How to use
Enter the distance you actually raced and choose kilometres or miles. Use a recent, sustained race effort rather than a relaxed training run. Enter the complete finish time, including any stops, as MM:SS or H:MM:SS. A value of 25:00 means 25 minutes; 1:25:00 means one hour and 25 minutes.
Choose a target distance and select Calculate. The main result is the predicted finish time. The comparison table uses the same recorded performance for all four distances; it is not four independent forecasts.
The Riegel formula
T₂ = T₁ × (D₂ ÷ D₁)1.06. Both distances use the same unit and time is calculated in seconds. The fixed exponent models a gradual loss of pace as distance increases. Peter Riegel’s 1981 paper describes a power-law model of endurance records; 1.06 is the conventional running-prediction exponent used here.
The half marathon is 21.0975 km and the marathon is 42.195 km. One mile is exactly 1.609344 km. Calculations keep these distances unrounded; displayed times are rounded to whole seconds and are still only estimates.
Worked examples
A 5 km race in 25:00 predicts a 10 km time of about 0:52:07. A 10 km race in 50:00 predicts a 5 km time of about 0:23:59. Predicting the same distance returns the recorded time.
The faint initial example can be calculated immediately. Focusing an example field clears it; leaving that field empty restores the example without restoring an old result. Your own entries remain unchanged.
Scope and limitations
The original paper discusses an endurance range of 3.5–230 minutes. We use that as a conservative tool boundary for recorded and predicted times, plus a 1.5 km to marathon distance scope. The input pace guard of 2:30–15:00 per km catches obvious entry mistakes; it is a product guardrail, not a universal physiological limit.
The formula cannot account for hills, heat, wind, fatigue, pacing, nutrition or an individual endurance profile. Large distance jumps are especially uncertain. Adequate preparation for the target distance is assumed; no training plan, medical advice or promised performance is provided.
Questions and answers
Why is a comparison cell blank?
An estimate outside the supported time range is replaced by a dash and an explanation, rather than presented as a precise prediction. A selected target outside that range also shows that explanation as the main result.
How do I save or revise a result?
Copy result copies the confirmed prediction, inputs, comparison and limitations as text. Editing any input clears the confirmed result and disables Copy until you calculate again. Clear removes your entries while keeping unit and target selections. Nothing you enter is uploaded by this calculator.
Sources
Riegel (1981), Athletic records and human endurance · Prediction and Quantification of Individual Athletic Performance of Runners (2016)