Race Time Predictor
Predict your finish time at a new distance from a recent race result, using Riegel’s formula, with the pace you’d need to hold.
Predicted finish time
Uses Riegel’s formula, T2 = T1 x (D2 / D1)^1.06. It is most reliable when the two distances are within a few-fold of each other and you have trained appropriately for the longer one; predictions for a marathon from a 5K, for example, tend to run optimistic.
From a 5K to your next distance
A recent race is a snapshot of your current fitness, and Riegel’s formula extrapolates from it to other distances. It is a handy reality check before you commit to a goal time — a way to see whether the 10K target in your head lines up with the 5K you just ran.
For running parents building back to racing, it also helps set expectations kindly: enter an honest recent effort and let the prediction, not last year’s personal best, shape the plan.
Frequently Asked Questions
How does the race time predictor work?
It uses Peter Riegel's formula, T2 = T1 × (D2 / D1)^1.06. It takes a recent race time (T1) at one distance (D1) and scales it to your target distance (D2). The 1.06 exponent captures the fact that you slow down slightly as distance grows.
How accurate is a Riegel prediction?
It is most reliable when the two distances are within a few-fold of each other — say predicting a 10K from a 5K, or a half marathon from a 10K — and when your training supports the longer distance. Predicting a marathon from a single 5K tends to be optimistic because endurance, fueling, and long-run training matter more as the race lengthens.
Which recent race should I use?
Use a genuine hard effort at a distance reasonably close to your target, run in similar conditions. A parkrun 5K is a common input. The fresher and more representative the effort, the better the prediction.
Why does a longer prediction show a slower pace?
The formula assumes you can't hold your 5K pace for a marathon, so predicted pace gets slightly slower as the distance rises. That reflects real physiology — sustainable pace drops as the event gets longer.
Can I use it to set a goal pace?
Yes, as a starting point. Take the predicted time, look at the predicted pace per kilometre, and sanity-check it against your training. It is a target to train toward, not a guarantee of race day.
Estimates and heuristics for general information — not medical or professional fitness advice. If you are returning to running after pregnancy, get medical clearance from your doctor or a pelvic-health professional first.