Race Time Predictor
Predict your finish time at any race distance from a recent result. Uses the Riegel and Cameron formulas plus a VDOT fitness score.
1.06 is the standard exponent for trained runners. Lower (1.04–1.05) for elites; higher (1.08–1.12) for beginners.
Estimated VDOT
fitness score
Riegel prediction
3:59:46
5:40 /km · 9:08 /mi
Predicted Race Times
| Distance | Riegel | Pace /km | Pace /mi |
|---|---|---|---|
| 5K | 0:25:00 | 5:00 | 8:02 |
| 10K | 0:52:07 | 5:12 | 8:23 |
| 15K | 1:20:06 | 5:20 | 8:35 |
| 10 Mile | 1:26:18 | 5:21 | 8:37 |
| Half Marathon | 1:55:00 | 5:27 | 8:46 |
| Marathon | 3:59:46 | 5:40 | 9:08 |
Riegel predictions use a fatigue factor of 1.06.
Predict your finish time at any race distance from a recent result. The predictor pairs the standard Riegel formula (F5), with its customisable fatigue factor, against the Cameron model (F6), which adjusts for distance-specific endurance. Enter a recent race — a 5K, 10K, half, or marathon — and you'll see predicted times for every standard distance, a Daniels VDOT fitness index (F7), and the pace per km and per mile for each projection. Below the calculator you'll find the mathematics behind both models, their divergence patterns, what a realistic fatigue factor looks like for your training level, and the accuracy boundaries of race prediction in general.
T2 = T1 × (D2 ÷ D1)^k · Riegel k=1.06 · Cameron uses a distance-adjusted coefficient- T1
- = Known race time (e.g., recent 5K result)
- D1
- = Known race distance in kilometres
- T2
- = Predicted race time at the target distance
- D2
- = Target race distance in kilometres
- k
- = Fatigue exponent — 1.06 (Riegel); 1.04–1.08 range depending on endurance profile
- VDOT
- = Daniels running fitness index derived from F7 using T1 and D1
Worked example — 25:00 5K → marathon prediction
- T1 = 25.000 min D1 = 5.0 km D2 = 42.195 km
- ratio = 42.195 ÷ 5.0 = 8.439
- ratio^1.06 = 8.439^1.06 = 9.318
- T2_riegel = 25.000 × 9.318 = 232.95 min → 3:52:57
- T2_cameron = 25.000 × 9.022 = 225.55 min → 3:45:33 (coefficient ~0.968 of Riegel at this ratio)
- VDOT (F7) ≈ 45.2
- = Marathon prediction 3:45:33 (Cameron) ↔ 3:52:57 (Riegel) — VDOT 45.2
Riegel (F5) and Cameron (F6) agree closely at 10K → half; they diverge most sharply when predicting marathon from a 5K or predicting 5K from a marathon, because the exponent k is a single value for Riegel but distance-aware for Cameron. Both assume consistent training, a similar course profile, and a similar effort level between anchor and target.
