BYTETOOLS

Race Time Predictor

Predict your 5K, 10K, half marathon and marathon times from one recent race using Riegel's formula, with pace per km and per mile for each.

Your race, as entered

22:30 for 5.00 km (3.11 mi)

That is 4:30 per km and 7:15 per mile.

T₂ = 22:30 × (D₂ ÷ 5.000 km)^1.06

22:30
5K
46:55
10K
1:43:30
Half marathon
3:35:48
Marathon

Predicted times at every distance

DistancePredicted timePace / kmPace / mileSpeed
800 m3:144:026:2914.88 km/h
1500 m6:174:116:4414.33 km/h
1 mile6:464:126:4614.27 km/h
3000 m13:064:227:0113.75 km/h
5Kyour race22:304:307:1513.33 km/h
8K37:024:387:2712.96 km/h
10K46:554:417:3312.79 km/h
12K56:554:457:3812.65 km/h
15K1:12:064:487:4412.48 km/h
10 miles1:17:414:507:4612.43 km/h
20K1:37:484:537:5212.27 km/h
Half marathon1:43:304:547:5412.23 km/h
30K2:30:195:018:0411.97 km/h
Marathon3:35:485:078:1411.73 km/h
50K4:18:205:108:1911.61 km/h

Riegel’s endurance formula, published by Peter Riegel in American Scientist (1981): T₂ = T₁ × (D₂ ÷ D₁)^1.06. The exponent above 1 is what encodes fatigue — doubling the distance costs you more than double the time. Worked check: a 20:00 5K predicts 41:42 for 10K, 1:32:00 for a half and 3:11:49 for a marathon, which matches the published Riegel tables.

Estimates only, and not medical or coaching advice. Riegel is most reliable when the two distances are within about a factor of four of each other and least reliable at the marathon, where it assumes you have done the endurance training to hold the pace — it routinely predicts marathon times that under-trained runners cannot reach. Heat, hills, course, pacing, fuelling and how fresh you were on the day all move the real result. Build up distance gradually, and check with a doctor before a big jump in training load if you have any health concerns.

What is the Race Time Predictor?

A race time predictor uses Riegel's endurance formula, T2 = T1 × (D2 ÷ D1)^1.06, to project a finish time at a new distance from a recent race result. A 20:00 5K predicts about 41:42 for 10K and 3:11:49 for a marathon.

  • Fifteen race distances from 800 m to 50K in one table
  • Riegel's formula named and shown, with an editable exponent
  • Pace per kilometre and per mile plus average speed for every row
  • Highlights the race you entered so the maths is easy to check
  • Rejects times faster than world-record pace instead of printing nonsense
  • Runs entirely in your browser — no upload, no account

How to use the Race Time Predictor

  1. 1

    Choose metric or imperial units.

  2. 2

    Select the distance you raced, or enter a custom distance.

  3. 3

    Enter your finish time in hours, minutes and seconds.

  4. 4

    Read your predicted times for every other distance.

  5. 5

    Adjust the fatigue exponent if you know you fade or hold pace unusually well.

About the Race Time Predictor

The ByteTools Race Time Predictor projects your finish times across fifteen standard race distances from a single recent result. It uses Riegel's endurance formula, T2 = T1 × (D2 ÷ D1)^1.06, published by Peter Riegel in American Scientist in 1981 — the exponent above one is what encodes fatigue, so doubling the distance costs more than double the time.

Enter the distance you raced and your finish time and you get a full table: predicted time, pace per kilometre, pace per mile and average speed for everything from 800 m to 50K, with your own race highlighted. The fatigue exponent is editable, so you can lower it towards 1.03 if you hold pace unusually well over long distances or raise it if you tend to fade.

Metric and imperial are both supported and everything is calculated in your browser with nothing uploaded. These are estimates only and not medical or coaching advice. Riegel is most reliable between distances within about a factor of four of each other and least reliable at the marathon, where it assumes you have actually done the endurance training to hold the pace.

Frequently asked questions

How do you predict a marathon time from a 5K?

Apply Riegel's formula: multiply your 5K time by (42.195 ÷ 5) raised to the power 1.06. A 20:00 5K gives 20 × 8.439^1.06, which comes to about 191.8 minutes, or 3:11:49. Be aware that this assumes marathon-specific endurance training you may not have done.

How accurate is Riegel's formula?

Good between neighbouring distances, less good across big jumps. It works best when the two distances are within roughly a factor of four of each other — 5K to 10K, or 10K to half marathon. Predicting a marathon from a 5K stretches it a long way, and studies of real race data find it systematically optimistic for the marathon in runners with low weekly mileage.

What is the fatigue exponent?

The 1.06 in the formula. It expresses how much your pace decays as distance grows: an exponent of exactly 1 would mean you could hold the same pace forever, and higher values mean faster decay. Riegel derived 1.06 from a large set of race results; well-trained endurance runners often behave closer to 1.05, and less trained ones closer to 1.08 or above.

Why is my real marathon time slower than the prediction?

Almost always because of endurance rather than speed. Riegel only knows how fast you are, not how much long-run and weekly mileage you have banked, and the marathon punishes that gap harder than any other distance. Fuelling, heat, hills and going out too fast in the first 10 km account for most of the rest.

Can I use this for a custom distance?

Yes. Select the custom option and type any distance from 400 m to 200 km in kilometres or miles, and the whole prediction table is recalculated from it.

How do I convert a pace between km and miles?

Multiply your pace per kilometre by 1.609 to get pace per mile. This tool shows both columns for every distance, so you can read either without doing the conversion yourself.

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