What could you run at the other distances?
One recent race time, projected across 5K, 10K, half and marathon by the two models coaches actually use — shown side by side, because where they disagree is the interesting part.
| Distance | Riegel | VDOT | Pace |
|---|
How to read this
The hero number is the midpoint of the two models for the race you asked about. Underneath are the two models themselves. When they agree to within a minute or two, the prediction is solid. When they disagree by five minutes over a marathon, that gap is the honest answer: it depends on endurance that a short race cannot measure.
Both models project fitness, not readiness. They answer "what is this engine worth over that distance?" — not "have you done the long runs?" The further the target is from the race you entered, the more the second question matters.
Predicting up is harder than predicting down
A marathoner asking about a 5K will usually run close to, or faster than, the prediction: the endurance is there and only speed is in question. A 5K runner asking about a marathon will usually run slower than either model, because the models assume a long-run base the 5K never tested. The two numbers in the table are a ceiling in that direction, not an estimate.
The pace column is the one to write on your hand
Pace to hold is the midpoint prediction divided by the distance. It is an even-split pace. If the course is hilly, hot or windy, the honest plan is a little slower than this for the first half and no faster than it for the second.
Where these numbers come from
Riegel's formula is T2 = T1 × (D2 / D1)1.06, the same exponent Riegel published in 1981. The VDOT equivalence is the Daniels & Gilbert model: your race is converted to an aerobic index, and the index is solved back into a time at the new distance. Both are the code the Outrunr app runs when it prices a goal — ported to JavaScript and pinned to the app's own test fixtures, so the website and the app cannot disagree.
The app adds the rule neither model has: a raced personal best at the goal distance inside six months beats any projection. If you have one, trust it over this page.
Common questions
Which prediction should I believe?
Whichever model is nearer the distance you actually train for. Riegel assumes you are equally prepared for every distance; VDOT assumes a typical decline in the fraction of your aerobic ceiling you can hold as the race gets longer. For a 5K runner asking about a 10K they are nearly identical. For a 5K runner asking about a marathon, both are optimistic, and the gap between them is a fair measure of how much the answer depends on endurance you may not have built yet.
Why is my marathon prediction so fast?
Because every projection from a short race assumes the long-run endurance to go with it. A runner whose longest run is 12 km will not run the marathon their 5K predicts, however good the 5K. Treat the marathon figure as what your aerobic fitness is worth if the training were done — which is a useful goal, and a terrible pacing plan.
What is the Riegel formula?
T2 = T1 × (D2 ÷ D1)1.06. Peter Riegel published it in 1981 from race results across distances, and it is still the most widely used prediction in running. The exponent 1.06 is the rate at which runners slow as the distance grows; every doubling costs about 4.2% in pace.
What is VDOT?
A performance-equivalent aerobic index from Jack Daniels and Jimmy Gilbert. It converts a race into the oxygen cost of that velocity, divided by the fraction of maximum a runner can sustain for that duration, and then solves the same equation backwards for another distance. It is the model behind most published training-pace tables.
How recent does the race need to be?
Inside about three months, and run as a genuine effort. A time from last year describes last year's fitness. The Outrunr app uses the same rule: a raced personal best at the goal distance inside six months is trusted over a projection; otherwise it projects from the best recent fitness it can find.
Is anything stored?
No. The calculation runs in your browser. Nothing is sent anywhere and nothing is saved.
More free tools
parkrun® is a registered trade mark of parkrun Limited. Outrunr is not affiliated with or endorsed by parkrun. VDOT is a trade mark of its owner. Outrunr implements the published Daniels & Gilbert formula and is not affiliated with them.
A prediction is where the plan starts.
Outrunr takes the same projection, scores how likely your goal is against the weeks you have, and builds the block that closes the gap — then re-prices every pace each time you race.