How accurate is this race time predictor?
The Riegel formula is below. For trained runners it is usually within a few percent over one step in distance.
T2 = T1 × (D2 / D1)1.06
The bigger the jump, the more your endurance base, fuelling and pacing matter. Predictions assume similar conditions, and that you trained for each distance.
What's the limit?
Predictions get less reliable as the gap between distances grows.
A 5K to marathon prediction is too optimistic without the long runs. It is too cautious for a marathoner's tune-up 5K.
Treat the marathon prediction as a best case, not a promise.
See it as a chart
The VDOT chart lines up equal 5K, 10K, half marathon and marathon times for VDOT 30 to 70. It uses Jack Daniels' formula, not Riegel's.
Expect the two to differ a little, most of all at the marathon. The comparison above shows both side by side.
Once you have a target, the marathon, half marathon, 10K and 5K pace charts give the pace and splits to hit it.