heartRateVariabilitySDNN: HealthKit quantity type
HKQuantityTypeIdentifierHeartRateVariabilitySDNN · read from Apple’s documentation on 2026-10-02
HKQuantityTypeIdentifier.heartRateVariabilitySDNN (Objective-C HKQuantityTypeIdentifierHeartRateVariabilitySDNN) is a HealthKit quantity type in Apple's "Vital signs" group. Apple's abstract: “A quantity sample type that measures the standard deviation of heartbeat intervals.” Apple's discussion says its samples use time units and measure discrete values, so in HKStatisticsQuery it takes .discreteAverage, .discreteMin or .discreteMax. It is available from iOS 11.0 and watchOS 4.0. This site's verified cross-platform matrix pairs it with HeartRateVariabilityRmssdRecord on Android Health Connect in its HRV row, with the caveat “The big one: the HRV Apple Watch records into HealthKit is SDNN, while Health Connect's record is RMSSD.”
What Apple says heartRateVariabilitySDNN is
A quantity sample type that measures the standard deviation of heartbeat intervals.
Apple’s abstract, quoted from its heartRateVariabilitySDNN reference page, read 2026-10-02. Apple’s discussion of the type runs 68 words.
- Swift
HKQuantityTypeIdentifier.heartRateVariabilitySDNN- Objective-C
HKQuantityTypeIdentifierHeartRateVariabilitySDNN- Family
HKQuantityTypeIdentifier- Apple's topic group
- Vital signs
- Group page
- Vital signs
- Apple's reference
- heartRateVariabilitySDNN on developer.apple.com
Availability
The versions Apple’s documentation data gives for heartRateVariabilitySDNN, platform by platform, as read on 2026-10-02.
| Platform | Introduced | Deprecated |
|---|---|---|
| iOS | 11.0 | — |
| iPadOS | 11.0 | — |
| Mac Catalyst | 13.0 | — |
| macOS | 13.0 | — |
| visionOS | 1.0 | — |
| watchOS | 4.0 | — |
The rest of that release: HealthKit Types Introduced in iOS 11.
Sum or average, and in which units
Apple states both of these in the prose of the type’s discussion, not as properties, so they are derived from Apple’s sentence and the sentence is shown with each.
Aggregation style
discrete — in an HKStatisticsQuery, use .discreteAverage, .discreteMin or .discreteMax. Apple’s sentence:
These samples use time units (described in ) and measure discrete values (described in ).
Unit family
time — every type in this family is on the HKUnit reference.
Apple states it in the same sentence as the aggregation style, quoted above.
Empty parentheses and a dangling “described in” mark links in Apple’s sentence to other documentation pages: the generator keeps the sentence’s text, not its links, and the quote is left as stored rather than repaired.
Can apps write heartRateVariabilitySDNN?
Apple’s documentation for heartRateVariabilitySDNN does not state that its samples are read-only. Apple states read-only status only in prose, and its silence is not a statement either way, so this page makes no claim that apps can save it.
On Android Health Connect
The verified cross-platform matrix pairs heartRateVariabilitySDNN with HeartRateVariabilityRmssdRecord in its HRV row.
The big one: the HRV Apple Watch records into HealthKit is SDNN, while Health Connect's record is RMSSD. They are different measures and are not interconvertible — do not normalize one into the other. iOS 27 added HKQuantityTypeIdentifier.heartRateVariabilityRMSSD, but as of October 2, 2026 Apple's documentation gives it no description, so it is not paired with Health Connect's record here: what it holds, how it is computed and whether Apple Watch writes it are not stated.
The metric guide for this row: HRV on every platform.
Metadata keys Apple ties to it
The HKMetadataKey constants whose own Apple text names heartRateVariabilitySDNN, with the sentence that does, from Apple’s metadata-key pages read 2026-10-04.
- HKMetadataKeyAlgorithmVersionvalue: NSNumber
Note: In watchOS 8, the system uses this key for heartRateVariabilitySDNN and HKHeartbeatSeriesSample samples generated by Apple Watch.
Questions
- Should I sum or average heartRateVariabilitySDNN in HealthKit?
- Average it, or take its minimum or maximum — do not sum it. Apple's documentation for heartRateVariabilitySDNN describes it as measuring discrete values, so an HKStatisticsQuery over it takes .discreteAverage, .discreteMin or .discreteMax. Apple's sentence, as read on 2026-10-02: "These samples use time units (described in ) and measure discrete values (described in )."
- Which unit family does Apple give heartRateVariabilitySDNN?
- Time. Apple's discussion of heartRateVariabilitySDNN says its samples use time units: "These samples use time units (described in ) and measure discrete values (described in )."
- Does heartRateVariabilitySDNN have a Health Connect equivalent?
- This site's verified cross-platform matrix pairs HealthKit's heartRateVariabilitySDNN with HeartRateVariabilityRmssdRecord in its HRV row. The caveat recorded with that row: The big one: the HRV Apple Watch records into HealthKit is SDNN, while Health Connect's record is RMSSD. They are different measures and are not interconvertible — do not normalize one into the other. iOS 27 added HKQuantityTypeIdentifier.heartRateVariabilityRMSSD, but as of October 2, 2026 Apple's documentation gives it no description, so it is not paired with Health Connect's record here: what it holds, how it is computed and whether Apple Watch writes it are not stated.
- Which HealthKit metadata keys mention heartRateVariabilitySDNN?
- HKMetadataKeyAlgorithmVersion. Apple's text for HKMetadataKeyAlgorithmVersion: "Note: In watchOS 8, the system uses this key for heartRateVariabilitySDNN and HKHeartbeatSeriesSample samples generated by Apple Watch."
- Which iOS version added heartRateVariabilitySDNN?
- iOS 11.0. Apple's availability data for heartRateVariabilitySDNN lists iOS 11.0, iPadOS 11.0, Mac Catalyst 13.0, macOS 13.0, visionOS 1.0 and watchOS 4.0.
Sources checked
- Apple — HKQuantityTypeIdentifier.heartRateVariabilitySDNN · read 2026-10-02
- Apple — HKQuantityTypeIdentifier · read 2026-10-02
- Apple — HKMetadataKeyAlgorithmVersion · read 2026-10-04
Apple’s abstract and sentences are quoted for identification; the page layout, the capsule and the cross-platform pairing are AIFitnessAPI’s. Generated from Apple’s documentation by scripts/fetch-healthkit-identifiers.mjs, not typed by hand — the same data as the full identifier table.