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Half marathon predictor

The half marathon (21.0975 km) rewards smart pacing as much as fitness. Feed any recent race to the Riegel formula and get a realistic target time plus goal pace.

Half marathon predictor

Riegel formula from any recent race.

Predicted half marathon time
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Enter a recent race distance and time.

Predicted pace
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Riegel exponent
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Riegel formula: T2 = T1 x (D2 / D1)^1.06. Half marathon = 21.0975 km.
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The Riegel formula

T2 = T1 x (D2/D1)^1.06 projects a known race to 21.0975 km. Predictions from a recent 10K are the most reliable; from a 5K they run slightly optimistic because endurance becomes the limiter.

Pacing the half well

Hold the predicted pace through 15K, then race the last 6K. Going out even 10 seconds per km too fast in the first half is the classic way to blow up at 18K.

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Half Marathon Pred FAQs

What is a good half marathon time?
Under 2:00 is a great recreational goal, under 1:45 is strong, under 1:30 is advanced. Your prediction shows what is realistic now.
Can a 10K predict my half marathon?
Yes, well. A 50:00 10K predicts roughly 1:50:20 for the half. Make sure the 10K was a true maximal effort.
Why do half predictions from 5K feel optimistic?
The 1.06 exponent slightly underestimates the endurance cost for runners without long run fitness. Add 2 to 3 minutes if your longest run is under 16 km.
How should I pace using the predicted time?
Divide by 21.1 for goal pace per km, run the first half a touch easy, and negative split if you feel strong after 15K.
Does course profile matter?
A lot. Predictions assume similar terrain; a hilly half can add 3 to 8 minutes versus a flat one at the same fitness.
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