Do your Apple Watch and Oura numbers mean the same thing?

No. Apple Watch reports SDNN. Oura reports RMSSD, and so do WHOOP and Garmin. Both are in milliseconds and both come from the gaps between your heartbeats, but they measure different things about those gaps. You can’t swap them, convert one into the other or average them.

Why it’s an easy mistake

Both apps call the number HRV. Both put “ms” after it. Many people wear a watch by day and a ring at night, so they see two numbers for the same night and assume one device is broken. Usually neither is.

Is a day’s temperature range the same as how fast it changes?

No, though both are in degrees. Picture a day that warms steadily from 10 to 20 degrees. The range is big, but each hour is only a little warmer than the last. Now picture a day that flickers between 14 and 16 every hour. The range is small, but every hour jumps.

SDNN is like the range: how widely all the gaps between beats spread over the recording. RMSSD is like the hour-to-hour change: how much each gap differs from the one just before.

Six heartbeats, two answers

Take two sets of six gaps between beats, in milliseconds, both at about 59 beats a minute.

On the first set, RMSSD is double SDNN. On the second, SDNN is bigger. Same heart rate, opposite answers. That’s why no formula turns one into the other. The gap between them depends on the pattern of your beats, which changes with breathing, sleep stage and how long the recording runs.

A 2017 review explains why. RMSSD mostly picks up the quick, breath-by-breath swing of the vagus nerve. SDNN also picks up slower waves, which count for more over longer recordings. The same review warns that norms from 24-hour, 5-minute and shorter recordings can’t be swapped for one another.

Even RMSSD devices differ

Two WHOOP scientists wrote in 2025 that a fair comparison needs the same measurement, the same window and the same way of averaging. Apple Watch against Oura fails on all three.

The best case for converting

“Both come from the same heartbeats, so there must be a ratio.” The two often move the same way, because both get bigger when the gaps vary more. Moving together isn’t the same as a fixed ratio. The worked example above flips the ratio with the same heart rate. A 2026 guideline also lists the input signal, the recording length and breathing among the things that change HRV results, all of which differ between a watch and a ring.

What changed with Series 12?

Apple Watch Series 12 and Ultra 4 now show Recovery HRV and overall HRV, measured as often as every five minutes. Apple’s developer documentation now lists an RMSSD data type from iOS 27, alongside the long-standing SDNN one. Apple hasn’t said how Recovery HRV or overall HRV is calculated. Until it does, keep each in its own history, apart from your ring.