How to use the race time predictor
- Select your recent race distance from the presets (5K, 10K, HM, Marathon) or enter a custom distance in km.
- Enter your finish time in hours, minutes, and seconds.
- The calculator instantly shows predicted finish times for 8 distances — from 800m to marathon — using both the Riegel formula and Jack Daniels VDOT method.
- Use the Jack Daniels column for longer distance jumps (e.g. 5K → marathon). Use Riegel as a quick cross-check for shorter jumps.
- Use the training paces section to dial in your easy, threshold, and interval paces based on current fitness.
What is a race time predictor?
A race time predictor takes a recent race result — your time and distance — and uses a mathematical model to estimate what time you would run at a different distance, assuming equal fitness and preparation.
The prediction is not a goal. It is an equivalent performance — the time a runner of your current aerobic fitness would be expected to run if they trained appropriately for the target distance. A runner with a 20-minute 5K who has done no long runs will not run a 2:54 marathon, even though that is what the formula predicts.
Race time predictors are most accurate when the source and target distance are within 2–3× of each other (e.g. 5K → 10K, 10K → half marathon). Accuracy decreases for large jumps like 5K → marathon.
The Riegel formula — exponent 1.06 explained
Peter Riegel published the formula in a 1981 American Scientist article after analysing world record progressions across distances. The formula is:
- T₁ = recent race time, D₁ = recent race distance
- T₂ = predicted time, D₂ = target distance
- 1.06 = the fatigue exponent — performance degrades slightly faster than linearly with distance
If you double the distance, you do not double the time — you multiply by 21.06 ≈ 2.085. A 20:00 5K runner would predict a 41:41 10K, not exactly 40:00. The exponent 1.06 is fixed and does not adjust for age, sex, or event type — which is one of its key limitations for large distance jumps.
Race time predictor chart — 5K to marathon
Quick-reference table using the Jack Daniels VDOT method. Find your 5K time and read across for predicted half marathon and marathon times:
| 5K time | 10K (Riegel) | Half Marathon | Marathon |
|---|---|---|---|
| 18:00 | 37:32 | 1:19:47 | 2:45:55 |
| 20:00 | 41:42 | 1:28:40 | 3:04:34 |
| 22:00 | 45:52 | 1:38:05 | 3:23:14 |
| 25:00 | 52:07 | 1:51:59 | 3:51:38 |
| 28:00 | 58:23 | 2:05:57 | 4:20:17 |
| 30:00 | 1:02:33 | 2:14:33 | 4:38:51 |
| 35:00 | 1:12:58 | 2:37:04 | 5:25:31 |
| 40:00 | 1:23:24 | 2:59:36 | 6:12:10 |
| 45:00 | 1:33:49 | 3:22:08 | 6:58:51 |
Jack Daniels VDOT race equivalency table
VDOT scores let you compare performances across distances. Find your score below to see equivalent times at every standard race distance:
| VDOT | 1 Mile | 5K | 10K | Half | Marathon |
|---|---|---|---|---|---|
| 30 | 9:11 | 29:51 | 1:02:05 | 2:17:21 | 4:49:17 |
| 35 | 7:55 | 25:33 | 53:03 | 1:57:49 | 4:08:01 |
| 40 | 6:58 | 22:21 | 46:24 | 1:42:17 | 3:36:58 |
| 45 | 6:15 | 19:56 | 41:21 | 1:31:05 | 3:13:00 |
| 50 | 5:41 | 17:57 | 37:15 | 1:21:55 | 2:54:35 |
| 55 | 5:12 | 16:22 | 33:57 | 1:14:44 | 2:39:21 |
| 60 | 4:49 | 15:04 | 31:16 | 1:08:48 | 2:27:03 |
| 65 | 4:29 | 13:58 | 29:00 | 1:03:46 | 2:16:58 |
| 70 | 4:13 | 13:01 | 27:01 | 0:59:33 | 2:08:41 |
Riegel vs Jack Daniels: which formula is more accurate?
| Factor | Riegel Formula | Jack Daniels VDOT |
|---|---|---|
| Formula | T₂ = T₁ × (D₂/D₁)^1.06 | Invert VDOT aerobic power score |
| Accuracy (5K↔10K) | Very good | Very good |
| Accuracy (5K→marathon) | Overestimates (too optimistic) | More conservative, usually more accurate |
| Speed-focused athletes | Overestimates endurance | Better accounts for physiological limits |
| Training paces | Not provided | Yes — 5 zones (easy, MP, threshold, interval, rep) |
| Adjusts for age/sex | No | No (use age-graded calculator for that) |
| Best used for | Quick checks, similar distances | All distances, training plan setup |
How Garmin and Strava predict race times
Garmin race time predictor uses your VO₂max estimate (derived from heart rate and pace data across your recent runs) combined with your training load to predict race times for standard distances. It is more dynamic than the Riegel or VDOT methods because it updates continuously with your training — but it requires a heart rate monitor and several weeks of data to become reliable. Early Garmin predictions are often wildly inaccurate.
