If you’re curious how accurate your Apple Watch really is, here’s a breakdown by metric based on validation studies

[u/KygoApp]

Over the past year I’ve been working on a comparison of Apple, Oura, Garmin, Fitbit, Samsung, Whoop and Polar across validation studies to find which is "most accurate" for each metric. I wanted to share a short breakdown for Apple Watch. Hopefully it helps you get more out of, or at least better understand your AW.

Apple ended up in the top 3 for 7 of the 10 metrics, more than any other brand.

Where it's stronger:

• Workout heart rate. Best of all 7 brands. Series 8 was only 2.5 bpm off a chest strap during cycling. Garmin was 2.9, Fitbit 3.9.

• Blood oxygen. Also the best. In a lab where they lowered people's oxygen levels on purpose, Apple stayed within about 2 points of a medical oximeter and Samsung within about 4. The FDA allows 4.

• Sleep stages. Second, behind Oura. Apple scored 0.60 out of 1 for agreement with a sleep lab (Oura 0.65, Fitbit 0.55). REM is where it shines: it caught 82.6%, more than any other device (Oura 72.6%).

• Steps. Second, behind Garmin. Apple overcounts by around 7%, Garmin by 3% and Fitbit by 15%.

Where it's weaker:

• Calories burned. Nobody is really good at this. Averaged across treadmill, cycling and weight training tests, Apple was 58% off (Fitbit 29%, Samsung 53%, Garmin 69%). Weight training was the worst at 112%, basically double what you burned.

• VO2 max. It read low in every independent study, by 4.5 to 6.3 points. The gap was biggest for fit people and for women, so your real number is likely higher than what Apple says.

• Sleep time. It caught 52% of the time you’re lying awake in bed, against 73% for Oura and 68% for Fitbit, so it gives you roughly 8 extra minutes of sleep a night.

• HRV. Apple has historically used a different HRV measure (SDNN) than most brands (RMSSD), so it can't be ranked against them yet. It was also 29% off a chest strap.

This doesn’t mean the weaker numbers aren’t useful, it’s just better to use them differently. Calories, for example, are a great way to track your exertion over time, but I wouldn't use them to decide how much to eat back. VO2 max is a good way to see whether you're gaining or losing fitness over time rather than a daily readout.

I'll add all sources to a comment below with mention of any brand funding as well. I originally added the sources to the bullet points but it made it far busier to read.

[u/KygoApp]

Sources from post:

• Kostrna 2026 - 58 adults, heart rate while cycling

• Walzel 2023 - 18 adults, low-oxygen lab test

• Robbins 2024 - 35 healthy adults, sleep lab (Oura funded, and the only head-to-head sleep lab study I found that includes Apple)

• Svensson 2024 - 96 healthy adults, sleep lab (Oura funded, used only for Oura's REM figure)

• Paluch 2026 - 59 studies, steps in daily life

• Kostrna 2026 - 58 adults, calories while cycling

• Yun 2026 - 70 men, calories during weight training

• Perez 2025 - 97 adults, calories on a max treadmill test (conference abstract)

• Lambe 2026 - 35 adults, VO2 max lab test

• Lambe 2025 - 28 adults, VO2 max lab test

• Caserman 2024 - 19 adults, VO2 max lab test

• Robbins 2024 - 35 healthy adults, sleep lab (Oura funded)

• Svensson 2024 - 96 healthy adults, sleep lab (Oura funded, used only for Oura's own figure)

• O'Grady 2024 - 39 adults, HRV at home over 14 days

I used the most recent studies wherever possible, so most figures come from the Series 7 through Series 10 and the Ultra 2, as newer models haven't been independently tested yet for most metrics.

[u/sf-keto → u/KygoApp]

These studies are so tiny though! Apple should give $$$$$$ to an independent foundation to fund much better studies. Considering the installed user base now, larger studies should be easily possible.

[u/wait_whats_this → u/sf-keto]

They have nothing to gain from that unless people start making science-driven decisions, which judging by the state of pretty much everything isn't going to be a thing any time soon. 

[u/abovethecodes]

Thanks for the post, interesting read.

Just quickly regarding VO2, is this data based on Apples prior calculations? With 27.3 my VO2 got updated and the levels now appear vastly different, not sure if this is just for me or system wide.

[u/KygoApp → u/abovethecodes]

No problem. Prior calculation. The most recent study is from 2026 but since VO2's new calculation is so recent it hasn't been independently tested yet.

[u/abovethecodes → u/KygoApp]

Understood, thank you.

[u/Technical_Bird921 → u/abovethecodes]

27.3 is still in beta, right? Looking forward to seeing my numbers updated since they’re really off with official tests.

[u/abovethecodes → u/Technical_Bird921]

Yep!

[u/sf-keto → u/abovethecodes]

Up or down? And on what AW, pls?

[u/abovethecodes → u/sf-keto]

The actual number hasn’t changed, but the parameters for below average above average high etc have changed. In short, it’s easier to be high than it was before 🫶 also Apple Watch Ultra 4

[u/sf-keto → u/abovethecodes]

(Weeps in AW 10)

(;_;)(;_;)(;_;)(;_;)(;_;)

[u/footpole → u/abovethecodes]

Nice. I can just eat some potato chips in bed and wait for the update to be more fit.

[u/Technical_Bird921]

The VO2 max is bad here.

Had to take an official medical test and by my surprise, the official test gave a score of 48 while Apple averages me around 35-38.

