How Accurate Is Your Watch's VO2max?
How accurate your watch's VO2max estimate is depends on a figure the watch never displays: the error bar around it. A systematic review published in Frontiers in Sports and Active Living in December 2025 gathered the 13 studies that have tested consumer wearables against laboratory gas analysis. Across those studies, the mean absolute percentage error for VO2max estimates ran from under 5% at the best end to 15.78% at the weakest. On a VO2max of 55, that spread is roughly 3 to 8 points — and where you land inside it depends on your fitness, your heart rate sensor, and how the reading was collected.
So here is the calibration, metric by metric. VO2max, lactate threshold pace, threshold heart rate, and the bike-versus-run question a triathlete runs into that a runner never does. For each one: what the device estimates, how it gets there, how far off it has measured, and what to do with it in a training week.
What the VO2max Estimate Is Built From
The estimate on most multisport watches comes from an algorithm developed by Firstbeat Technologies, which the review describes as exploiting the relationship between heart rate and speed during submaximal running. The watch records your heart rate at a known external workload and works backward to an oxygen uptake figure.
Two inputs carry most of the load. The first is heart rate. The second is external workload, and its source differs by sport: Garmin's own documentation states that for cycling, "external workload data is provided by your power meter and is measured in watts," while running uses GPS speed from the watch itself.
The Third Input, and How Much It Moves the Number
The quieter input is your maximum heart rate, and the review is specific about how much it matters: "If HRmax is underestimated by 15 beats/min, the error in the V̇O2max result is approximately 9%. If the HRmax is overestimated by 15 beats/min, the error in the V̇O2max estimate is 7%. If the person's actual HRmax is known, the error in the V̇O2max estimate drops to 5%."
That puts a meaningful share of the total error range inside one editable field — an athlete running on a default age-predicted HRmax carries a wider band than one who has measured it.
Across all 13 studies, the authors of seven concluded the device was valid or acceptable for VO2max estimation. One study reported a MAPE under 5%, four landed between 5% and 10%, and one underestimated VO2max by a mean of 4.51 mL/kg/min at a MAPE of 15.78%. That is the honest shape of the evidence: usable, with a band around it that varies by study and by athlete.
Athlete Takeaway Pull your actual max heart rate from a hard race file or a maximal session and enter it manually in your device profile. The review puts the VO2max error near 5% when true HRmax is known, against roughly 7–9% when it is off by 15 beats. If you want to tighten the rest of your heart rate setup while you are in there, start with how heart rate training zones are built.
The Accuracy Shifts as You Get Fitter
This is the part that matters most to a competitive age-group athlete: the estimate performs differently at different fitness levels.
Engel, Masur, Sperlich and Düking published a validation in the European Journal of Applied Physiology in 2025 using 35 endurance athletes with a mean VO2max of 60.1 ± 8.2 mL/min/kg. They split the group at 59.8 and reported each half separately. In the moderately trained half, MAPE was 2.8–4.1%. In the highly trained half, MAPE was 9.4–10.4%, with the watch underestimating VO2max by 6.3 mL/min/kg. Agreement dropped alongside it, from an ICC of 0.63–0.66 in the moderately trained group to 0.34–0.41 in the highly trained one.
The systematic review lands in the same place in one line: "Usefulness in elite endurance sports is questionable." The author adds that validity "may differ when the sample consists of highly trained runners with higher V̇O2max values or higher speed at LT."
Two consequences follow. A strong age-grouper sits in the population where the estimate is least precise, and there it tends to err low. If you have been quietly annoyed that your watch reports a number below what your race times suggest, the measured direction of the error is on your side of that argument.
Athlete TakeawayIf your reported VO2max sits around 60 or above, read the absolute figure as a bracket and give your attention to which way it moves across a training block. The same discipline serves you in tracking what changes with age — direction over months tells you more than any single reading.
Lactate Threshold: The Pace Estimate Holds Up Better Than the Heart Rate Estimate
Five of the 13 studies looked at lactate threshold estimates. Three demonstrated validity. That is a thinner evidence base than the VO2max work, and the two halves of the threshold estimate performed differently enough to deserve separate treatment.
The clearest result came from the Garmin Fenix 6 in trained athletes. Speed at lactate threshold reached validity: MAPE 7.52%, concordance correlation coefficient 0.79. Heart rate at lactate threshold, from the same device and the same athletes, did not — the review states plainly that "HR at LT was not considered valid, as the correlation analysis showed a CCC < 0.70."
There is a known pattern in where the heart rate figure goes astray. The review cites Carrier and colleagues, who "identified biases in LT estimates in individuals with very high LT values or HR at LT significantly different from approximately 175 bpm." An athlete whose threshold heart rate sits well outside that region has more reason to check it against a field test.
The pace estimate is a reasonable place to begin a threshold session. The heart rate estimate is worth confirming before you build a zone structure on it — which is a solved problem, because a field test hands you a threshold you measured yourself. Our guide to the 30-minute critical swim speed test applies the same logic in the water, and the aerobic work those zones organize is covered in the Zone 2 guide.
