Race Time Predictor

Race predictors answer a tempting question — "what could I run a marathon in?" — from a result you already have. Nearly all of them, ours included, use the Riegel formula, a one-line model from 1977 that has held up remarkably well. Used honestly it is a genuinely useful planning tool; used naively it writes checks your training hasn't cashed.

Open Pace Calculator

The Formula

T1 is your known time at distance D1; T2 the predicted time at D2. The exponent is the whole story: if pace never decayed the exponent would be exactly 1 (double distance, double time). Real endurance decays — the 1.06 says doubling the distance costs about 6% more than double the time, matching observed results across well-trained runners from 1,500 m to the marathon.

A Worked Example

From a 25:00 5K, the pace calculator predicts:

Note the built-in realism: the predicted pace slows at every step up — no one is expected to hold 5K pace for 42 km. What those paces mean in finish-time terms across distances is laid out in the running pace chart.

When It Holds — and When It Flatters

Riegel's hidden assumption is equivalent training for the target distance. The 10K prediction from a 5K is usually solid — the events are physiologically close. The marathon prediction from a 5K assumes you have marathon endurance, which a 5K result says nothing about; runners without the long-run base typically finish 10–20 minutes slower than predicted. Two practical rules: predict from the closest distance you have raced recently, and treat cross-spectrum predictions as ceilings. Then verify in training — cumulative checkpoint times for any goal pace come from the split table described in the pace calculator guide.

Frequently Asked Questions

What is the Riegel formula?
T2 = T1 × (D2/D1)^1.06 — proposed by Peter Riegel in 1977. The 1.06 exponent captures how pace decays as race distance grows; doubling the distance costs about 6% more than double the time.

Why is my marathon prediction too fast?
Riegel assumes equivalent training at the target distance. Predicting a marathon from a 5K assumes marathon-level endurance volume; without long-run mileage, real results come in slower than predicted.

Which baseline race gives the best prediction?
The closest recent distance to the target. A half marathon predicts a marathon far better than a 5K does, because the extrapolation gap is smaller.