Google Fitbit Air HRV: how it works, vs WHOOP

Documentation checked: 22 October 2026, on Google’s own pages only. Google renamed the Fitbit app to Google Health this year and changes its features often.

What is the Google Fitbit Air?

Google announced the Fitbit Air on 7 May 2026 as “our smallest and most affordable tracker”, a “screenless” tracker that pairs with the Google Health app, which replaced the Fitbit app (Google, Fitbit Air announcement). The tracker is a pebble 34.9 by 17 by 8.3 mm that weighs 5.2 g, or 12 g with its band (Google Store, tech specs). You press it out of the band to swap bands (Google Health Help, Get started).

Instead of a screen it has a status light, and you double-tap it to check the battery (Google Health Help, Get started). Everything you read, HRV included, is in the phone app, which needs Android 11 or iOS 16.4 or newer and a Google Account.

What the hardware has, from Google’s specification page (Google Store, tech specs):

• Sensors: an optical heart rate monitor, an accelerometer and gyroscope, red and infrared sensors for blood oxygen (SpO2), and a temperature sensor whose readings appear as skin temperature variation in the app.
• Heart rate storage: heart rate data at 2-second intervals.
• Battery: up to 7 days, 90 minutes for a full charge, and a day’s use from 5 minutes of charging. Google recommends charging “every few days”.
• Price: from $99.99 in Google’s US store on 22 October 2026 (Google Store). The Stephen Curry Special Edition was $129.99 at launch (Google, Fitbit Air announcement).

Does Fitbit Air measure HRV?

Yes. Google’s vitals page lists HRV for the Google Fitbit Air, alongside breathing rate breathing rate Breathing rate How many breaths you take a minute. It moves HRV on its own: slower, deeper breathing makes the beat-to-beat swing larger, and below about 9 breaths a minute it shifts from the HF band into LF. More about Breathing rate , skin temperature variation, SpO2 and resting heart rate resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate , and says Google uses “the RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD formula to determine heart rate variability from heart rate data” (Google Health Help, vitals). RMSSD measures how much the gap between one heartbeat and the next changes (Task Force, 1996).

Google’s heart rate help page covers Pixel Watch and Fitbit devices and gives wear instructions for the Fitbit Air. Its HRV section describes one method for Fitbit devices: the latest value comes from “the longest sleep period over the past 24 hours”, and “only sleep periods greater than 3 hours are considered” (Google Health Help, heart rate). Google does not describe a different method for the Air.

Which sleep Fitbit takes HRV from

Three example days

• Sleep the HRV comes from
• Sleep not used

1. A full night

   HRV from this night.

2. A night and a nap

   HRV from the night, the longest sleep. The nap is not used.

3. A short night

   No new HRV: no sleep lasted more than 3 hours.

Three 24-hour tracks from noon to noon. First, a full night of 7 hours 30 minutes: the HRV comes from that night. Second, a 1-hour afternoon nap and a 6-hour-15-minute night: the HRV comes from the night, the longest sleep, and the nap is not used. Third, a short night of 2 hours 30 minutes: no sleep lasted more than 3 hours, so there is no new HRV value.

The Air therefore gives you one HRV value per night, taken from sleep. A nap does not replace the night, and a night under 3 hours leaves no new value. The vitals page adds that most metrics need at least 3 hours of quality sleep, and that your personal range for each metric comes from up to 30 days of data (Google Health Help, vitals).

To see it, open the Google Health app, tap the Health tab, then Vitals, then Heart rate variability (HRV). The detail view graphs your nights by week, month, quarter or year (Google Health Help, heart rate). Fitbit and Pixel Watch HRV covers the method in more detail, for every Google wearable.

What Google has not published

Google’s pages leave out several things you would want for HRV:

• which part of the longest sleep the RMSSD comes from, how the values in it are combined, and how beats are cleaned before the calculation;
• whether the Air’s “new algorithms”, which Google’s store page credits for “more accurate, precise tracking”, change the HRV calculation (Google Store);
• any HRV accuracy figures for the Air;
• a way to see the beat-to-beat intervals behind the number.

