What this result means
The race time predictor maps a result you have onto a distance you do not. This page maps a goal you want onto a distance you can race soon. Enter the goal distance and time, then the known distance. The output is the time you must run at that known distance if Riegel is the model.
It is a planning check, not a qualifying standard. Hitting the required 10K does not guarantee the marathon if the long runs never happened.
A 3:30:00 marathon goal implies a required half and 10K under Riegel. A 46:20 10K predicting 3:33:09 for the marathon is close but still short of 3:30. The inverse tells you the 10K that would map to 3:30.
A 1:30:00 half goal requires a faster 10K than a 1:42 half. Use nearby distances; a required 5K for a first marathon is a noisy check.
Calculation method
Riegel: T2 = T1 × (D2 ÷ D1)^1.06. Required T1 at D1 for goal T2 at D2 is T2 × (D1 ÷ D2)^1.06, which is predictTime with the distances swapped. Cite Riegel 1981.
The implementation round-trips: predicting then requiring returns the original time within floating-point error.
Inverse Riegel does not know your training. A required half for a marathon goal is a fitness checkpoint, not a license to skip long runs. VDOT equivalents are a second opinion on the race-equivalent page.
Frequently asked questions
How is this different from the race time predictor?
The predictor goes from a result you have to other distances. This tool goes from a goal you want to the time you need at a distance you can race.
What 10K do I need for a 3:30 marathon?
Enter 42.2 km and 3:30:00 as the goal, then 10 km as the known distance. The number is a Riegel estimate, not a guarantee.
Does this use VDOT?
No. Required performance is inverse Riegel. VDOT equivalents are on the race equivalent calculator.
Is this a training plan?
No. It is a planning checkpoint: the time Riegel says you need at a known distance. Hitting that 10K does not replace marathon long runs.