Predict your marathon time from a half marathon result

The basic multiplier method

The simplest prediction uses a fixed multiplier: multiply your half marathon time by 2.1. A runner who completes a half marathon in 1:45 would predict 3:40 for the marathon (105 minutes × 2.1 = 220.5 minutes). This accounts for the aerobic and muscular demands increasing beyond simple doubling of distance.

Why the multiplier isn't always 2.0

Doubling 13.1 miles gives 26.2 miles, but doubling pace doesn't work. The marathon depletes glycogen reserves differently, mental fatigue increases, and pacing strategy changes. Faster runners (sub-1:30 half marathons) often see multipliers closer to 2.05; slower runners (over 2:00) may see 2.15–2.2 as the aerobic system works at higher relative intensity over longer duration.

Adjusting for training readiness

• Well-trained for marathon (12+ weeks structured training): use 2.05–2.08 multiplier
• Moderate marathon prep (8–10 weeks training): use 2.1–2.12 multiplier
• New to marathon distance: use 2.15–2.2 multiplier
• Racing a half marathon, not training for it: add 0.03–0.05 to multiplier (effort was maximal, not sustainable)

Course and condition factors

If your half marathon was on a fast, flat course and your marathon will be hilly or exposed, add 5–10 minutes to the prediction. Conversely, if the marathon is known to be fast (like London), subtract 3–5 minutes. Weather on race day also shifts prediction by several minutes; heat and wind typically cost 30 seconds per mile or more.

When the prediction breaks down

1. You raced the half marathon all-out without marathon training. Predictions assume you've built aerobic and muscular endurance specific to 26.2 miles.
2. Your half marathon was a supported workout, not a goal race. An honest effort time is essential.
3. You're new to racing or have irregular training history. Predictions work best with consistent running patterns.
4. You have unaddressed injuries or biomechanical issues. These often surface only in the final 6–8 miles of a marathon.

Using your prediction as a training anchor

Your predicted marathon time guides pacing for long runs and tempo work. If you predict 3:45, you'd run marathon-pace intervals at roughly 8:33 per mile. Nail those workouts at that pace, and your prediction becomes realistic. Miss them consistently, and you'll need to adjust upward by 30 seconds to 2 minutes per mile.

Data-driven adjustment during training

The most accurate marathon prediction updates as you train. RunV-style coaching tracks how you respond to marathon-pace work, long runs and hard efforts over 8–12 weeks. If you're consistently running marathon-pace repeats faster than predicted, the estimate moves down. If you're struggling to hit target paces in week 16 long runs, the prediction adjusts upward. Real training data beats any formula.