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Race Time Predictor

Predicts race times at every common distance from one known result using Riegel’s formula, with pace per km and per mile.

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In short Race Time Predictor is a free online tool on inputtesthub.com that predicts race times at every common distance from one known result using Riegel’s formula, with pace per km and per mile. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.

How to use the Race Time Predictor

  1. Enter your figures in the input fields.
  2. The result updates as you type, together with the full working.
  3. Check the breakdown table to see exactly how the number was reached.

About the Race Time Predictor

The Race Time Predictor predicts race times at every common distance from one known result using Riegel’s formula, with pace per km and per mile. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.

What people use it for

  • Setting a target time for a longer race.
  • Comparing results across distances.
  • Checking whether a marathon goal is realistic.

Details

Standard exponent1.06
For untrained long distanceUse 1.10
Predicts upOptimistically
Half marathon21,097.5 m

The maths behind it

This tool uses Riegel: T₂ = T₁ × (D₂ ÷ D₁)^1.06. Raising the exponent towards 1.10 gives more realistic long-distance predictions. Every intermediate value is shown on screen, so the result can be checked by hand.

Riegel assumes you have trained for the target distance, and that assumption is what breaks marathon predictions. Predicting up from a 5 K routinely gives a marathon time 10–20 minutes faster than reality for anyone who has not done the long runs.

Frequently asked questions

Do I need an account to use the Race Time Predictor?
No charge and no sign-up. There is no free tier to run out of because there is no paid tier — inputtesthub.com is funded by advertising rather than by metering the page.
Can you see what I put into this page?
No. The Race Time Predictor runs entirely in your browser using JavaScript. Nothing is transmitted, logged or stored, which you can verify yourself in the Network tab of your browser's developer tools.
Can I check the maths myself?
It uses Riegel: T₂ = T₁ × (D₂ ÷ D₁)^1.06. Raising the exponent towards 1.10 gives more realistic long-distance predictions. The full working is printed under the result, so you can follow every step rather than trusting a black box.
What problem does the Race Time Predictor solve?
In practice it gets used for three things: setting a target time for a longer race. Comparing results across distances. Checking whether a marathon goal is realistic. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Any caveats I should know about?
Riegel assumes you have trained for the target distance, and that assumption is what breaks marathon predictions. Predicting up from a 5 K routinely gives a marathon time 10–20 minutes faster than reality for anyone who has not done the long runs.
What are the reference figures here?
Standard exponent is 1.06. For untrained long distance is Use 1.10. Predicts up is Optimistically. These are printed on the page itself, so you do not have to take them on trust.
Can I rely on this for exact work?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.

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