How this HRV calculator works
Heart rate variability (HRV) is how much the time between your heartbeats varies, in milliseconds (ms). It reflects how your nervous system is steering your heart, and it falls with age, so a number only makes sense next to people your own age.
Wearables report one of two measures, and each has its own reference:
- SDNN, which Apple Health shows, is compared with 1,906 healthy adults aged 25 to 74 in Germany, measured for 5 minutes at rest with an ECG (Voss 2015).
- RMSSD, which Oura, WHOOP, and Fitbit show, is compared with 8.2 million Fitbit users aged 20 to 61, measured in 5-minute windows between 6 and 7 AM (Natarajan 2020).
You get two numbers:
- Your percentile: where you rank among people of your sex and age. At the 50th, half of them have a higher HRV. HRV is skewed toward high values, so the calculator fits a log-normal curve to each published age group and draws straight lines between groups.
- Your HRV age: the age at which your HRV would be the median for your sex.
For example, a 48-year-old woman with an Apple Watch SDNN of 42 ms ranks in the 66th percentile for her age and has an HRV age of 38.
HRV chart by age
These are the published values the calculator uses, in ms.
SDNN by age (Apple Watch): mean ± standard deviation, 5-minute resting ECG.
| Age | Women | Men |
|---|---|---|
| 25–34 | 48.7 ± 19.0 | 50.0 ± 20.9 |
| 35–44 | 45.4 ± 20.5 | 44.6 ± 16.8 |
| 45–54 | 36.9 ± 13.8 | 36.8 ± 14.6 |
| 55–64 | 30.6 ± 12.4 | 32.8 ± 14.7 |
| 65–74 | 27.8 ± 11.8 | 29.6 ± 13.2 |
RMSSD by age (Oura, WHOOP, Fitbit): median, with the middle 50% in brackets, 6 to 7 AM.
| Age | Women | Men |
|---|---|---|
| 20–21 | 56 (37–85) | 66 (45–96) |
| 25–26 | 48 (32–73) | 54 (35–79) |
| 30–31 | 45 (31–67) | 49 (34–71) |
| 35–36 | 41 (29–60) | 43 (31–62) |
| 40–41 | 37 (26–52) | 38 (27–54) |
| 45–46 | 33 (24–46) | 34 (25–48) |
| 50–51 | 31 (22–42) | 31 (23–42) |
| 55–56 | 29 (22–40) | 29 (21–39) |
| 60–61 | 28 (21–38) | 27 (20–37) |
The two tables come from different people, devices, and measures, so don't compare numbers across them. Before 25 for SDNN and 20 for RMSSD, and after 74 and 61, the calculator uses the nearest age group.
What is a good HRV?
For your age, any HRV above the median is higher than typical. But there's no single good number, because HRV varies a lot from person to person. In the Fitbit data, the middle half of men aged 40 to 41 had a morning RMSSD anywhere from 27 to 54 ms.
Your own trend tells you more than any chart. That's why recovery scores compare your HRV with your own baseline, not with other people.
Does HRV decrease with age?
Yes, and fastest when you're young. In the Fitbit data, HRV fell fastest from 20 to 40 and more slowly after that. RMSSD fell faster than SDNN. The median morning RMSSD for men dropped from 66 ms at 20 to 21 to 38 ms at 40 to 41, then to 27 ms at 60 to 61.
The decline levels off later in life. In a 24-hour study of 260 healthy people aged 10 to 99, RMSSD fell to 47% of its level in the teens by the 50s, and then held steady. In the German study, the effect of age disappeared between 65 and 74.
Do men or women have higher HRV?
It depends on the measure and the age:
- RMSSD: in the Fitbit morning data, men were higher until about 46. The two were about equal at 50, and after that women were higher.
- SDNN: Fitbit men were higher at every age. In the German study, the difference between men and women wasn't significant in any decade.
Apple Watch HRV: SDNN vs RMSSD
Apple Health shows HRV as SDNN, the standard deviation of the time between heartbeats. Oura, WHOOP, and Fitbit use RMSSD, which tracks beat-to-beat changes and better reflects the rest-and-digest side of your nervous system. Apple Health has stored HRV as SDNN since iOS 11. iOS 27 adds a data type for RMSSD to HealthKit, so apps may start reading RMSSD from Apple Health too.
The two measures give different numbers from the same heartbeats, so a 45 ms SDNN and a 45 ms RMSSD don't mean the same thing. Pick the measure your device shows.
Read Apple Watch results as a rough guide. Apple Watch takes readings at scattered times through the day and night, and Apple doesn't publish how long each one lasts. Longer recordings show more variability, and SDNN from recordings of different lengths shouldn't be compared directly. No large study has published Apple Watch SDNN by age yet, so the calculator uses 5-minute resting SDNN, the standard short-term reference.
When to measure HRV
Time of day matters. In the Fitbit data, men aged 40 to 41 had a median RMSSD of 38 ms between 6 and 7 AM and 26 ms between 6 and 7 PM. The researchers advise comparing readings from the same time of day.
