Endurometry
Use pace for structured work on the flat; use heart rate when heat, hills or fatigue move the cost. They are not two fitness scores. Karvonen Z2 for age 35 / rest 55 is 132–145 bpm — about 72–79% of max, not 60–70%. When they disagree, believe the condition, not the watch name.
Last updated October 5, 2026
A Tuesday 6:11/km easy run and a Tuesday 140 bpm easy run can be the same session — or they can fight. Pace is time per kilometre. Heart rate is how hard the heart is working to deliver that pace. What each zone is for lives in the heart-rate zones guide and in VDOT explained. This page is when to trust which number.
Two maps of the same week
The training pace calculator turns a race into min/km bands (VDOT 41.5 from a 10K in 48:30 → Easy mid 6:11/km). The heart-rate zone calculator turns rest and max into bpm. They are independent: HR is not derived from VDOT. Default HR example, age 35, rest 55, Tanaka:
| Language | Easy / Z2 | Threshold / Z4 |
|---|---|---|
| Pace (VDOT 41.5) | 6:11/km mid (5:41–6:48) | 5:04/km mid |
| Karvonen (Tanaka 184) | 132–145 bpm | 158–171 bpm |
| Same Z2 as % of max | 72–79% HRmax | — |
A watch that paints Z2 as 60–70% of max will call 140 bpm “tempo.” It is still Karvonen Z2. Compare bpm, not the zone label. Equations: methodology.
Follow pace when the course is honest
Flat road or track, cool air, structured work: pace is the cleaner target. Intervals and threshold sessions are defined by duration at a speed, not by how long the optical sensor takes to catch up. Convert the session with the pace calculator, then ignore a lagging wrist HR for the first 45 seconds of each rep.
Follow heart rate when the cost moved
Heat, humidity, hills, altitude, dehydration, caffeine, poor sleep and some medicines (including beta blockers) raise HR at the same pace. Cardiac drift does the same on a long easy run: km 18 at 6:20 can sit 10–15 bpm above km 3. If you chase 6:11 on a 28 °C day, you have turned Easy into Z3. Cap the bpm (stay in 132–145 in the example) and let pace slow. That is the point of HR on recovery days — not to hit a split.
Do not referee both in one session
Picking the faster of pace and HR “because I feel good” is how easy weeks become moderately hard. Choose the constraint before you start: pace for the quality session, HR cap for the easy and the long. If they still disagree after a cool, flat 20 minutes, re-check rest (morning, lying down) and max (measured beats Tanaka). Fox for the same 35-year-old is 185 max and Z2 133–146 — a few beats, not a new plan.
Related Tools
Heart Rate Zone Calculator
Calculate your five running heart-rate zones from age, resting heart rate and max heart rate using the Karvonen method.
Training Pace Calculator (VDOT Zones)
Get your VDOT from a recent race and see easy, marathon, threshold, interval and repetition pace zones plus heart-rate zones.
Running Pace Calculator
Enter distance and finish time to get your pace per km and mile, average speed and per-kilometre splits.
Related guides
Heart Rate Zones Explained
What each heart rate zone is for, how to work out your own zones from max and resting HR, and when a chest strap beats a wrist sensor.
VDOT explained
VDOT is Daniels’ endurance index from a race, not a lab VO2max. How Endurometry gets 41.5 from a 10K in 48:30 and turns it into Easy, Threshold and Interval paces.
How the numbers are calculated
Frequently Asked Questions
Why is my easy pace in Z3 on the watch?
Often the watch uses %HRmax. Karvonen Z2 is ~72–79% of max in the default example. Heat and drift also push you up. Slow down or ignore the label and look at bpm.
Should long runs follow pace or HR?
Cap HR. Drift will make even 6:20 look “too slow” on the watch while the heart is already working like Z3. Time on feet in Z2 beats a pretty average pace.
Can I use wrist HR for intervals?
It lags. Pace (or a chest strap) for the work bouts; HR is useful in the recovery jog if it actually comes down.
Is this medical advice?
No. Zones here are a training map. Arrhythmia, medication and pregnancy need a clinician, not a calculator.
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