Race time predictor

One honest race result is the best evidence of your fitness. Enter it and see what it predicts for every distance, by two independent models.

h:mm:ss or mm:ss.

Predicted marathon3:49:475:27/km · 8:46/mile · VDOT 40.0
DistanceVDOT modelRiegel 1.06Avg pace /kmAvg pace /mile
Mile7:077:134:277:10
5K24:0623:594:487:44
10K50:0050:005:008:03
15K1:17:011:16:515:088:15
10 miles1:23:021:22:485:098:17
Half marathon1:50:521:50:195:158:26
30K2:40:582:40:135:218:37
Marathon3:49:343:50:015:278:46
50K4:33:124:35:215:298:50

Ask the race-plan coach

An AI coach reads the numbers above — your VDOT, predictions and training paces — and writes a pacing plan and key sessions for your goal race. It only talks running.

Paces per
Free, no sign-up. Uses AI; check advice against how you feel.

The one number that decides your marathon prediction

Riegel noticed that across running, swimming and cycling records, time grows a little faster than distance: double the distance and you take slightly more than twice as long. That “slightly” is the exponent. Drag it.

With exponent 1.06, a 25:00 5K predicts a 3:59:47 marathon.

1.00 means no fatigue at all — you’d hold 5K pace for 42 km, which nobody can. 1.06 is Riegel’s fit for trained athletes. Runners on low weekly mileage often behave more like 1.08–1.10, which is why a 5K alone overpromises the marathon.

1.003:30:59
1.043:49:46
1.063:59:47
1.084:10:14
1.104:21:08
1.124:32:31

Two models, one answer

VDOT comes from Jack Daniels and Jimmy Gilbert’s 1979 work Oxygen Power. It estimates the oxygen cost of your race speed and divides it by the fraction of VO2max a runner can sustain for that duration, producing one fitness number that maps to times at every distance. The VDOT calculator turns the same number into training paces.

Riegel is a pure power law fitted to world records. It knows nothing about physiology, which is its strength: it is simple, transparent and easy to make more conservative.

Both assume you are equally trained for the goal distance. If you raced a 5K off 25 km a week, add time to anything over 21 km. Then pick a goal and plan it on the marathon pace chart or the split calculator.

Questions runners ask

How accurate are race time predictions?

For distances close to the one you raced (5K to 10K, 10K to half) predictions are usually within 1–3%. Marathon predictions from a 5K are the least reliable because they assume you have the endurance training for 42 km; runners on low mileage often finish 5–10% slower than predicted.

What is the Riegel formula?

Published by engineer Pete Riegel in 1981: T2 = T1 × (D2 ÷ D1)^1.06. The exponent 1.06 captures how much you slow as distance increases. Raising it to 1.08–1.10 gives more conservative predictions for less-trained runners.

Which prediction should I trust, VDOT or Riegel?

They usually agree within a minute or two up to the half marathon. For the marathon, VDOT (from Jack Daniels’ oxygen-cost tables) tends to be a little more optimistic. When they differ, plan to the slower number unless your long runs and weekly mileage are strong.

Can I predict my marathon time from a half marathon?

Yes, and it’s the best single predictor. A common rule of thumb is double your half time and add 10 minutes; the Riegel formula gives a very similar answer — a 1:45 half predicts about 3:39.

Does the predictor account for hills or heat?

No. All predictions assume a flat course in cool conditions (around 10–12 °C). Hot, humid or hilly races can add several minutes to a marathon.

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