Your Garmin or Polar watch has been giving you a VO2max estimate for years. You've watched it go up after a good training block, drop after a recovery week, and fluctuate in ways that don't always match how you feel.
That number has one serious problem: it is an estimate derived from an algorithm, not a measurement of your physiology. And the training paces most runners use are either based on it, or on nothing at all.
I spent two training cycles chasing paces based on a Garmin VO2max of 54. When I finally ran a proper 10K time trial, my race-derived VDOT came out at 49 — five points lower. Every threshold session I'd been running for months was too hard. I wasn't building lactate threshold. I was just accumulating fatigue and wondering why I wasn't improving.
Jack Daniels built VDOT to solve this. It doesn't measure what your lungs can theoretically process. It measures what you can actually do in a race — and uses that to set training paces that fit your real fitness, not an algorithm's guess.
What VO2max actually measures
VO2max is the maximum volume of oxygen your body can consume per minute per kilogram of body weight (ml/kg/min). It represents the ceiling of your aerobic engine — the highest rate at which your cardiovascular system can deliver oxygen to working muscles.
In a lab, it's measured with a metabolic cart, a treadmill, and a graded exercise test that pushes you to exhaustion. The result is as accurate as exercise physiology gets.
The number on your watch is not that measurement. It is an estimate.
How your watch estimates VO2max
Consumer devices estimate VO2max using heart rate data during runs or workouts. The underlying algorithm compares your heart rate to your running speed and infers an aerobic capacity from the relationship.
The assumptions this requires:
- That your heart rate data is accurate (wrist optical HR is unreliable at high intensity)
- That the HR/pace relationship is stable across conditions (it isn't — heat, caffeine, elevation, fatigue, and stress all affect it)
- That your runs are representative enough to calibrate the model (short easy runs produce poor estimates)
- That the algorithm's general population parameters apply to you specifically (they often don't)
The result is an estimate that can be off by 5–10 points in either direction. For most runners, the watch number is a trend indicator — useful for seeing whether fitness is going up or down over months. It is not a reliable basis for setting training paces.
Where VDOT comes from
Jack Daniels developed VDOT in collaboration with exercise physiologist Jimmy Gilbert in the 1970s. The concept appears in Daniels' Running Formula and is built on a simple observation: if you want to know someone's effective aerobic capacity, the most accurate measure is what they just ran in a race.
VDOT is not VO2max. It is a number that represents the VO2max equivalent implied by your race performance, accounting for running economy. Two runners with the same lab-measured VO2max may have different VDOTs — because one of them converts oxygen into forward motion more efficiently.
The formula takes your race time and distance, and outputs a VDOT value that reflects your functional capacity as a runner, not just your physiology in isolation.
Why the two numbers diverge
Here's a concrete example of how the difference plays out:
| Runner | Lab VO2max | Watch VO2max | Race-derived VDOT |
|---|---|---|---|
| Runner A | 58 | 61 | 52 |
| Runner B | 54 | 49 | 55 |
Runner A has a high VO2max but relatively poor running economy — maybe inefficient form, heavier frame, or limited neuromuscular speed. Their watch slightly overestimates. Their actual race performance implies a lower effective capacity.
Runner B has a more moderate VO2max but exceptional running economy. Their watch underestimates because their heart rate stays lower at fast paces than the algorithm expects. Their race results show more capacity than the lab or the watch would predict.
Setting training paces from VO2max — lab or watch — ignores running economy entirely. VDOT doesn't.
The conversion: what your watch number implies for training
If you want a rough conversion from watch VO2max to an equivalent VDOT for a runner with average running economy:
| Watch VO2max | Approximate VDOT | Easy pace (/km) | Threshold pace (/km) |
|---|---|---|---|
| 45 | 43–45 | 6:35–7:10 | 5:05–5:15 |
| 50 | 48–50 | 6:10–6:45 | 4:45–4:55 |
| 55 | 53–55 | 5:45–6:20 | 4:25–4:35 |
| 60 | 57–59 | 5:25–5:55 | 4:10–4:20 |
| 65 | 62–65 | 5:05–5:35 | 3:55–4:05 |
These are approximations. The actual conversion depends on your running economy, which only a recent race can reveal.
When your watch gets it wrong
There are four common situations where watch VO2max estimates become actively misleading:
1. Heat and humidity In summer, your heart rate runs higher at any given pace. The algorithm interprets this as lower fitness. Your watch VO2max drops by 3–5 points in July without any real change in your aerobic capacity. Runners who train by watch metrics in summer often believe they're getting slower when they're not.
2. High altitude Heart rate increases at altitude for reasons unrelated to fitness. The algorithm can't distinguish between an altitude-driven HR elevation and a fitness decline.
3. Illness and stress HRV (heart rate variability) and resting HR changes from illness, poor sleep, or psychological stress affect the algorithm's baseline. You can watch your watch VO2max trend downward during a difficult work month even while your aerobic fitness remains stable.
4. New runners During the first 2–3 months of running, watch VO2max estimates are particularly unstable because the algorithm doesn't have enough calibration data. Some new runners see wildly optimistic or pessimistic numbers that bear little relationship to their actual fitness.
Which number to use for training
For setting training paces, use VDOT derived from a recent race — not your watch estimate.
To calculate your VDOT:
- Take your most recent race result (5K, 10K, half marathon, or marathon — the more recent and the longer the better)
- Use a VDOT calculator to find your number
- Look up your training paces from the VDOT table
The result gives you five exact paces — E, M, T, I, and R — calibrated to your actual race fitness, with running economy already accounted for.
If you don't have a recent race: The next most reliable option is a time trial on a flat, measured course. Run a 5K or 10K at full effort, treat the result as a race time, and calculate VDOT from that.
Watch VO2max is useful for one thing: tracking trends over months. If it goes up 3 points over a 16-week training block, that's a real signal. If it drops 2 points in a week, that's probably noise.
For day-to-day training decisions — what pace to run today, whether a session went well, how hard to push Thursday's tempo — VDOT from your last race is the number that matters.
For further reading on the science behind VDOT, the original framework is detailed in Daniels' Running Formula (3rd edition). Garmin's own documentation acknowledges the limitations of optical HR accuracy at high intensity, which directly affects VO2max estimation quality.
Zarkus calculates your VDOT from your most recent race result and sets your training paces from there — not from your watch's estimate. As you complete sessions and your fitness changes, VDOT is recalculated and your paces update automatically. If you want training paces that reflect where your fitness actually is, join the waitlist below.