Free running calculators

Race Time Predictor

Enter a recent race and the distance you're training for. You get a predicted finish time from three different models, how far apart they are, and the pace to run on the day.

How the prediction works

Every runner slows down as races get longer, and the slowdown follows a fairly regular curve. A race time predictor takes one point on your curve (a race you've actually run) and reads off another. Different researchers have fitted that curve in different ways, so this calculator runs three of the best-known models and shows all of them. The big number is their average.

Riegel's formula

Pete Riegel, an engineer and marathoner, published the simplest and most widely used model in 1977 and refined it in American Scientist in 1981:

T₂ = T₁ × (D₂ ÷ D₁)^1.06

Double the distance and your time a little more than doubles: 2^1.06 is 2.085, so a 25:00 5K predicts a 52:07 10K. Riegel fitted the exponent to world records between roughly 3.5 minutes and 4 hours, and it holds up well inside that window.

Cameron's model

Dave Cameron fitted a curve to elite performances from the mile to the marathon. Instead of one exponent, it uses a correction factor that shrinks as the distance grows (distance d in miles):

f(d) = 13.49681 − 0.048865·d + 2.438936 ÷ d^0.7905

T₂ = T₁ × (D₂ ÷ D₁) × f(D₁) ÷ f(D₂)

Cameron's model slows you down a little more than Riegel's over long jumps, so it usually gives the slowest marathon prediction of the three.

Daniels VDOT

Jack Daniels and Jimmy Gilbert's model (from Oxygen Power, 1979) works through physiology. It estimates the oxygen cost of running your race pace, divides by the share of your aerobic capacity you can hold for that long, and calls the result your VDOT, a working VO2max for racing. The prediction is the time at the target distance that scores the same VDOT. It's the same model behind the pace tables in Daniels' Running Formula.

Example: a 25:00 5K

The three models side by side for a 25-minute 5K. They agree within seconds up to the half marathon and drift apart at the marathon, which is why the calculator shows the range and not only one number.

5K race time equivalents

Predicted 10K, half marathon and marathon times for common 5K times, averaging the three models. Use the calculator above for any other distance or time.

Why marathon predictions run fast

All three models assume you are equally well trained for both distances. A fast 5K shows your speed, but a marathon also depends on endurance built from months of long runs and weekly mileage. Runners on low mileage often finish marathons several minutes, sometimes much more, behind a prediction from a short race.

For the most reliable prediction, use a race close to your target: a half marathon predicts a marathon far better than a 5K does. The calculator flags long jumps like 5K to marathon so you know to treat that number as a best case.

Getting a useful prediction

• Use a recent, all-out race. A result from the last six to eight weeks reflects your current fitness. A training run or a race you jogged will undersell you.
• Match the course and the weather. The models don't know about hills, heat or wind. A hilly summer 10K will predict a slower flat autumn marathon than you can run, and the other way around.
• Train for the distance. The prediction is what you could run if you're as ready for the target as you were for the race you entered. It's a target, not a promise.
• Start at the slower end. For a first attempt at a distance, pace your first half at the slower end of the range and speed up if you feel good.

Frequently asked questions

Which race time predictor is most accurate?

For neighbouring distances, such as 5K to 10K or 10K to half marathon, all three models agree within about 1%, so any of them is accurate. Over longer jumps no model is reliably best: Riegel and VDOT suit runners with a good endurance base, while Cameron's more cautious marathon numbers often suit everyone else.

Can I predict my marathon time from a half marathon?

Yes, and it's the best race to use. A common rule of thumb is to double your half marathon time and add 10 to 20 minutes; the three models put it at double plus roughly 7 to 16 minutes, depending on your pace and the model, which assumes you have done the long runs a marathon needs.

What does my VDOT number mean?

VDOT is Jack Daniels' measure of your current running fitness, worked out from a race result. Higher is fitter: a 25:00 5K scores about 38, a 20:00 5K about 50. Coaches use it to set training paces, and it goes up as your races get faster.