For the fairest comparison, use your overnight or morning average over a week, not one reading. Apple Watch shows individual readings in the Health app, so look at the weekly average.
What is HRV age?
The age at which your HRV would be the median for your sex. An HRV age below your real age means your HRV is higher than typical for your age.
| SDNN | HRV age, women | HRV age, men |
|---|---|---|
| 25 ms | Over 70 | Over 70 |
| 30 ms | 57 | 60 |
| 35 ms | 49 | 49 |
| 40 ms | 42 | 42 |
| 45 ms | 31 | 32 |
| 50 ms | Under 30 | Under 30 |
Track your HRV with Ness
A percentile tells you where you stand. Your trend tells you what to do today. Ness reads your Apple Watch HRV from Apple Health and compares it with your own history:
- Recovery score: your overnight HRV and sleeping heart rate against your own 14-night baseline, so a rough night shows up the next morning.
- Biological age: the years your HRV adds or takes off, next to VO2 max, resting heart rate, and more.
Ness is an AI health coach for iPhone and Apple Watch. It also scores your sleep, strain, and stress every day, logs meals from a photo, and answers questions about your own data.
FAQ
What is a normal HRV by age? For Apple Watch SDNN, the average in a resting reference study was about 48.7 ms for women and 50 ms for men at 25 to 34, falling to 27.8 and 29.6 ms at 65 to 74. For RMSSD, the median morning value in 8.2 million Fitbit users was 45 ms for women and 49 ms for men at 30 to 31, and 31 and 31 ms at 50 to 51. Normal covers a wide range around these.
What is a good HRV for my age? Anything above the median for your age and sex is higher than typical. Enter your age, sex, and HRV above to see your percentile. Your own trend over weeks matters more than where you rank.
Is 40 a good HRV? It depends on your age and the measure. As an Apple Watch SDNN, 40 ms ranks in the 36th percentile for a 30-year-old man and in the 66th percentile for a 50-year-old man. As an RMSSD, it ranks in the 41st percentile for a 30-year-old woman and in the 69th percentile for a 50-year-old woman.
What is a normal HRV on Apple Watch? No large study has published Apple Watch HRV by age. Apple Watch shows SDNN, and resting 5-minute SDNN averaged 45.4 ms for women and 44.6 ms for men aged 35 to 44 in a study of 1,906 healthy adults. Apple Watch readings are shorter, so treat that as a rough guide and watch your own trend.
Why is my HRV different on Apple Watch and Oura or WHOOP? Apple Watch shows SDNN, while Oura, WHOOP, and Fitbit show RMSSD. They're different calculations, measured at different times, so the numbers don't match. Compare each one only with its own history.
Does HRV go down with age? Yes. It falls fastest between 20 and 40, then more slowly, and levels off later in life. RMSSD falls faster than SDNN.
Do women have lower HRV than men? Not always. In 8.2 million Fitbit users, men had a higher morning RMSSD until about 46, and women had a higher one after 50. SDNN was higher in men at every age. In a resting ECG study of 1,906 adults, the differences weren't significant.
What is HRV age? The age at which your HRV would be the median for your sex. For example, an Apple Watch SDNN of 35 ms gives women an HRV age of 49 and men an HRV age of 49.
Does one low HRV reading mean something is wrong? Usually not. HRV changes with the time of day and from one reading to the next. In the Fitbit data, evening values were well below morning ones. Look at your weekly average, and talk to your doctor if your HRV stays low along with symptoms.
Does this calculator save my data? It runs in your browser and sends nothing anywhere. It remembers your entries on this device so you can check again next month. Delete them from the fields anytime.
Related reading
- Apple Watch HRV: What It Means and How to Read It
- Biological Age Calculator
- VO2 Max Calculator
- Apple Watch Recovery Score: How to Check It on Any Model
- Strain vs Stress vs Recovery: What Each Score Actually Measures
Sources
- Natarajan A, et al. Heart rate variability with photoplethysmography in 8 million individuals: a cross-sectional study30246-6). Lancet Digital Health, 2020.
- Voss A, et al. Short-term heart rate variability: influence of gender and age in healthy subjects. PLOS ONE, 2015.
- Umetani K, et al. Twenty-four hour time domain heart rate variability and heart rate: relations to age and gender over nine decades00554-8). Journal of the American College of Cardiology, 1998.
- Shaffer F, Ginsberg JP. An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health, 2017.
- Apple. heartRateVariabilitySDNN and heartRateVariabilityRMSSD. Apple Developer Documentation.
- Fitbit. Heart Rate Variability: Get HRV Summary by Date. Fitbit Web API.
- Oura. Heart Rate Variability. Oura Help.
- WHOOP. Heart Rate Variability: How to Read and Improve Your HRV. WHOOP.
This calculator is for general wellness and education. It isn't medical advice. HRV isn't a diagnosis. If you have symptoms or a heart condition, talk to your doctor.