5K to Marathon Conversion Calculator
Your 5K time is a strong predictor of Marathon performance. This calculator converts your track fitness to a road racing target by factoring in the increased endurance, pacing discipline, and aerobic at 75-85% VO2max required at Marathon distance.
Quick answer: a 20:00 5K converts to about 3:34:13 for the marathon (typical range 3:20:00 to 3:55:03).
5K to Marathon conversion table
Estimates only – individual results vary with training, conditions and event experience.
| 5K time | Predicted Marathon | Range |
|---|---|---|
| 14:00 | 2:29:57 | 2:20:00–2:44:32 |
| 14:45 | 2:37:59 | 2:27:30–2:53:21 |
| 15:30 | 2:46:01 | 2:35:00–3:02:10 |
| 16:15 | 2:54:03 | 2:42:30–3:10:59 |
| 17:00 | 3:02:05 | 2:50:00–3:19:47 |
| 17:45 | 3:10:07 | 2:57:30–3:28:36 |
| 18:30 | 3:18:09 | 3:05:00–3:37:25 |
| 19:15 | 3:26:11 | 3:12:30–3:46:14 |
| 20:00 | 3:34:13 | 3:20:00–3:55:03 |
| 20:45 | 3:42:14 | 3:27:30–4:03:52 |
| 21:30 | 3:50:16 | 3:35:00–4:12:41 |
| 22:15 | 3:58:18 | 3:42:30–4:21:29 |
| 23:00 | 4:06:20 | 3:50:00–4:30:18 |
| 23:45 | 4:14:22 | 3:57:30–4:39:07 |
| 24:30 | 4:22:24 | 4:05:00–4:47:56 |
| 25:15 | 4:30:26 | 4:12:30–4:56:45 |
| 26:00 | 4:38:28 | 4:20:00–5:05:34 |
| 26:45 | 4:46:30 | 4:27:30–5:14:23 |
| 27:30 | 4:54:32 | 4:35:00–5:23:11 |
| 28:15 | 5:02:34 | 4:42:30–5:32:00 |
| 29:00 | 5:10:36 | 4:50:00–5:40:49 |
| 29:45 | 5:18:38 | 4:57:30–5:49:38 |
How it works
The 5K→Marathon conversion blends Riegel's power-law scaling with VDOT oxygen-cost validation. With a 8x distance jump, the predictor applies additional correction factors for the major physiological shift between these events. Where both distances fall within aerobic range, critical speed modeling adds a third reference point. The result is a predicted time with a confidence range.
Training advice
Use a recent race or time-trial 5K for the best prediction. To reach your Marathon target, build high mileage and fueling strategy progressively. Marathon demands specific preparation — your 5K fitness provides the foundation, but high mileage and fueling strategy closes the gap.
Frequently asked questions
How accurate is a 5K to Marathon prediction?
With a recent all-out 5K time, predictions are typically within 4-8% of actual Marathon performance. Accuracy improves when the 5K effort is from the past 6-8 weeks under normal conditions.
What training should I do to reach my predicted Marathon time?
Focus on high mileage and fueling strategy. Your 5K fitness provides the speed foundation — building aerobic at 75-85% VO2max capacity through progressive training is the key to hitting the target.
Can I use an old 5K time for this prediction?
Recent times (within 6-8 weeks) give the most reliable predictions. Older results may not reflect your current fitness, especially if your training has changed significantly.