Free running tools.
Four calculators built on Jack Daniels' VDOT methodology — the same engine the Why Not app uses to build adaptive training plans. No signup, no ads, instant results.
These are the exact Daniels VDOT calculations the Why Not app runs after every workout, given away free as standalone web tools. Punch in a recent race result and the math tells you what your fitness is worth: a single VDOT score, five training paces calibrated to it, and predicted finish times at every common race distance.
Each tool works on its own — start wherever your question is. Everything is computed live in your browser from our own implementation of the published formulas; nothing you type is stored, sent anywhere, or tied to an account. If you just need a number before tomorrow's tempo run, get it and go.
VDOT Calculator
The flagship. Enter one race result and get your VDOT score, all five Daniels training paces, and equivalent race times from 5K to marathon — everything the other tools do, in one place.
Open →Race Time Predictor
Turn one finish time into predicted times for the 5K, 10K, half marathon, and marathon — so you know what your recent race says about the next one.
Open →Training Paces
Five Daniels paces — easy, marathon, threshold, interval, and repetition — from a VDOT score or a recent race. Know exactly how fast each workout should be.
Open →Pace Converter
Flip between min/mi and min/km without mental math, or enter a goal time for any distance and get the pace you'll need to hold.
Open →FAQ
VDOT is the fitness score at the heart of Daniels' Running Formula — an "effective VO2max" calculated from a race result rather than a lab test. Because it's derived from actual performance, it quietly folds in running economy, which is why it usually differs from a treadmill-measured VO2max. As a reference point, a 20:00 5K works out to roughly VDOT 49.8, while elite marathoners sit around 75–85.
Yes — completely. There's no signup, no email gate, no ads, and no usage limit. Everything runs in your browser, and nothing you enter is stored or sent anywhere. The calculators exist because the math shouldn't be the paid part: Why Not, the app, charges for the adaptive coaching built on top of this engine, not for the formulas themselves.
For runners who train appropriately for the target distance, VDOT-based predictions are remarkably close — often within a percent or two. The honest caveat comes from Daniels himself: an equivalent time assumes you'd prepare specifically for that race. A sharp 5K PR predicts your marathon potential, not what you'd run off pure 5K training with no long runs in the bank.
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.
Try Why Not free for 7 days