Athlete Takeaway Use the watch's threshold pace as an opening bracket for threshold work, and set your threshold heart rate from a field test rather than from the device estimate. If your threshold HR sits far from about 175 bpm, make that field test the higher priority.
Bike Numbers and Run Numbers: The Multisport Question
A runner gets one VO2max figure. A triathlete gets two, and they rarely match. Garmin says so directly in its own material: "If you are an active cyclist and runner, you will notice that you have one VO2 max for cycling and a similar but slightly different VO2 max value for running."
Two separate things drive that gap, and it helps to keep them apart.
The Inputs Are Different
Running feeds the algorithm GPS speed. Cycling feeds it watts from a power meter. Those signals have different failure modes: GPS pace wobbles under tree cover and on switchbacks, while power depends on a calibrated meter. Ride without one and there is no cycling VO2max figure. Power earns its place for reasons well beyond this metric, which we get into in the guide to pacing an IRONMAN bike leg with power.
The Physiology Is Different Too
Mode-specific differences in measured VO2max are well established in the laboratory. Basset and Boulay, in the European Journal of Applied Physiology in 2000, tested triathletes, runners and cyclists on both a treadmill and a cycle ergometer. The group as a whole recorded significantly higher VO2max running, and the differences between tests averaged 10.5% for runners, 6.1% for triathletes and 2.8% for cyclists. Worth stating plainly: that study used six athletes per group, so treat those figures as an order of magnitude, not a constant.
What Has Not Been Measured
Here the evidence runs out, and we would rather say so than fill the gap. In the 2025 review, running was the predominant activity across the included studies; only two of the 13 involved cycling, and none compared a running-derived estimate with a cycling-derived estimate in the same athletes. So the cycling number inherits whatever error the running validation work found, plus whatever cycling adds on top, and the size of that second part remains unestablished. Anyone quoting you a precise bike-versus-run accuracy figure is going beyond the published data.
Handling it is straightforward, though: each sport's number belongs to a series of its own. Comparing your cycling VO2max to your running VO2max measures two algorithms and two physiologies at once, which neither figure was built to answer.
Athlete TakeawayTrack cycling VO2max against your own past cycling VO2max, and running against running. Two separate lines, each compared only with itself. For the bike line to exist at all you need a power meter — we make the case for one in our athlete questions round-up on power meters and gear.
How to Use These Numbers in a Training Week
The error ranges above are workable once you know them. Four habits get the most out of the data.
Give the algorithm two runs, not one. The review reports that in Düking and colleagues' study, the Garmin Forerunner 245 improved after a second non-fatiguing run, with MAPE falling from 5.58% to 1.06%. The recommendation follows directly: "it is recommended to perform at least two continuous submaximal runs with the smartwatch to enhance the validity of V̇O2max assessment."
Wear a chest strap for the runs that feed it. The review is unambiguous: "Measurement of HR with a chest belt sensor is more accurate than with an optical sensor, especially during high-intensity activities." Since heart rate is one of the two primary inputs, sensor quality propagates straight into the output.
Read it as a six-week line. Here is the arithmetic, using the review's own figures. At a MAPE of 7–10% on a VO2max of 60, the band around any single reading is roughly 4 to 6 mL/kg/min wide. A move from 56 to 57 sits comfortably inside that. Six weeks of readings drifting the same direction is worth something; two readings a week apart is measurement noise doing what measurement noise does. The same discipline applies to every metric you track, which is the subject of our guide to reading training data without overthinking it.
Read it next to something else. This is the review's own recommendation, and it is a good one: VO2max and threshold estimates "should always be interpreted alongside additional objective information from the WT that indicates the athletes' performance and health status (e.g., HRV), as well as in correlation with the RPE." A VO2max line that has been flat for a month means one thing when your HRV is stable and sessions feel normal, and something else when both have slipped. If you want a system for that, HRV-guided training is where to start, and the fitness, fatigue and form framework puts the load side of it in order.
Athlete Takeaway Run two easy outdoor sessions a week with a chest strap, and read the VO2max line on a six-week horizon alongside your HRV trend and how the sessions have felt. Those three together tell you more than any one of them at full resolution.
The Bottom Line
Your watch is running an estimation, and estimation comes with error bars. The 2025 systematic review puts the VO2max band between under 5% and 15.78% across 13 studies, with seven judging the estimate valid or acceptable. Engel and colleagues show that band widening to 9.4–10.4% in highly trained athletes, skewed low. Threshold pace reached validity at a CCC of 0.79; threshold heart rate fell short of it, below 0.70. And the bike-versus-run question every triathlete runs into has yet to be addressed head-on: two of 13 studies included cycling, and none compared the two.
The practical version fits on an index card. Enter your real max heart rate. Wear a chest strap. Give the algorithm two easy runs a week. Compare each sport only with itself. Read the trend over six weeks next to your HRV and your session feel.
If you want help turning your data into a season plan — setting zones from numbers you can rely on, reading the trends that matter, and building the training around them — that is the work we do at NVDM Coaching. We would genuinely enjoy looking at your numbers with you. Set up a call with us now, and we will help you be ready for your best race day!
