Race Time Predictor
Enter a recent race — see your predicted finish times at every common distance, from 1500m through the marathon. Built on Jack Daniels' Running Formula, which captures the fact that you can sustain a higher percentage of your maximum aerobic capacity for shorter races.
Race Time Predictor
Predicted race times
| 1500m | 5:25 |
| 1 mile | 5:51 |
| 3000m | 11:35 |
| 5kyour input | 20:00 |
| 8k | 32:47 |
| 10k | 41:28 |
| 15k | 1:03:46 |
| 10 mile | 1:08:45 |
| Half marathon | 1:31:50 |
| Marathon | 3:11:17 |
Predictions assume similar training, course, and conditions. Longer-distance predictions also require adequate endurance base — a sharp 5K doesn't guarantee the marathon time if your long-run mileage hasn't been built up.
How race predictions work
The idea behind race time prediction is that your current running fitness — often summarized as a VDOT score — determines a relationship between distance and time. Enter one data point (a recent race) and the formula solves for where you should finish at every other distance.
Two models dominate the field. The older Riegel formula (T₂ = T₁ × (D₂/D₁)1.06) is simple but tends to over-predict longer distances from short inputs. The Jack Daniels VDOT model, which this tool uses, accounts for how race duration changes the percentage of your maximum aerobic capacity you can sustain — making it more realistic for long-distance predictions.
What the predictions assume
- Full-effort, recent input race. A workout or under-raced time will under-represent your fitness.
- Similar conditions. A flat road race predicts other flat road races. Trails, heat, altitude, and hills all change actual finish times.
- Adequate endurance base.Long-distance predictions assume you have the weekly mileage and long runs behind you to sustain the predicted pace. A 20-minute 5K translates to a ~3:10 marathon only if you've built the endurance to race 26.2 miles — not just to run 3 miles fast.
- Reasonable pacing.Predictions assume even effort. Positive-split marathons that fade in the final 10K will miss the model's target.
Predictions vs. training paces
Race predictions tell you what your current fitness can produce. They do not tell you how to train to get there. For workout-specific paces (easy, marathon, threshold, interval, repetition) based on your current VDOT, use the VDOT calculator. To convert between distance, time, and pace for any given run, use the pace calculator.
Frequently asked questions
Why does my predicted mile time seem too slow?
The mile is short enough that absolute speed (neuromuscular, not aerobic) matters a lot. If you have specific mile speed from track work, you may out-perform the prediction. The formula predicts a fitness-equivalent mile, not your actual peak speed.
My 5K is sharp but my marathon prediction looks optimistic.
That's the endurance caveat in action. The model can only see your current VDOT, not your training history. If you haven't built the long-run base, the marathon prediction is aspirational — you'd need to build mileage first.
Should I trust the half-marathon prediction from a 10K?
Yes, more than the marathon prediction. The half is close enough in physiology to the 10K that the model is reliable, provided you've raced a few halves worth of long runs.