How the race time predictor works

Enter a recent, honestly raced result and select the endurance profile that best reflects your preparation. The calculator estimates how that performance might translate to other common distances. It also returns the average pace required for each prediction, in both minutes per kilometre and minutes per mile.

This is the Riegel formula. T is time, D is distance and R is the fatigue factor. A lower exponent assumes that you preserve more of your speed as distance increases. The standard setting uses 1.06, a widely used general estimate. Strong endurance uses 1.04; developing endurance uses 1.08.

Which result should you enter?

Use the most recent performance that reflects your current fitness. A properly paced race on a measured course is more useful than an old personal best, a casual training run or a workout interrupted by traffic. Predictions are generally more credible when the known race and target race are reasonably close in distance.

• For a 10K target, a recent 5K or 10K is usually useful.
• For a half marathon, a recent 10K or half marathon is preferable.
• For a marathon, a half-marathon result is more informative than a short sprint, but long-run durability still matters enormously.
• Do not use a result distorted by illness, injury, extreme weather or a course that was substantially short.

How to choose your endurance profile

Strong endurance

Choose this only if your weekly volume, long-run consistency and race history show that you lose relatively little speed as distance increases. It is not simply the ambitious option.

Balanced / standard

This is the best starting point for most runners. It provides a neutral mathematical estimate without assuming exceptional endurance or a major endurance gap.

Developing endurance

Use this when your short-distance speed is ahead of your long-run durability, when you are new to the target distance, or when training volume has been inconsistent.

Why marathon predictions are often optimistic

A fast 5K proves speed, not necessarily marathon readiness. The marathon adds glycogen management, muscular durability, hydration, fueling practice and the ability to hold form for several hours. Two runners with the same 10K time can therefore produce very different marathon results. Treat a marathon prediction as an upper-level fitness reference, then validate it against long runs, sustained marathon-pace work and recovery.

Turn the prediction into a pacing plan

Use the predicted time to define a realistic range, not a single all-or-nothing target. Convert the central estimate into average pace, then consider course elevation, expected temperature, wind, aid stations and GPS error. Starting slightly controlled is usually safer than trying to bank time early.

During the race, compare cumulative time at official markers instead of reacting to every instant-pace fluctuation. A sensible plan should also include an effort ceiling for the opening kilometres and a decision point later in the race.

Frequently asked questions

Is this the same as a VO₂ max prediction?

No. This calculator uses one performance result and a distance-fatigue relationship. It does not measure oxygen uptake, running economy or laboratory physiology.

Can I predict a marathon from a 5K?

You can generate an estimate, but uncertainty is high. A half marathon plus evidence of consistent long-run training is a stronger basis.

Why is my watch prediction different?

Watch algorithms may use heart rate, training load, VO₂ max estimates and proprietary models. Different inputs and assumptions naturally produce different answers.

Should I race exactly at the predicted pace?

Only if the estimate is supported by training and adjusted for conditions. Use it as one input, not as a substitute for judgement.

This tool is for general education and does not replace individual coaching or medical advice.