The readiness score on Fitbit Air

HRV also feeds the readiness score readiness score Readiness score A single number, often out of 100, that a device builds from your HRV, resting heart rate, sleep and other inputs to suggest how ready you are for the day. The weighting is not published. More about Readiness score , which Fitbit used to call Daily Readiness. It runs from 0 (low) to 100 (high): 65 or above is high, 30 to 64 moderate, and 29 or below low. Google names three inputs, heart rate variability, resting heart rate and sleep over the past week, and says a drop in HRV can signal that your body needs a break. You need 7 nights of wear before the first score, and Google suggests a month for a more accurate baseline baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline . Google lists the Google Fitbit Air among the devices that get readiness in the app (Google Health Help, readiness).

Google does not publish how it weighs the three inputs. A low score can come from resting heart rate or sleep while your HRV is normal, so look at the HRV behind it. Recovery scores explained compares the score with other makers’.

Does Fitbit Air need a subscription?

Not for HRV, on the evidence of Google’s own pages. The Google Store’s Fitbit Air page lists the Google Health app’s “Features when paired with a Pixel Watch or Fitbit device”, among them “cardio load, readiness” and “heart rate, HRV, breathing rate and quality, skin temperature”. It lists separately what Google Health Premium adds, among them the Gemini-based coach, adaptive fitness plans, detailed sleep insights, daily guidance, advanced Health Guardian features and a library of workout videos (Google Store). The readiness help page mentions Premium only for coaching built on the score (Google Health Help, readiness).

Google’s getting-started page for the Air says you need Premium “to use advanced health tracking features”, without listing them (Google Health Help, Get started). We found no Google page that puts HRV or readiness behind Premium.

Premium costs $9.99 a month or $99 a year (Google, Google Health announcement). Google says every Fitbit Air comes with three months free; Google’s store terms say the trial “automatically renews for $9.99/mo” unless you cancel (Google Store). The help page adds that your account gets no new trial if it has redeemed a Premium trial before, including one that came with an older Fitbit or Pixel Watch (Google Health Help, Premium).

Is Fitbit Air accurate for HRV?

We found no published validation study of the Fitbit Air, so there is no accuracy figure for its HRV. Google’s store page notes that, as with all heart rate tracking, accuracy “is affected by physiology, location of device and different movements” (Google Store, tech specs).

What exists is for other Fitbit models. It does not carry over to the Air: Google has not said whether the Air shares their sensor or software.

• Fitbit Versa 4, a 2025 pilot study. 8 people, mostly older adults (average age 64) awaiting shoulder surgery, wore a Versa 4 and an ambulatory ECG ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG for at least three nights. The median overnight RMSSD was 21.7 ms from the Fitbit and 20.6 ms from the ECG, not a significant difference. Across 5-minute stretches the ECG read about 8% lower than the Fitbit, a statistically significant gap the authors judged unlikely to matter clinically. They reported strong correlation and acceptable agreement and, with so few people, called this preliminary support (Nasr and colleagues, 2025).
• The 2025 home study of wearables. Dial and colleagues tested Oura, WHOOP, Garmin and Polar devices against a chest-strap ECG over 536 nights. No Fitbit was in it (Dial and colleagues, 2025).

The device validation tracker lists both studies with their numbers. Until a study of the Air appears, read its HRV the way you would read any wearable’s: as a trend against your own past nights.

Fitbit Air vs WHOOP for HRV

Both are bands that measure HRV while you sleep and turn it into a morning score. They differ in the part of the night they read, how much evidence there is, where you can wear them, and how you pay.

From Google’s and WHOOP’s pages (see Sources), checked on 21 and 22 October 2026.

