RACEPACEKIT

One race in.
Every distance out.

Enter your most recent all-out race and get realistic predicted times — and paces — for 5K, 10K, the half and the marathon. Then turn any prediction into a full split plan with one click.

Recent race resultRiegel method
Distance you raced
Your finish time
hrs
:
min
:
sec
Your endurance profile
Your projections

Predicted times

The math

The Riegel formula

In 1977, engineer and runner Peter Riegel published a deceptively simple power law that has been the backbone of race prediction ever since:

T2 = T1 × (D2 ÷ D1)1.06
25:00 5K → 25:00 × (10 ÷ 5)1.06 = 52:08 for the 10K

The exponent is the interesting part. If pace never decayed with distance, the exponent would be exactly 1 and a 25:00 5K would mean a 50:00 10K. The extra 0.06 encodes fatigue: every doubling of distance slows your sustainable pace by roughly 4%. That one number matches decades of real-world results across distances remarkably well — for runners who are properly trained for the target distance.

That last clause is doing heavy lifting

Riegel assumes your endurance is as developed as your speed. Predicting up in distance is therefore honest only if your training says so. The classic disappointment is the 5K-to-marathon jump: the formula might print 3:59, but on 40 km per week of training, race day usually says otherwise — not because the math is wrong, but because the assumption is. Two fixes:

  • Predict from the nearest distance. A half marathon predicts the marathon far better than a 5K does. Short predicts short, long predicts long.
  • Adjust the exponent to your profile. That is what the selector above does: 1.05 if you are speed-strong with a big aerobic base, 1.06 as the standard, 1.08 if your weekly mileage is modest and long races historically bite you.

From prediction to plan

A prediction is a starting point, not a finish line. The productive workflow: race something short and honest, project it here, pick the target that matches your training reality, then click Plan this race on that card — it opens the race pace calculator with your predicted time preloaded, ready to generate splits and a printable pace band. Doing race-pace treadmill rehearsals? The treadmill converter translates the plan into belt speed.

One more honest note: predictions describe fitness, not race day. Heat, hills, wind and fueling all tax the number. Treat the projection as your B goal, set an A goal slightly faster and a C goal you would still be proud of — then pace for B and let the last 10K decide.

Questions runners ask

FAQ

How are race times predicted from another distance?

With the Riegel power law: T2 = T1 × (D2 ÷ D1)^1.06. Your known result scales to the new distance, with the exponent modeling how sustainable pace decays as distance grows.

Why was my real marathon slower than the prediction?

Because marathon results depend on endurance-specific training that a 5K result cannot see. Predict from the half instead, or use the 1.08 profile if your mileage is modest — the formula is only as honest as its assumption that you are trained for the target distance.

Which race should I use as the input?

Your most recent all-out effort, ideally inside the last 8–10 weeks, on a reasonable course and day. The closer its distance to the target, the better the projection.