Race Time Predictor
Run one race, predict four. Enter a recent finish time and get equivalent performances at 5K, 10K, half marathon and marathon from Daniels' VDOT curves — the same math the Why Not app trains you with. Free, instant, nothing stored.
Enter a recent race
One honest caveat before you plan a race around these numbers: equivalency assumes you would train appropriately for the target distance — that's Daniels' own qualifier, printed right alongside his tables. A 5K PR measures your engine; it says what your marathon could be after months of marathon-specific mileage, long runs and fueling practice — not what next month's marathon will produce. Without that work, the prediction is optimistic, often by several minutes. It cuts the other way too: endurance-trained runners regularly beat what their short-distance results imply, and speed-trained runners outrun predictions made from their marathon.
How race equivalency works
Every race you run is a measurement. Finish a 10K in 48:30 and you've told the Daniels model two things: how fast you were moving, which fixes the oxygen cost of the effort, and how long you sustained it, which fixes the fraction of your aerobic ceiling a human can hold for that duration. Combine the two and you get a single fitness score — VDOT. Our VDOT calculator does exactly that step on its own.
Prediction is the same calculation run in reverse. Holding your VDOT fixed, we ask: what finish time at 5K — or the half, or the marathon — would imply this exact same score? Because the duration changes, the sustainable intensity changes with it. You can hold roughly 95% of VO2max for a 5K but only around 80% for a marathon, so predicted paces fall as the distance grows, and the curves already know by how much. One number, four honest projections. The same score also sets your five training paces, which is where the model earns its keep between race days.
Equivalent race times by VDOT
No calculator needed for a quick read: the table below shows what equal fitness looks like across the four distances at representative VDOT values, computed from the same formulas the calculator uses. Find the row nearest one of your recent results and read across.
| VDOT | 5K | 10K | Half Marathon | Marathon |
|---|---|---|---|---|
| 30 | 30:41 | 1:03:49 | 2:21:17 | 4:49:49 |
| 35 | 26:59 | 56:02 | 2:04:13 | 4:16:06 |
| 40 | 24:06 | 50:01 | 1:50:54 | 3:49:37 |
| 45 | 21:49 | 45:13 | 1:40:14 | 3:28:16 |
| 50 | 19:56 | 41:20 | 1:31:31 | 3:10:40 |
| 55 | 18:22 | 38:06 | 1:24:16 | 2:55:55 |
| 60 | 17:03 | 35:22 | 1:18:09 | 2:43:22 |
| 65 | 15:55 | 33:02 | 1:12:54 | 2:32:35 |
FAQ
Because the prediction assumes you would train appropriately for the target distance — Daniels states this caveat himself. The math measures your aerobic engine and reports what it could deliver over 26.2 miles after a proper marathon build: months of mileage, regular long runs, and fueling practice. It knows nothing about whether you've actually done that work. A 5K specialist's marathon prediction is a statement of potential, not a race-day promise — and the bigger the jump in distance from your input race, the more optimistic the number tends to be.
Your most recent all-out effort on an accurately measured course — ideally from the last two or three months, since fitness drifts. A recent honest 10K beats a year-old 5K PR. If you have several current results, prefer the one closest in distance to the race you're predicting: a half marathon predicts a marathon better than a 5K does, because the two efforts share more of the same physiology.
No. The equivalency assumes comparable courses and conditions — reasonably flat, firm footing, sane weather. A hilly trail race makes you look slower than you are, so the implied VDOT comes out too low and every prediction built on it will be too slow. The reverse holds for a short course or a big net downhill. Use a road or track result for input, and treat the predictions as applying to similar courses.
No — it answers a different question entirely. Age-grading compares your time against the best achievable performance for your age and sex and returns a percentage, so a 55-year-old and a 25-year-old can compare efforts fairly. Race equivalency ignores age and sex altogether: it takes your own result and asks what the same fitness implies at other distances. One ranks you against a standard; the other maps you onto yourself.
Methodology based on Daniels' Running Formula (Jack Daniels & Jimmy Gilbert). This site is not affiliated with or endorsed by the authors. Every number on this page is computed from our own implementation of the published formulas — nothing is transcribed from the book's tables.
Your paces, recalculated every week.
This page is the static version of the math. The Why Not app runs the same Daniels VDOT engine after every run you log — tightening your paces as you get fitter, easing off when you're cooked, and rebuilding your plan when life gets in the way.
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