Runners moving up from half marathon to full marathon almost always want the same answer before they've raced the distance: what should my marathon goal be, based on what I already know I can run? Two common models answer that question, and they disagree with each other more than most runners expect.

Method one: Riegel's formula

The Riegel endurance formula, the same model behind most race predictors on this site, treats the half-to-marathon jump as a straightforward exponential fatigue calculation — multiply your half marathon time by roughly 2.085 (rather than a flat 2.0) to account for the extra fatigue of doubling the distance.

Method two: the coaching rule of thumb

A widely used alternative, common among marathon coaches, is simpler and more conservative: double your half marathon time, then add a cushion — commonly 10-20 minutes for a first-time or moderately trained marathoner — to account for factors Riegel's pure math doesn't capture well at this specific distance jump: glycogen depletion past 18-20 miles, fueling and hydration logistics that don't exist in a half marathon, and the mental toll of a distance nearly twice as long.

Where they land, and why the gap matters

Worked example: A 1:45:00 half marathon predicts to roughly 3:38:30 under Riegel's 2.085 multiplier. The coaching rule — double plus a 15-minute cushion — lands closer to 3:45:00. That's a 6-7 minute gap between two reasonable, commonly used methods, which matters enormously for pacing strategy on race day.

The gap exists because Riegel's formula assumes you're equally well-trained for the marathon distance as you are for the half — an assumption that holds for experienced marathoners repeating the distance, and holds much less well for someone racing 26.2 miles for the first time, where fueling execution and long-run experience matter as much as raw fitness.

Which to trust

First-time marathoners are generally better served trusting the more conservative, cushion-added number — going out at the Riegel-predicted pace and blowing up at mile 20 is a far worse outcome than finishing a few minutes slower than the aggressive prediction. Experienced marathoners with a solid long-run base and practiced fueling can lean closer to the Riegel number. Run both through the Half → Marathon Predictor and treat the spread between them as your safety margin, not a discrepancy to resolve.