on long-run mileage that shorter race times do not reveal.