Marathon Time Projected from Half

Marathon time projected from half: Riegel and practical rule

There are two common ways to project the marathon from the half. The practical rule is the quick one: double your half time and tack on 10-15 minutes to cover the famous "wall" after km 30. The Riegel formula is more precise, T(42.2) = T(21.1) · 2^1.06, where the 1.06 exponent captures how performance drops off non-linearly. Take a half at 1:30 (90 min) as an example. Riegel gives 90 · 2^1.06 = 90 · 2.085 ≈ 188 min ≈ 3:08 for the marathon, while the practical rule gives 1:30 · 2 + 12 min = 3:12. Most of the time the two land within 5-10 min of each other.

Applications: training planning and goal setting

This shows up in marathon training planning, where you check 8-12 weeks out whether the target pace is realistic. It helps with the goal-race decision too, telling you if sub-3:30, sub-3, or sub-2:30 is within reach off your current half. An amateur sub-3 tends to need a HM under 1:25, and an elite sub-2 like Kipchoge sits around a 59 min HM. One caveat: the projection holds up best when terrain and weather match. A flat HM rarely predicts a hilly marathon well.

FAQ

Why isn't the marathon exactly double the half? The second half runs slower because fatigue piles up, glycogen runs low (that's the "wall" at km 30), and your body spends more on thermoregulation.

How reliable is Riegel? Pretty reliable for runners who've done marathon-specific training. Skip the long runs above 30 km, though, and the projection comes in under your real time.

What if I never ran a marathon? Treat Riegel as a starting point. A first marathon usually comes in 5-10 min over the projection, mostly because pacing takes experience to get right.