Marathon Time Predictor
Doubling a half marathon time is not the answer.
Doubling a half marathon time is not the answer. States the assumption instead of hiding it.
As h:mm:ss or mm:ss
1.06 is standard; a well-trained endurance runner may be nearer 1.05
Predicted time for 42.195 km
3:28:30
4:56 / km · Reasonable
Riegel's formula raises the distance ratio to the power 1.06 rather than scaling linearly, and that exponent above 1 is what encodes the fact that pace slows as distance rises. Doubling from a half to a full marathon predicts 8:30 slower than simply doubling the time. The prediction assumes training appropriate to the target distance — a fast half-marathon runner who has never run beyond 25 km will not hit the marathon figure, because the formula models physiology rather than preparation.
How the Marathon Time Predictor works
Predicted race time at one distance from a performance at another, using Riegel's formula. The exponent above 1 is what encodes that pace slows as distance rises, which linear scaling cannot.
Also known as: predict my marathon time · half marathon to marathon prediction · riegel formula calculator · race equivalency calculator
Why the exponent is above 1
Riegel's formula is T2 = T1 × (D2/D1)^1.06. If the exponent were exactly 1, doubling the distance would double the time — which would predict marathon times nobody has ever run.
The exponent above 1 encodes that pace slows as distance rises. Doubling from a half to a full marathon multiplies the time by 2^1.06, which is about 2.08 rather than 2 — roughly eight minutes on a 1:40 half.
Different exponents fit different runners. Well-trained endurance athletes fit closer to 1.05; less endurance-trained runners closer to 1.08. Fitting your own from two known races is more useful than any published default.
What the formula cannot know
It models physiology, not preparation. A fast half-marathon runner who has never run beyond 25 km will not hit the predicted marathon time, and the shortfall is training rather than talent.
It also assumes a comparable course and comparable conditions. A hilly marathon predicted from a flat half will be slower, and neither the formula nor any of its alternatives adjusts for that.
Accuracy degrades as the extrapolation grows. Within a factor of two between distances it is reasonable; predicting a 100 km from a 5 km is arithmetic without meaning, which is why the page says so rather than returning a confident number.
Building the training the prediction assumes
Marathon plans commonly run 16 to 20 weeks from a reasonable base, with the long run as the key session. Building it takes time that cannot be safely compressed.
Most plans stop the longest run at 30 to 35 km rather than the full distance, deliberately. Running the full distance in training costs more in recovery than it returns in confidence, and the race-day taper is what closes the gap.
VDOT and similar systems work the other way — deriving training paces from a race result rather than predicting one. Both rest on the same relationship between race performance and aerobic capacity, viewed from opposite ends.
Where to go next
The Marathon Time Predictor question rarely arrives on its own. These are the ones that usually come with it:
- Running Pace Calculator — Pace is a reciprocal — it cannot be averaged like speed.
- Race Split Calculator — Nearly every record has been set on a negative split.
- Triathlon Time Calculator — Transitions are free time almost nobody trains.
- BMI Calculator — Body mass index with category and healthy weight range.
Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.
Frequently asked questions
How do I predict a marathon time from a half?
Riegel's formula: multiply by 2 raised to the power 1.06, which is about 2.08. A 1:40 half predicts roughly a 3:28 marathon, not 3:20.
What is Riegel's formula?
T2 = T1 × (D2/D1)^1.06. The exponent above 1 captures the fact that pace slows as distance increases; a linear scaling would predict times nobody has run.
How accurate is it?
Reasonable when the two distances are within a factor of two, and increasingly optimistic beyond that. It also assumes training appropriate to the target distance.
Why does my prediction feel too fast?
Because the formula models physiology rather than preparation. A fast half-marathon runner who has never run beyond 25 km will not hit the marathon prediction, and the shortfall is training rather than talent.
Should I use a different exponent?
Well-trained endurance runners often fit closer to 1.05 and less endurance-trained runners closer to 1.08. Fitting your own from two known races is more useful than any default.
Does the formula work for shorter distances?
It works downward too — predicting a 5 km from a 10 km — and it is less useful there because short races depend more on speed than endurance, which the formula does not model.
What other prediction formulas exist?
Cameron's and Daniels' VDOT are both widely used and give similar answers within a reasonable range. All of them model physiology rather than training status.
How long should marathon training take?
Commonly 16 to 20 weeks from a reasonable base for a first marathon. The long run is the key session and building it takes time that cannot be compressed safely.
How far should my longest run be?
Typically 30 to 35 km for most training plans, deliberately short of the full distance. Running the full distance in training costs more in recovery than it gains in confidence.
Why do I run slower in a marathon than a half?
Glycogen depletion, accumulated muscular fatigue and dehydration all compound over the distance. That is exactly what the Riegel exponent encodes.
What is VDOT?
A measure of running fitness derived from race performance, used to set training paces and predict equivalent times. It is effectively a race-derived estimate of aerobic capacity.
Can I predict from a parkrun time?
Yes, with caution — the extrapolation from 5 km to 42 km is large and assumes marathon-specific training. Treat the result as a ceiling rather than a target.
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Open