Which part of the night each reads

WHOOP gives a “dynamic average during sleep” weighted towards your last slow-wave (deep) sleep stage, in the words the Dial study quotes, and its developer documentation names the field for RMSSD (WHOOP developer documentation). Fitbit takes the longest sleep period and does not say which part of it counts. HRV changes from one sleep stage to the next: in a sleep-lab study of 15 healthy young men, the median RMSSD was 67.3 ms in slow-wave sleep and 79.7 ms in REM sleep (Herzig and colleagues, 2018). So the same night can give two different numbers on the two devices.

The model below shades the part of one simulated night each device reads. For Fitbit it shades the whole sleep, because Google does not say more.

Same night, different numbers

A teaching model, not a measurement

Fitbit WHOOP

Sleep stages lights out to waking

• Awake
• REM
• Light
• Deep

HRV every 5 minutes (RMSSD) 20 to 60 ms

0h002h004h006h008h00

Fitbit

47.1 ms RMSSD

The longest sleep of the past 24 hours, if it lasted more than 3 hours. Google does not say how the values in it are combined; the model averages all of it.

Same night, same heart: Fitbit 47.1 ms and WHOOP 54.6 ms, because each device averages different hours.

Another night Night 1 of 5

The buttons work when JavaScript is on. The window for each device is listed in the description.

Two charts over eight hours of simulated sleep. The top chart is a hypnogram stepping between awake, REM, light and deep sleep, with deep sleep mostly in the first half and REM mostly in the second. The bottom chart is a line of HRV values, one per five minutes, that rise gently through the night and dip during wakeful minutes. One button per device shades the part of the night that device averages. Fitbit: 47.1 ms RMSSD. WHOOP: 54.6 ms RMSSD. Same night, same heart: Fitbit 47.1 ms and WHOOP 54.6 ms, because each device averages different hours.

Readiness vs Recovery

WHOOP Recovery is calculated when you wake, as a percentage from 0 to 100%, from resting heart rate and HRV, plus blood oxygen and skin temperature for WHOOP 4.0 members (WHOOP developer documentation). Fitbit’s readiness uses HRV, resting heart rate and a week of sleep, against a baseline from at least 7 nights (Google Health Help, readiness).

What is in a readiness score?

From the makers' own documentation

Fitbit readiness score (formerly Daily Readiness) 0 (low) to 100 (high): high 65 or above, moderate 30 to 64, low 29 or below. Seven nights of wear set the first baseline.

• ✓ HRV : named by the maker Heart rate variability.
• ✓ Resting heart rate : named by the maker Resting heart rate.
• – Breathing rate : not named Not named by the maker.
• ✓ Sleep : named by the maker Sleep patterns over the past week.
• – Temperature : not named Not named by the maker.
• – Activity or load : not named Not named by the maker.

Also: Activity was an input in the earlier version and is not in the updated algorithm.

Fitbit's readiness score (formerly Daily Readiness) names 3 of the six inputs: HRV, resting heart rate and sleep. Google names the three inputs and does not publish the formula.

Source: Understanding your readiness score in the Google Health app (Google Health Help)

WHOOP Recovery 0–100%, calculated when you wake.

• ✓ HRV : named by the maker Heart rate variability; the API field for it is named hrv_rmssd_milli.
• ✓ Resting heart rate : named by the maker Resting heart rate.
• – Breathing rate : not named Not named by the maker.
• – Sleep : not named Not named by the maker.
• ✓ Temperature : named by the maker Skin temperature, for WHOOP 4.0 and later.
• – Activity or load : not named Not named by the maker.

Also: Blood oxygen (SpO2) on WHOOP 4.0 and later. Sleep and breathing rate are not named in the developer documentation.

WHOOP's Recovery names 3 of the six inputs: HRV, resting heart rate and temperature. WHOOP lists the measurements that "factored into the score" but not how they are combined.

Source: Recovery (WHOOP developer documentation)

A list of scores, one per device, with six inputs for each: HRV, resting heart rate, breathing rate, sleep, temperature and activity or load. Fitbit's readiness score (formerly Daily Readiness) names 3 of the six inputs: HRV, resting heart rate and sleep. Google names the three inputs and does not publish the formula. WHOOP's Recovery names 3 of the six inputs: HRV, resting heart rate and temperature. WHOOP lists the measurements that "factored into the score" but not how they are combined.

