Why does the tool use a 1.06 exponent instead of simply doubling?

The 1.06 exponent reflects fatigue accumulation over longer distances. Doubling half-marathon time assumes linear pace degradation, but empirical data show that pace slows more steeply as distance increases beyond lactate-threshold duration. Riegel's model captures this nonlinear slowdown with an exponent derived from aggregate race-performance data.

How recent should my half-marathon time be for an accurate prediction?

Predictions are most reliable when the input race occurred within 8–12 weeks of the target marathon and reflects current fitness. Older performances may not account for gains or losses in aerobic capacity, neuromuscular efficiency, or body composition. Seasonal detraining or injury layoffs reduce the validity of older race times.

Can this calculator predict ultra-marathon times?

No. The Riegel formula with exponent 1.06 is calibrated for road-race distances from 5K to marathon. Ultra distances introduce additional variables—terrain technicality, mandatory aid-station stops, sleep deprivation—that fall outside the model's scope. Separate prediction equations exist for ultra events, typically with higher exponents (1.08–1.10).

What if my half-marathon was hilly or had poor weather?

The tool applies a mathematical ratio without adjusting for course difficulty or environmental conditions. A half-marathon completed on a hilly course or in heat may underestimate marathon potential if the goal race is flat and cool. Conversely, a fast, downhill half may overestimate marathon capability. Comparing performances on similar courses improves accuracy.

Does the prediction change if I add marathon-specific training?

The calculator outputs a static estimate based on current half-marathon performance. Marathon-specific training—progressive long runs, tempo work at goal pace, glycogen-depletion runs—can improve the fatigue resistance that Riegel's exponent approximates. Structured training may enable results faster than the initial prediction, while inadequate long-run volume may lead to slower outcomes.

Running

Marathon Time Predictor

Estimate your marathon finish time from a recent half-marathon performance using Riegel's formula.

Last updated: 22 April 2026

What this tool does

This calculator estimates marathon finish time from a recent half-marathon performance using the Riegel formula, a widely-used endurance prediction model published in Runner's World in 1977. It applies a fatigue exponent of 1.06 to extrapolate race time across the distance ratio (21.0975 km to 42.195 km), taking half-marathon time in minutes as input and returning a predicted marathon time with average pace. The estimate assumes similar training status, race conditions, and pacing strategy between the two distances, and tends to be most accurate for runners who have trained specifically for the marathon distance.

Inputs

Result

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Formula Used

T_{42.2} = T_{ha l f} \times (\frac{42.2}{21.1})^{1.06}

T_{ha l f}

Half-marathon time in minutes

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How Marathon Time Predictor works

This calculator applies the Riegel formula to estimate full-marathon (42.195 km) finish time from a completed half-marathon performance. The tool multiplies your half-marathon time by a distance-ratio factor raised to a fatigue exponent of 1.06, reflecting the nonlinear relationship between race distance and sustainable pace. The output displays predicted marathon time, average pace per kilometer, and a reference copy of your input half-marathon time.

The formula

The calculation uses Riegel's exponential model: T₂ = T₁ × (D₂ / D₁)^1.06, where T₁ is your half-marathon time (minutes), D₁ is 21.0975 km, D₂ is 42.195 km, and 1.06 is the fatigue exponent. The tool converts the result to hours:minutes:seconds format and computes average pace by dividing total seconds by 42.195.

Where this method is most accurate

Riegel extrapolation tends to perform best when the input race was completed at maximal aerobic effort, recent (within 8–12 weeks), and conducted under similar conditions (temperature, elevation, course profile) expected on marathon day. The 1.06 exponent represents an aggregate across trained runners; individual fatigue curves vary with training volume, long-run frequency, and lactate-threshold conditioning. Predictions may overestimate performance for athletes new to marathon distance or underestimate for those with marathon-specific training blocks.

What this tool does not do

This calculator generates a mathematical projection; it does not prescribe training plans, race-day pacing strategies, or individualized taper protocols. It does not account for weather variability, fueling mishaps, injury history, or cumulative fatigue from weekly mileage. The tool does not replace structured long runs, tempo sessions, or coached programming designed to build marathon-specific endurance.

Disclaimer

This tool provides educational estimates for informational purposes only. It is not medical advice, coaching instruction, or a performance guarantee. Race outcomes depend on training status, environmental conditions, and physiological adaptation. Consult a qualified running coach or sports-medicine professional before undertaking marathon training or adjusting race-day goals.

Questions

Why does the tool use a 1.06 exponent instead of simply doubling?: The 1.06 exponent reflects fatigue accumulation over longer distances. Doubling half-marathon time assumes linear pace degradation, but empirical data show that pace slows more steeply as distance increases beyond lactate-threshold duration. Riegel's model captures this nonlinear slowdown with an exponent derived from aggregate race-performance data.
How recent should my half-marathon time be for an accurate prediction?: Predictions are most reliable when the input race occurred within 8–12 weeks of the target marathon and reflects current fitness. Older performances may not account for gains or losses in aerobic capacity, neuromuscular efficiency, or body composition. Seasonal detraining or injury layoffs reduce the validity of older race times.
Can this calculator predict ultra-marathon times?: No. The Riegel formula with exponent 1.06 is calibrated for road-race distances from 5K to marathon. Ultra distances introduce additional variables—terrain technicality, mandatory aid-station stops, sleep deprivation—that fall outside the model's scope. Separate prediction equations exist for ultra events, typically with higher exponents (1.08–1.10).
What if my half-marathon was hilly or had poor weather?: The tool applies a mathematical ratio without adjusting for course difficulty or environmental conditions. A half-marathon completed on a hilly course or in heat may underestimate marathon potential if the goal race is flat and cool. Conversely, a fast, downhill half may overestimate marathon capability. Comparing performances on similar courses improves accuracy.
Does the prediction change if I add marathon-specific training?: The calculator outputs a static estimate based on current half-marathon performance. Marathon-specific training—progressive long runs, tempo work at goal pace, glycogen-depletion runs—can improve the fatigue resistance that Riegel's exponent approximates. Structured training may enable results faster than the initial prediction, while inadequate long-run volume may lead to slower outcomes.

Sources & Methodology

Applies the Riegel formula T₂ = T₁ × (D₂/D₁)^1.06 with a fatigue exponent of 1.06, extrapolating from half-marathon (21.0975 km) to marathon (42.195 km) distance. Developed by Peter Riegel and published in Runner's World (1977).

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