| 5K | 10K | 15K | 10 mi | Half | Marathon |
|---|---|---|---|---|---|
| 16:00 | 33:18 | 50:56 | 54:48 | 1:13:22 | 2:34:22 |
| 18:00 | 37:29 | 57:17 | 1:01:39 | 1:22:32 | 2:53:39 |
| 20:00 | 41:39 | 1:03:39 | 1:08:30 | 1:31:42 | 3:12:55 |
| 22:00 | 45:49 | 1:10:01 | 1:15:21 | 1:40:52 | 3:32:12 |
| 25:00 | 52:04 | 1:19:34 | 1:25:38 | 1:54:37 | 4:01:07 |
| 28:00 | 58:19 | 1:29:07 | 1:35:54 | 2:08:23 | 4:30:01 |
| 30:00 | 1:02:29 | 1:35:29 | 1:42:46 | 2:17:33 | 4:49:18 |
| 33:00 | 1:08:44 | 1:45:02 | 1:53:02 | 2:31:18 | 5:18:12 |
| 36:00 | 1:14:59 | 1:54:35 | 2:03:19 | 2:45:04 | 5:47:06 |
| 40:00 | 1:23:19 | 2:07:18 | 2:16:55 | 3:03:25 | 6:25:44 |
Riegel vs. Cameron: When They Diverge
Riegel (1977)
- Single fatigue exponent k = 1.06 across all distance ratios
- Simple, widely adopted, easy to sanity-check by hand
- Closest to reality for jumps of 1.5×–3× the anchor distance
- Tends to overpredict marathon time from a short-distance race
- Tends to underpredict 5K from a marathon result
The workhorse — trust it in the 10K→half and half→marathon windows
Cameron (1998)
- Distance-aware coefficient; the effective exponent shifts with event length
- Historically tuned on elite road data; middle distances most accurate
- Matches Riegel closely inside the 10K–half band
- Predicts faster marathon times from short efforts than Riegel does
- Softens the penalty when extrapolating well beyond the anchor
Use as a second opinion — most valuable when extrapolating marathon from 5K
What k means, by runner profile
Elite / highly trained
1.04–1.06
High lactate threshold fraction, well-developed aerobic base; small penalty going long
Experienced recreational
1.05–1.07
Consistent weekly mileage with at least one long run each week
Riegel default
1.06
The canonical 1977 value; a sensible starting point for most adults
Newer runner with a short base
1.08–1.12
Moderate weekly mileage, limited long runs; larger fall-off with distance
Beginner / couch-to-5K graduate
1.10–1.15
Aerobic engine still developing — half and marathon predictions will likely be optimistic
Understanding Your VDOT Score
What VDOT is
Daniels' pseudo-VO2 max index — a single number that captures current running fitness. Derived from a recent race result rather than a lab test. Scale runs from about 30 (new runner) to 85 (elite).
How to use it
VDOT feeds directly into training paces — Easy, Marathon, Threshold, Interval, Repetition. Pair this predictor with the Training Pace Calculator to translate the same VDOT into daily run targets.
Its limitations
VDOT assumes you've raced close to your current fitness ceiling — use a hard, recent effort rather than a tempo run. It also doesn't account for heat, altitude, or course elevation; adjust expectations on race day accordingly.
| Anchor → Target | Typical accuracy | Notes |
|---|---|---|
| 5K → 10K | ±2% | Most reliable short jump; similar energy system |
| 10K → 15K | ±3% | Close to threshold on both ends; still tight |
| 5K → half | ±4–6% | Endurance starts to matter; Cameron narrows error |
| 10K → half | ±3–5% | Canonical jump — both models perform well |
| 5K → marathon | ±8–12% | Highest-error jump; long-run volume becomes decisive |
| 10K → marathon | ±5–8% | Assumes a 2+ month marathon build with weekly long runs |
| half → marathon | ±3–5% | Most accurate marathon prediction if fuelling is dialled in |
| marathon → 5K | ±5–7% | Reverse direction underpredicts speed — top-end not trained |
Explore More Running Tools
Training Pace Calculator
Feed a VDOT into Easy, Tempo, Interval, and Repetition targets.
VO2 Max Running Calculator
Estimate VO2 max directly from a recent race using Daniels' pseudo-VO2 curve.
Marathon Pace Calculator
Once you've predicted a marathon time, build the km-by-km pacing plan.
Age-Graded Running Calculator
Compare predicted times across age and sex using WMA factors.
Half Marathon Pace Calculator
The most reliable stepping stone to a marathon prediction.
Sources
- 1.Athletic records and the fatigue factor (F5) — Riegel, P. S. — American Scientist, 69(3), 285–290, 1981 (accessed 2026-04-21)
- 2.Cameron Model for race time prediction (F6) — Cameron, D. F. — CRS Performance Tables, 1998 (accessed 2026-04-21)
- 3.Daniels' Running Formula, 4th ed. — VDOT tables (F7) — Daniels, J. (Human Kinetics), 2013 (accessed 2026-04-21)
- 4.World Athletics current world records — World Athletics (accessed 2026-04-21)