5K to Marathon Race Equivalency
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 the Daniels/Gilbert oxygen cost 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 → Marathon Equivalency Table
| 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 Daniels/Gilbert oxygen cost equation (1979). Predictions assume distance-specific training is in place.
How accurate is this prediction?
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.
Equivalency predicts potential — training determines actuals
This table shows what your aerobic capacity predicts at a different distance. StrideIQ tracks whether your training is actually developing the distance-specific fitness needed to meet that potential — long-run adaptation, pacing control, threshold stamina — from your own workout data.
Common questions
How accurate is the 5K to marathon equivalency?
The Daniels/Gilbert formula 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.
What is the marathon equivalent of a 20:00 5K?
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.
Why does my marathon time differ from the prediction?
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.