Enter your 5K time to see what it predicts for the marathon.
Your 5K time reveals your current aerobic capacity. This table shows what that aerobic capacity predicts for the marathon — the equivalent time a runner with the same fitness would run, assuming full marathon-specific preparation. The formula is StrideIQ’s RPI equation; the caveat is that marathon performance requires distance-specific training that raw aerobic fitness alone does not provide.
Quick answer: A 22:00 5K runner (RPI 44.5) has equivalent Marathon fitness of 3:30:12 (8:01/mi). Range in this table: 16:00 5K → 2:33:24 Marathon through 35:00 → 5:28:14.
| 5K Time | RPI | Marathon Equivalent | Equiv Pace |
|---|---|---|---|
| 16:00 | 64.6 | 2:33:24 | 5:51/mi(3:38/km) |
| 17:00 | 60.2 | 2:42:56 | 6:13/mi(3:52/km) |
| 18:00 | 56.3 | 2:52:29 | 6:35/mi(4:05/km) |
| 19:00 | 52.9 | 3:01:49 | 6:56/mi(4:18/km) |
| 20:00 | 49.8 | 3:11:19 | 7:18/mi(4:32/km) |
| 21:00 | 47 | 3:20:48 | 7:40/mi(4:46/km) |
| 22:00 | 44.5 | 3:30:12 | 8:01/mi(4:59/km) |
| 24:00 | 40.2 | 3:48:42 | 8:43/mi(5:25/km) |
| 26:00 | 36.6 | 4:07:00 | 9:25/mi(5:51/km) |
| 28:00 | 33.5 | 4:25:24 | 10:07/mi(6:17/km) |
| 30:00 | 30.8 | 4:43:48 | 10:49/mi(6:43/km) |
| 35:00 | 25.6 | 5:28:14 | 12:31/mi(7:47/km) |
Computed via StrideIQ’s RPI equation. Predictions assume distance-specific training is in place.
The 5K → marathon prediction assumes comparable marathon training: 18–20 mile long runs, marathon-pace work, consistent high-volume build. Without this, most runners underperform their aerobic equivalency by 10–20 minutes. Use this as a potential ceiling and a training goal, not a race-day prediction without distance-specific preparation.
The RPI equation is highly accurate for predicting aerobic potential — it gives the marathon time that the same aerobic capacity would support. The accuracy gap in practice comes from marathon-specific training: long runs, glycogen economy, and pacing experience. Runners with full marathon preparation tend to run within 5–10 minutes of their equivalency. Runners who are primarily 5K-trained typically underperform by 15–25 minutes due to insufficient long-run adaptation.
A 20:00 5K gives an RPI of 49.8. The equivalent marathon time is 3:11:19 at 7:18/mi. This assumes full marathon training. Without distance-specific long runs and marathon-pace work, expect to run 15–20 minutes slower.
The most common cause is insufficient marathon-specific training. The 5K equivalency predicts aerobic potential, not distance-specific readiness. The marathon demands adaptations — long-run glycogen economy, marathon-pace stamina — that 5K training does not develop.