Race time predictor

How race time prediction works

A race time predictor takes one race you have run and estimates what you could run over another distance. It works because the way speed falls off with distance is fairly predictable for trained runners. This calculator uses two established methods side by side, so you can see where they agree and where they do not. It is part of the free LeCoach running calculators.

Method 1: Riegel. Peter Riegel, an engineer, described race times across distances with one formula, first in Runner's World in 1977 and later in American Scientist in 1981:

T2 = T1 × (D2 ÷ D1)^1.06

T1 and D1 are your known time and distance; D2 is the target distance. The exponent 1.06 means that every doubling of distance makes you about 4 % slower per kilometre.

Method 2: VDOT. Jack Daniels and Jimmy Gilbert built a model from the oxygen cost of running speed and the share of VO2max a runner can sustain for a given time. Your race gives a VDOT score, and the model finds the time at every other distance that matches the same score. The VDOT calculator turns the same score into training paces.

Worked example

A runner finishes a 10K in 50:00. The predictions:

For most distances the two agree within a minute. Larger gaps tell you the prediction is less certain.

Which prediction should you trust?

Both methods assume you are equally well trained for both distances. That assumption is the main reason predictions go wrong.

• Shorter than your race: both methods are usually reliable. If you can run a good 10K, you have the aerobic fitness for a good 5K, although a mile also needs speed you may not have trained.
• Up to about twice your race distance: reasonable, if you have done some longer runs.
• Much longer than your race, especially the marathon: treat the prediction as a best case. A marathon depends on endurance, fuelling and pacing that a 10K does not test. Many runners with modest weekly mileage run their first marathon 5 to 10 % slower than predicted from a 10K or half.

A practical approach: predict a marathon from a recent half marathon rather than from a 5K, and use the slower of the two predictions as your goal if you are newer to the distance. If you run low weekly mileage, increase the Riegel exponent to 1.08 in the advanced settings for a more cautious estimate.

How to get a good prediction

• Use a recent race, ideally within the last 6 to 8 weeks, run all-out on a flat course in good conditions.
• Use a race, not a training run. A hard training run is rarely a true maximal effort.
• Use the closest distance. The further you extrapolate, the less reliable the number.
• Check your training. If your target is a marathon, you need long runs of 28 to 35 km and enough weekly volume. The weekly training volume planner helps you build that.

Common mistakes

• Using a downhill or short-measured course. It inflates every prediction.
• Ignoring conditions on race day. Heat, wind and hills slow the target race too. A predicted time assumes similar conditions.
• Racing the prediction from the gun. Start a little slower than the predicted pace and build. Use the running pace calculator to plan your splits.
• Forgetting fuelling. For races over 90 minutes, a fuelling plan is part of hitting the time. The race nutrition calculator gives you carbs and fluid per hour.

When predictions shift

• After a training block focused on speed, predictions from a recent 5K become more reliable for short races and less for long ones.
• As you age, the shape of your speed curve stays similar, so predictions remain useful.
• For trail and ultra races, these road models do not apply. Terrain and elevation matter more than distance.

LeCoach plans your running or cycling towards your race date and explains in plain language how your fitness is trending, so a goal time is something you check against your training, not a guess.