How fast could you run
the next one?

It depends almost entirely on how far you extrapolate. Between neighbouring distances (10K to half marathon) the Riegel formula holds up well. Across a big jump — 5K to marathon — it gets optimistic, and our own data says systematically so: across 22,422 daily Garmin predictions, every single athlete in our cohort implied a steeper slowdown than Riegel assumes, median 1.131 against his 1.06. That is why this calculator shows both curves instead of pretending there is one answer.

Peter Riegel's 1977 formula: T2 = T1 × (D2 / D1)^1.06. The 1.06 exponent accounts for the fact that you slow down slightly per kilometer over longer races. It remains one of the most widely used and tested race predictors despite being almost 50 years old.

Only if it was a maximal effort over the stated distance. Race performance reflects pacing, fueling, and mental effort that regular training runs don't. Time trials and races are what the formula is designed for.

Training paces here are based on your current race fitness. If you train harder or easier than these paces feel, trust how you feel over the formula. These paces assume healthy recovery; in a high-stress week, easy pace should drift 15–20 seconds slower.

Not directly. Riegel assumes flat, consistent terrain. For trail races, add 10–30% to the predicted time depending on elevation gain and technicality. Ultras (50+ miles) break the formula entirely — fueling and pacing dominate.

They are answering slightly different questions. Garmin works from your actual training — VO2 max trend, recent paces, lactate threshold — while this works from one race result. The interesting part is that they disagree in a consistent direction: we measured the fade Garmin builds in and it is steeper than the textbook formula for every athlete we looked at. In 11 real races where we had a prediction from the day before, Garmin came within a median of 0.6%. Eleven races is a case series, not proof, but it is the only direct evidence we have and it does not favour the textbook.