Strava race time predictor uses a similar approach, estimating your fitness from recent activity data. Both Garmin and Strava predictions are conceptually equivalent to the VDOT method — they estimate your aerobic ceiling and project from there — but with the advantage of continuous real-time data.
The Riegel and VDOT methods used here are single-point predictions: one race result in, one set of predictions out. They are more transparent and controllable, and work without a wearable device.
Limitations of race time predictors
All race time predictors make assumptions that may not hold for every runner:
- Training specificity: A 20-minute 5K runner who has only done short runs will not run the predicted marathon time without marathon-specific long runs and fuelling practice.
- Speed vs endurance profile: Riegel's 1.06 exponent is an average. Pure sprinters degrade faster over long distances; elite ultrarunners degrade slower.
- Course and conditions: Hilly courses, heat, wind, and altitude all affect finish time independently of fitness.
- Pacing strategy: Going out too fast is the single most common reason runners miss their predicted marathon time.
- Recency of source race: A race from 6 months ago reflects older fitness. Use the most recent race you have fully recovered from.
How to use your predicted time to set a goal pace
Once you have a predicted finish time, divide by the race distance to get your goal pace per km. For example, a predicted marathon time of 3:51:38 over 42.195 km works out to 5:29/km. This becomes your even-split target.
Most coaches recommend starting 5–10 seconds per km slower than goal pace for the first third of a marathon, then settling into goal pace through the middle, and running the final third on feel. This is the classic negative-split approach and is the most reliable way to avoid blowing up in the final 10 km.
Use the Running Splits Calculator to generate a full per-km split card from your predicted finish time.
Frequently asked questions
What is the Riegel formula for race time prediction?
Peter Riegel published his formula in a 1981 American Scientist article: T₂ = T₁ × (D₂ ÷ D₁)^1.06. T₁ is your known race time, D₁ is the known distance, D₂ is the target distance, and 1.06 is the fatigue exponent — reflecting that performance degrades slightly more than linearly with distance. The formula is simple, widely used, and accurate for short distance jumps (e.g. 5K to 10K).
How accurate is a race time predictor?
For distance jumps within 1–2× (e.g. 5K to 10K, or 10K to half marathon), predictions are typically within 2–5%. For larger jumps (e.g. 5K to marathon), accuracy drops because marathon performance depends heavily on long-run training, fat oxidation, and pacing strategy that a 5K time cannot capture.
What is VDOT and how is it used to predict race times?
VDOT is Jack Daniels' shorthand for the equivalent VO₂max that produces a given race performance. It is calculated from your race time and distance using a formula that models the percentage of VO₂max sustainable over time. Once your VDOT is known, the same model is inverted to predict the time you would run at any other distance at that same relative effort. A VDOT of 50 corresponds roughly to a 3:25 marathon or a 40-minute 10K.
What is the difference between Riegel and Jack Daniels race prediction?
Riegel uses a simple power law (exponent 1.06) and works well for similar distances. Jack Daniels VDOT derives an aerobic fitness score and models the sustainable percentage of VO₂max, making it more accurate for large distance jumps like 5K to marathon. Daniels also provides five training pace zones; Riegel does not.
Why is my predicted marathon time faster than I actually ran?
Race time predictors assume equal preparation at both distances. If your source race was a 5K or 10K and you had not done marathon-specific training (long runs of 28–35 km, fuelling practice, race-pace work), you will run slower than predicted. The prediction reflects your aerobic ceiling, not your event-specific readiness.
Can I predict a 5K time from a 10K result?
Yes — the calculator works in both directions. Enter your 10K time as the source race and the 5K prediction will appear in the table. The Riegel formula gives T₅ₖ = T₁₀ₖ × (5/10)^1.06 ≈ T₁₀ₖ × 0.479. Note that speed-dominant runners will often outperform this prediction at 5K.
How does Garmin predict race times?
Garmin estimates your VO₂max from heart rate and pace data across your recent activities, then combines this with your training load history to project finish times for standard distances. It is conceptually similar to the VDOT method but updates continuously and requires a heart rate monitor. Early predictions (first 4–8 weeks) are often inaccurate; they improve as Garmin collects more data from your actual runs.
What is a good VDOT score?
VDOT 30 corresponds to a beginner runner (roughly 4:49 marathon). VDOT 40 is recreational competitive (3:37 marathon). VDOT 50 is a solid club-level runner (2:55 marathon, 37-minute 10K). VDOT 60 is sub-elite (2:27 marathon). Elite male marathoners operate at VDOT 75–85.
Is the race time predictor accurate for ultramarathons?
No — Riegel's 1.06 exponent was derived from track and road races up to the marathon. For distances beyond 42 km, fatigue, sleep deprivation, and fuelling constraints make performance degrade much faster than the formula predicts. Ultramarathon predictors use different exponents (typically 1.10–1.15) and terrain-adjustment factors.
How often should I update my race time predictor input?
Use the most recent race you have fully recovered from. If you have not raced recently, a time trial (e.g. a solo 5K effort on a flat course) is a reasonable substitute. Fitness changes significantly over 8–12 weeks of training, so predictions based on a race 6+ months old may no longer reflect your current ability.