Everyday utility
Predict 5K, 10K, half marathon, or marathon finish time from a recent race, then compare pace and splits.
When to use this
It is useful after a tune-up race, time trial, or recent personal best when you need a realistic target for another distance.
Default result
A 25:00 5K predicts a 52:07 10K at 5:13 per km with the default exponent.
Result interpretation
A 5K to 10K or 10K to half marathon jump usually gives a cleaner estimate than predicting a marathon from a short race.
Use the course adjustment for heat, hills, wind, trails, altitude, or a crowded start instead of treating the formula as exact.
The pace output is a starting point. Race strategy, fueling, warm-up, and training background still decide the day.
The default example starts with a recent 5K time of 25:00 and predicts a 10K using exponent 1.06.
Distance ratio = 10 / 5 = 2.
Predicted seconds = 1500 x (2 ^ 1.06) = 3,127.4 seconds.
Rounded predicted time = 52:07 for 10 km.
Pace = 5:13 per km, or 8:23 per mile.
An even halfway split would be 26:04.
The calculator uses T2 = T1 x (D2 / D1)^exponent. T1 is the recent race time, D1 is the recent race distance, D2 is the target race distance, and T2 is the predicted target time.
Choose half marathon as the recent distance, marathon as the target distance, and enter a recent half marathon time. The default exponent is a starting point; raise it if marathon endurance is a weakness.
Choose 10K as the recent distance and half marathon as the target. This jump is still endurance-sensitive, but it is usually more grounded than predicting a marathon from a 5K.
Use 1.06 first. Lower values make long-distance predictions faster; higher values make them slower. Keep the exponent editable when comparing athletes, training phases, or target distances.
Predictions are weaker when the recent race was not all-out, the target distance is much longer, the terrain changes, weather is severe, or the athlete is returning from injury.
Use the copy button to share the predicted time, pace, distance jump, and formula settings. Include the exponent and course adjustment so the result is interpretable.
| Reference point | Value | Source | Date | Verification note |
|---|---|---|---|---|
| Prediction formula | Riegel-style power law: T2 = T1 x (D2 / D1)^1.06 | Peter S. Riegel, Athletic Records and Human Endurance | As of June 19, 2026; article published May-June 1981 | The exponent is editable because individual endurance curves, terrain, weather, training, and distance jumps can differ. |
| Research context | Power-law running performance models remain a known benchmark method | Blythe and Kiraly, Prediction and Quantification of Individual Athletic Performance | As of June 19, 2026; arXiv version revised May 13, 2015 | The paper discusses individual performance prediction methods and the success of Riegel-style formulas as a documented benchmark. |
| Marathon distance | 42.195 km | World Athletics marathon discipline page | As of June 19, 2026 | Verify the exact event course distance and measurement before treating a prediction as a race plan. |
| Half marathon distance | 21.0975 km | World Athletics half marathon discipline page | As of June 19, 2026 | The calculator also includes 5K and 10K presets as exact kilometer inputs. |
It uses the Riegel-style power formula T2 = T1 x (D2 / D1)^exponent. The default exponent is 1.06, and you can edit it.
No. It is a planning estimate from one recent performance. Terrain, weather, pacing, injury, fueling, training, and course measurement can change the result.
Raise it when longer distances slow you down more than expected. Lower it when you are endurance-strong and the target distance is close to your recent race.
You can, but it is a large jump. Treat it as a rough upper-level estimate and validate it with longer workouts or recent longer races.
Use it for heat, hills, trail surface, altitude, wind, crowding, or known course difficulty. Positive values make the target slower; negative values make it faster.
The presets cover 5K, 10K, half marathon, and marathon, plus custom kilometer distances for unusual road, trail, track, or time-trial courses.
Decision path
Use the date calculator to count weeks between today, tune-up races, and race day.
Training timeIf training time is the blocker, compare small screen-time cuts with reclaimed hours.
Long viewUse a week-grid view if this race goal is part of a bigger life or health reset.
