How to Predict Your Race Time With the Riegel Formula

Why pace slows over longer distances

You cannot hold your 5K pace for a half marathon. As distance grows, your body shifts to a more sustainable effort, glycogen stores deplete and fatigue accumulates, so average pace drops. A prediction that simply doubled your time for double the distance would be far too optimistic.

The Riegel exponent of 1.06 encodes this slowdown. Because it is slightly above 1, every doubling of distance costs a little more than double the time. Well-trained endurance runners sometimes use a lower exponent near 1.05, while runners who fade badly over distance may fit better with a higher value.

Working through a 5K to 10K example

Say you ran a 5K in 25:00, which is 1,500 seconds. To predict a 10K, the target distance is double, so multiply by 2^1.06 ≈ 2.085. That gives about 3,128 seconds, or 52:08. Notice this is slower than a flat doubling to 50:00 — the extra two minutes is the endurance cost of the longer race.

The same 5K time predicts a half marathon by using D2 = 21.1 km and D1 = 5 km. The ratio 21.1 ÷ 5 = 4.22, raised to 1.06, is about 4.55, giving roughly 1:53:50. The calculator does this arithmetic instantly and lets you test different base races to see which gives the most realistic target.

When the prediction will be off

Riegel assumes you are equally prepared for both distances. If you predict a marathon from a fast 5K but have never run long, the formula will flatter you badly — endurance is trained, not inherited from speed. Predictions across a big distance jump, such as 5K to marathon, are the least reliable.

Weather, hills, altitude and how well you paced the base race all shift the result. Treat the prediction as a realistic goal band, not a guarantee. Use it to set your target pace, then confirm it with a build-up of race-specific long runs and tempo work.

Frequently asked questions

What is the Riegel formula?

It is a race-prediction equation, T2 = T1 × (D2 ÷ D1)^1.06, published by Pete Riegel in 1977. It scales a known race time to a new distance using an exponent that reflects how endurance limits pace.

Can I predict a marathon from a 5K?

You can, but the result is only trustworthy if you have trained the endurance for a marathon. A large distance jump exaggerates your fitness, so predict from the longest well-trained race you have instead.

Why is the exponent 1.06 and not exactly 1?

An exponent of exactly 1 would mean pace stays identical at any distance, which is not true — everyone slows down as races lengthen. The value 1.06 was fitted to real race data and captures that gradual slowdown well.

How accurate are these predictions?

For distances close together and equal training, they are usually within a couple of percent. Accuracy falls when the target distance is far from your base race, or when weather, hills or poor pacing change the day.