Half marathon to
marathon pace
Your half marathon time is the single best predictor of marathon performance — if you convert it correctly. The conversion math, the full pace table, and the pacing mistakes that ruin race day.
Multiply your half time by 2.085
Marathon time ≈ half marathon time × 2.085. Not × 2.0 — the marathon is harder than two halves stitched together. That 0.085 multiplier accounts for accumulated fatigue, glycogen depletion, and the well-documented slowdown in the second half of any race that's roughly twice as long.
T_marathon = T_half × 21.06 ≈ T_half × 2.085
The exponent 1.06 comes from Pete Riegel's 1977 race time predictor formula, which models how pace degrades with distance for trained runners. For pace (instead of time): marathon pace ≈ half marathon pace × 1.085, or roughly +8.5% slower per kilometer.
Worked example. Half marathon: 1:45:00 = 6300 seconds, 4:58 min/km pace.
- Marathon time = 6300 × 2.085 = 13136 sec ≈ 3:38:56
- Marathon pace = 4:58 × 1.085 ≈ 5:23 min/km
- That's 25 seconds per kilometer slower than half pace.
Half marathon → Marathon
Pre-calculated for every common half marathon time. Find yours and read off the marathon pace.
| Half time | Half pace (min/km) | Marathon time | Marathon pace (min/km) | Δ pace |
|---|---|---|---|---|
| 1:15 | 3:33 | 2:36:23 | 3:42 | +9 s |
| 1:20 | 3:48 | 2:46:40 | 3:57 | +9 s |
| 1:25 | 4:02 | 2:57:00 | 4:12 | +10 s |
| 1:30 | 4:16 | 3:07:24 | 4:26 | +10 s |
| 1:35 | 4:30 | 3:17:48 | 4:41 | +11 s |
| 1:40 | 4:44 | 3:28:12 | 4:56 | +12 s |
| 1:45 | 4:58 | 3:38:36 | 5:11 | +13 s |
| 1:50 | 5:13 | 3:49:00 | 5:26 | +13 s |
| 1:55 | 5:27 | 3:59:24 | 5:41 | +14 s |
| 2:00 | 5:41 | 4:09:48 | 5:56 | +15 s |
| 2:10 | 6:10 | 4:30:36 | 6:25 | +15 s |
| 2:20 | 6:38 | 4:51:24 | 6:55 | +17 s |
| 2:30 | 7:07 | 5:12:12 | 7:24 | +17 s |
Conversion uses Riegel's formula with the standard 1.06 exponent. For runners with weak long-run base, the marathon time can be 5-15 minutes longer than predicted.
Three mistakes that destroy the conversion
1. Running marathon at half pace
The biggest race-day error: feeling fresh at km 5, falling into half-marathon rhythm by km 10, blowing up at km 30. Half pace is 15-25 sec/km too fast for the marathon. Your aerobic system can't sustain it past 90 minutes without glycogen running out.
Fix: set the watch alarm. Pre-program goal marathon pace, monitor every kilometer for the first 25 km. The discipline of running 15-20 sec/km slower than feels comfortable is the single highest-impact race-day skill.
2. Predicting from an old half marathon
A half marathon from 12 weeks ago doesn't reflect marathon-specific fitness. The classic tune-up half is 4-6 weeks before race day, at race effort — that's the input data the formula was calibrated for.
Fix: if your last half was over 8 weeks ago, run a controlled half marathon-effort 15-21 km in the last 5 weeks to get fresh prediction data.
3. Ignoring the long-run base
A 1:30 half time predicts a 3:07 marathon — but only if your weekly mileage is over 50 km and your longest run is 30+ km. Without that base, the body runs out of glycogen at km 28-32 and adds 10-15 minutes of slowdown the formula can't see.
Fix: if you ran a fast half but skipped marathon-specific long runs, add 10 minutes to the formula prediction as a realistic goal.
Frequently asked questions
How much slower is marathon pace than half marathon pace?
How do I convert half marathon pace to marathon pace?
Can I double my half marathon time to get my marathon time?
How long before the marathon should I run the prediction half?
Does this work for a hilly half predicting a flat marathon?
Training plan
for your goal time
WattRun calibrates a 12-20 week marathon plan from your half marathon time. AI-built, adapts after every run, free.
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