How accurate is the Riegel formula?

Within a few percent between adjacent distances — a 5K predicting a 10K, or a 10K predicting a half. It becomes progressively optimistic over larger gaps, because it assumes only pace changes with distance while glycogen, training volume and musculoskeletal durability increasingly decide longer races.

Can I predict a marathon from a 5K?

You can compute it and you should not trust it. A 5K can be raced well off 20 miles a week; a marathon cannot, and the formula does not know your mileage. Predict a marathon from a half marathon instead — every step of extrapolation adds error in the flattering direction.

How do I set training paces from a race result?

Project the result to a one-hour equivalent with Riegel, which gives you threshold pace, then take every training band as a percentage of it: easy at 1.20 to 1.35 times, tempo at 1.03 to 1.09, threshold at 0.98 to 1.03, and intervals at 0.88 to 0.96.

Does muscle mass affect race predictions?

At longer distances, yes. Carrying extra mass costs energy per kilometre, and that cost compounds with duration — so a strong 5K predicts an optimistic marathon for a heavier athlete. Set long-distance goals from a half marathon result rather than a short one.

Race time predictor

The formula

t2 = t1 * (d2 / d1) ^ 1.06

Peter Riegel published this in 1977 from a regression across race results. The exponent of 1.06 encodes a simple fact: you slow down as distance increases, and you slow down at a fairly consistent rate.

That is it. There is no physiology in it — it is a curve fitted to what people actually run.

Where it is accurate and where it is not

Prediction	Reliability
5K from a 10K, or vice versa	Good — within a few percent
10K from a half marathon	Good
Half from a 10K	Reasonable
Marathon from a 5K	Optimistic, often badly
Ultra from anything	Do not

The failure mode is systematic and it is always in the same direction: the formula is optimistic over long extrapolations, because it assumes the only thing that changes with distance is pace.

Above roughly the half marathon, three other things start deciding the outcome and none of them is in the equation:

Glycogen. A marathon is long enough to run out. A 10K is not.
Training volume. A 5K can be raced well off 20 miles a week. A marathon cannot, and the formula does not ask about your mileage — which is why this calculator does.
Musculoskeletal durability. Four hours of eccentric loading is a different problem from twenty minutes of it.

What it is genuinely useful for

Setting training paces from a race result. This is the best use and it is what Base does with it internally: a hard twenty-minute effort projected to a one-hour equivalent, which becomes your threshold, which becomes every prescribed band. Without the projection, a twenty-minute effort's raw pace would set every zone too fast — and the damage lands hardest on the easy days.

Sanity-checking a goal. If your recent 10K predicts a marathon forty minutes faster than your target, your target is conservative. If it predicts one twenty minutes slower, your target needs either more training or revision.

Comparing your own results across distances. A 5K and a half that predict each other closely means your training is balanced. A large mismatch tells you which end you have neglected.

For hybrid athletes specifically

One adjustment worth making.

An athlete carrying meaningful muscle mass will typically underperform a Riegel prediction at longer distances relative to shorter ones, because the mass cost compounds with duration. A strong 5K predicting a marathon is optimistic for a 200 lb hybrid athlete in a way it is not for a 140 lb runner.

That is not a reason to stop lifting. It is a reason to set marathon goals from a half marathon result rather than from a 5K.

What to do next

Race the distance closest to your goal, then predict from that. Every step of extrapolation adds error, and the error is always in the flattering direction.

What one result says about every other distance.