Apple Health Integration

Apple Health is the only outside integration Datum has — there's no separate account to create and no direct connection to a specific wearable brand. Datum only reads what your other apps (and whatever device they're connected to) have already written into Apple Health; it doesn't talk to an Apple Watch, Whoop, Garmin, or similar device directly.

Connecting#

In Settings, turn on the Apple Health toggle (see the Settings screenshot) and grant the permissions iOS asks for — you'll see Apple's own system permission sheet, not anything Datum builds itself. Turning it back off any time stops syncing. This step is entirely optional; skipping it just means Datum works as a self-contained app with no outside data.

Connecting also does a one-time catch-up: any workout you already finished before connecting gets synced right then too, not just workouts logged from that point on. Settings shows "Syncing past workouts..." while this runs, then how many were caught up.

What syncs, and in which direction#

Workouts (Datum → Apple Health): each finished workout is written to Apple Health as a workout entry, so it shows up alongside anything else you log there.

Calories & heart rate (Apple Health → Datum): for each workout, Datum queries Apple Health for calorie and heart-rate data covering that workout's time window. If a connected device (an Apple Watch, or another app that writes to Health) recorded that data during your workout, it appears automatically — no manual entry needed.

When Apple Health has no calorie total for a workout, Datum estimates one and labels it Calories (estimate), with a note under it explaining what the figure was built from. It never stores an estimate or passes one back to Apple Health — what gets saved is only ever Health's own measured total.

Age and biological sex. Datum reads these from Apple Health for one purpose: the most accurate of the calorie estimates uses them. They're read on your iPhone, used in that calculation, and never stored by Datum or sent anywhere. If you'd rather not share them, decline — Apple Health doesn't even tell Datum you declined, and the estimate simply falls back to a method that doesn't need them.

Body metrics (both directions): weight, body fat %, and lean body mass sync both ways — see Body Metrics & Measurements for exactly what that looks like, including backfilling history from before you started using Datum.

Nutrition (Apple Health → Datum, read-only): calories, protein, carbs, fat, sugar, and water are imported from whatever app you use to log food (Datum doesn't have its own food-logging screen). This shows up in Reports' Nutrition section and in correlation reports — see Reports.

Activities#

Each logged class or cardio activity is written to Apple Health as its own workout entry, tagged with the right activity type (yoga, cycling, running, and so on) — separate from the strength workout entry for the rest of that session, if there was one. This means a single Datum session with an embedded activity (a treadmill warm-up before lifting, say) can produce two separate entries in the Health app rather than one blended one. That's deliberate: a single Apple Health workout can only carry one activity type, so folding a cardio warm-up into a strength workout's entry would misrepresent both what it was and the energy Apple's own Activity-ring accounting expects for it.

Live heart rate during an activity can also come from a paired Bluetooth heart rate monitor rather than Apple Health — see Classes & Cardio Activities for both live sources.

Deleting a synced workout#

Deleting a workout in Datum doesn't automatically remove it from Apple Health — the two are separate copies once written. If the workout you're deleting had synced to Health, Datum asks first whether you want it gone from there too (see History); the default is to leave your Health data alone, so nothing is ever removed from Health without you explicitly choosing "Delete from Health." An embedded session's activity gets deleted from Health the same way as its strength entry, in the same prompt.

Why some numbers might show up blank#

Datum never presents a guessed number as a measured fact. Heart rate is shown only when something actually recorded it — if you trained without a connected device, or you're testing in the iOS Simulator (which has no real sensors), that field stays empty rather than showing a made-up value. This is the intended, cautious behaviour, not a sync failure.

Calories are the one exception, and only because the figure is clearly marked. When Apple Health has a real total, that's what you see. When it doesn't, Datum estimates one from whatever it does have, in this order of preference:

1. Your heart rate, age and bodyweight — the most accurate method available.
2. Your heart rate relative to your resting heart rate, combined with how hard the sets felt (your logged RPE).
3. Your RPE, bodyweight and how long you trained.
4. Bodyweight and duration alone — the roughest figure Datum will show.

Every one of these is deliberately cautious, and will usually read lower than other apps. Calorie estimates are the number people eat back, so overestimating is the harmful direction. Datum uses the low end of published figures for weight training rather than the middle, always rounds down, and never lets an estimate stray outside what the research supports for resistance training — even when the underlying formula says otherwise. Treat these as a lower bound, not a measurement. Logging RPE on your sets moves you up the list and makes the figure better.

If you've never logged a bodyweight, Datum can't scale any of these, so it shows nothing at all.