Marathon Time Predictor
Predict your marathon finish time from any known race result using the Riegel formula. Shows predicted times from 1 mile to full marathon.
How to use this tool
- Enter known race distance, finish time — minutes and finish time — seconds in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your marathon time and the full breakdown beneath it.
The Riegel formula — T2 = T1 × (D2/D1)1.06 — predicts race finish times across distances using one known result. It is widely used by coaches for race pacing.
Predictions assume equivalent conditions and fitness. Not a substitute for a structured training plan.
Formula
Riegel formula: T2 = T1 × (D2 / D1)1.06
Where T1 is your known finish time, D1 is the known distance, and D2 is the target distance.
How it works
The Riegel endurance formula, published by Peter Riegel in 1977, predicts race times at other distances from a single known performance. The exponent 1.06 accounts for the physiological reality that pace slows at longer distances; a value greater than 1.00 means times scale faster than linearly with distance.
The formula is an empirical fit to road-race data and works best for distances between 1 mile and the marathon for well-trained runners. Predictions for untrained runners, hilly courses, or extreme temperatures will be less accurate. Use the chart to sense-check predicted times across multiple distances.
Worked example
- Known race: 10K (6.2 miles) in 60:00 (3,600 seconds). Predict marathon (26.2 miles).
- Ratio = 26.2 / 6.2 ≈ 4.226. Apply exponent: 4.2261.06 ≈ 4.608.
- Predicted marathon time = 3,600 × 4.608 ≈ 16,589 seconds = 4 hours 36 minutes 29 seconds. Hours = 4, remaining minutes = 36.
Predicted marathon finish: 4 hours 36 minutes (marathonHours: 4, marathonMinutes: 36).
Common mistakes to avoid
- Extrapolating from a 1-mile time trial — the Riegel exponent of 1.06 underestimates fatigue over marathon distance when the base distance is under 5K.
- Using a race time from an unusually fast or slow course (net downhill, extreme heat, high altitude) without adjusting, biasing the marathon prediction.
- Treating the prediction as a target race pace — the formula describes what you can physiologically sustain, not an optimal pacing strategy for race day.
Key terms
- Riegel formula
- An empirical endurance-scaling formula (T2 = T1 × (D2/D1)^1.06) that predicts race times at new distances from a known performance.
- Fatigue exponent
- The value 1.06 in the Riegel formula, representing how much pace degrades as race distance increases.
- Race predictor
- A tool or formula that estimates finish time at one distance based on a result from a different distance.
- Equivalent performance
- A finish time at a different distance that represents roughly the same level of fitness or effort as a known result.
- Pace drop-off
- The inevitable slowing of average pace as race distance increases, captured by the exponent > 1 in the Riegel formula.
Frequently asked questions
- How accurate is the Riegel formula?
- For distances within 2–3x of your reference race it is generally accurate within 5%. Accuracy decreases for very long extrapolations.
- What exponent should I use?
- The standard exponent is 1.06. Some calculators use 1.07 for ultra distances. This tool uses 1.06.