Neither company publishes its weights. A readiness of 60 and a Recovery of 60% are not the same thing.

Accuracy

WHOOP 4.0 was in the 2025 home study: its overnight RMSSD had a concordance of 0.94 with the chest-strap ECG and an average error of 8.17%, or 4.17 ms, and the authors rated its agreement as acceptable (Dial and colleagues, 2025). The current WHOOP 5.0 and MG were not tested, and neither was any Fitbit. No study yet lets you rank the two on HRV accuracy.

What each costs over three years

What you pay to see HRV, from each maker’s US store:

Fitbit Air from Google’s US store on 22 October 2026; Polar H9 and WHOOP One from the makers’ US pages on 21 October 2026. WHOOP’s Peak plan was $239 a year. Tax, shipping and the phone each needs are not included.

What HRV tracking costs over three years

US prices, 21 – 22 October 2026

• ECG chest strap, no subscription
• Optical band or watch, no subscription for HRV
• Device included in a yearly membership

Each price, and the page it was read on, is in the table.

A step chart of the money spent on HRV tracking from the first day to the end of year 3, one line per option. A device bought outright is a flat line; a yearly membership steps up at the start of each year. Polar H9: $69.90 on the first day, $69.90 by the end of year 3. Google Fitbit Air: $99.99 on the first day, $99.99 by the end of year 3. WHOOP One membership: $199 on the first day, $597 by the end of year 3.

The chart leaves out Google Health Premium, which HRV does not need. If you keep the coach after the free three months, Google charges $9.99 a month or $99 a year (Google, Google Health announcement). The chest strap is there for scale. It measures the heart’s electrical signal, and Polar documents that the H9 sends the beat-to-beat intervals HRV apps need, but you wear it for a short morning reading rather than all night; the cheapest way to track HRV compares more options.

Getting your data out

• Fitbit Air: Google points you to Google Takeout, with Google Health selected, for a copy of your data; the page names activity, exercise, sleep and heart rate, not HRV (Google Health Help, export). On Android, the Google Health app can write HRV to Health Connect, where other apps can read it (Google Health Help, Health Connect). Google’s list of data the app writes to Apple Health does not include HRV (Google Health Help, Apple Health).
• WHOOP: an in-app export sends CSV files, one of them with Recovery, resting heart rate and HRV for each day (WHOOP Support, export).

Wearing both

You can wear both, one on each wrist or the WHOOP on your arm. Expect two different numbers. Pick one as your main trend and compare the other only for direction: both should rise and fall on roughly the same days.

Tips for tracking HRV with Fitbit Air

• Wear it to bed every night. No sleep over 3 hours, no HRV for that day. With up to 7 days of battery, charge it while you are awake.
• Wear it the way Google describes. On top of the wrist, behind the wrist bone, with just enough room to slide your little finger under the band (Google Health Help, Get started).
• Keep one device per trend. You can pair a Fitbit Air and a Pixel Watch with the Google Health app at the same time and filter the view by device (Google Health Help, Get started). Google’s announcement mentions wearing a Pixel Watch by day and the Air for sleep (Google, Fitbit Air announcement). If both ever record your nights, read each device’s HRV separately.
• Give it time. Readiness needs 7 nights, and the personal range uses up to 30 days. How long until my HRV baseline is reliable lists the waiting times by brand.
• Compare weeks, not nights. A single night can jump around. Overnight vs morning HRV explains why a sleep value is not comparable with a short awake reading.

When to talk to a doctor

Google states that the Air’s heart rate and sleep features are not intended for medical purposes (Google Store). Its HRV and readiness score diagnose nothing. Talk to a doctor if low HRV or low scores come with feeling unwell, lasting fatigue, palpitations, dizziness or chest pain, whatever the number.