Race Time Predictor
Recent race time → predicted time at another distance, plus pace targets. Uses the Riegel formula (1981), the industry standard.
Predicted
Estimated finish time
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Pace per mile
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Pace per km
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Predictions across distances
The Riegel formula
Pete Riegel (1981, Runner's World) fit empirical race data and found that the 1.06 exponent — slightly above 1 — captures the typical "tax" of going longer. Race pace drops as distance grows, but not as much as a naive linear extrapolation would suggest.
When Riegel is accurate
Best between 5K and marathon. Less accurate for very short sprints (where pace is dominated by neuromuscular speed) and for ultras (where fueling, terrain, and time on feet matter more than aerobic capacity).
The catch: training matters
Riegel assumes you've trained for the target distance. A solid 5K runner who hasn't done long runs above 10 miles will not run the Riegel-predicted marathon — they'll blow up at mile 18. The formula tells you what your current aerobic capacity supports; the marathon also requires endurance you have to build separately.
Other predictors
Daniels' VDOT tables are more nuanced (account for VO₂ max, training zones, lactate thresholds) but require more inputs. McMillan's calculator is similar to Riegel under the hood. For quick predictions in races you've trained for, Riegel is reliable enough.