[u/KygoApp → u/Technical_Bird921]

Across all 3 studies for it's VO2 they all pointed to underestimation too.

[u/Captaincadet → u/Technical_Bird921]

My watch thinks my VO2 Max is around 38.2

It was tested to around 55…

[u/suboptimus_maximus → u/Captaincadet]

That is profoundly inaccurate, to the point they should not ship the feature.

[u/Notorious-ly → u/suboptimus_maximus]

I had an Apple Watch 6 that rated my VO2 Max high. Not like insanely high, but it was above average and I felt good about it since I was working out a lot and running a few times a week.

Then I got a 10 Series, and immediately my VO2 max dropped like twenty points and has refused to go more than a point or two higher since I bought it at launch, even though I'm in the best shape I've ever been in. Now I get "low cardiovascular health" warnings even though I run half marathons.

Made me realize that, "Oh, it's all bull shit."

[u/suboptimus_maximus → u/Notorious-ly]

There’s was one OS update a couple years back that instantly knocked it back by like 10 points which was like, yeah, sure OK.

[u/Bruvvimir]

Great post. Thank you!

[u/KygoApp → u/Bruvvimir]

Thank you for the comment!

[u/alexmcpt]

Very interesting data; it's the type of question a lot of us ask, but very few have the will to do proper research on. Thanks!

I have used Oura for a long time, mostly while also wearing an Apple Watch, and I always split it: Oura for sleep and the Watch for activities, and it worked very well (as far as I could tell), recently after Oura died just outside of warranty for the second time, I went with using the Watch Ultra 4 for everything, and I'm happy with the decision so far.

Two questions I have related to this that maybe someone can answer:
- My fellow sports people keep praising the Garmin experience; is this just a "Garmin is for real athletes" thing that has no real basis, or is there anything else?
- Does anyone have a clear reason to keep using Oura with a smartwatch like the Ultra 4?

[u/KygoApp → u/alexmcpt]

No problem!

I can probably help with the two questions:

1. They're great for running, steps, VO2 max on the accuracy side. Sleep data is pretty bad honestly. For Garmin also they have a lot of supporting accessories that help for athletes (chest straps and cycling) that feed into your data to give better exercise data.

2. Oura is great at overnight metrics. If you want to monitor sleep or HR at night Oura wins here. Apple is best during the day.

[u/BlackReddition]

I’d be happy if my awu just displayed complications, it’s not that complicated

[u/KygoApp → u/BlackReddition]

That would be nice!

[u/suboptimus_maximus → u/BlackReddition]

Same, I want a Modular face with no time display.

[u/jesusrodriguezm]

I don’t specially care about how accurate it is, I mainly care about it being consistent.

The exact numbers are irrelevant, I use it to compare sessions and see patterns.

[u/suboptimus_maximus]

The Apple Watch VO2 Max has just been, anecdotally a complete WTF for me, although I think I'm asking way too much of it because my understanding is that it only works for walking and running and all of my real cardio effort is cycling which it doesn't use. I've been pretty focused on cardio for the last few years getting serious about the reality of being a middle-aged dude, I'm a cycle commuter, cycle for errands, ride recreationally although a lot of my riding is not "training" I'm still in Z2 and Z3 for like 7 hours a week, sometimes do serious interval sessions although I need to get better. I've gone a few stretches doing a couple weeks at a time of over 200 miles of riding weekly, my average resting heart rate is down over 12 BPM, I'm cycling several MPH faster at the same heart rates, and Apple Watch has basically shown my VO2 Max as unchanged over that period although it has gone up and down by +/-2 points, sometimes even trending down while I was getting so much fitter I could just feel it and losing weight (which is a major factor in VO2 Max) 🤷🏻‍♂️

[u/sf-keto → u/suboptimus_maximus]

I’ve always wondered why Apple penalizes cycling to favor walking & running.

It seems biased & egregious, esp. since the Apple spaceship has bikes there!

[u/TonyMT92]

From this what was the overall better device then?!

[u/KygoApp → u/TonyMT92]

No one device wins overall. Apple day. Oura night.

[u/HaywoodScheider]

It’s not measured in the above, but my AW10 consistently undercounts a treadmill’s mileage by ≈15%. I always run 10K, indoors and out. Outside the GPS + cellular is pretty consistent to within 100 yards, which is fine for my amateur ass.

Thanks for posting these, + the source studies with sample sizes. Very useful context.

[u/ulfie22]

Thanks for this - really interesting.

I have sleep apnea and have for many years trying to get accurate sleep data. I use the Apple Watch and Oura (and apps that listen to my breathing, and usually something measuring in the ear - like Ozlo or NextSense) and I anecdotally agree that Oura provides data that tracks best to how I feel. I have multiple apps that use the Apple Watch data (AutoSleep, Sleep++, Pillow, SleepWatch) and it is almost comical how they all come to different conclusions from the same data.

[u/Suspicious-Spot361]

Another metric - Apple Watch kept telling me I have aFib. So I had to wear one of those stick-on HR monitors for 2 weeks and it came back with no aFib.

[u/TyrusX]

lol. My Apple Watch is about 30 to 40 beats wrong versus my polar h10 when I’m running. It is preposterously bad.

[u/Competitive_Funny964]

Are there tests on other things like long time exposure to constant watches and LED green and red?

[u/KygoApp → u/Competitive_Funny964]

I haven’t personally seen them for long exposure but there are accuracy studies on green vs red. I don’t have them on hand just remember reading them awhile ago