The data has to come from somewhere.
A program you can't measure is a hope. Twelve reads your body composition off a connected scale and your training load off your watch, then does something with both.
Why we built on Withings
Withings has been making connected health hardware since 2008 and runs a real medical-device business alongside the consumer line. Two things made the decision for us.
A public API that isn't an afterthought
Their Health Data API is free, documented, OAuth 2.0, and pushes a notification the moment you weigh in rather than making us poll. Most scale makers either have no API or gate it behind a partnership call.
They publish every metric, not just weight
Fat mass, lean mass, muscle, bone, water, visceral fat, standing heart rate. A scale that only gives you a number is a scale you'll stop caring about in week three.
Withings and the Withings logo are trademarks of Withings SA, used here to identify compatibility. The logo is reproduced from the brand assets Withings publishes for this purpose. Twelve connects through the public Withings Health Data API. We are an independent developer, not affiliated with, sponsored by or endorsed by Withings.
We are applying to sell Withings devices directly
Twelve already reads from any Withings scale you own. We have applied through the Withings Partner Hub to carry their devices ourselves, so you could buy the scale and the programme together and have it arrive already paired to your account.
Until that is approved, the scales in our store are Twelve's own hardware. We do not use Withings product photography and we do not sell their devices yet. Buy directly from Withings →
What arrives, and what the program does with it
| Metric | Where it shows up |
|---|---|
| Weight | Trend line, rate per week, progress against goal |
| Body fat % | Composition chart — the number that says whether the loss is fat |
| Lean mass | The guardrail. If this falls with the scale, the deficit is too steep |
| Muscle mass | Cross-checked against your lifting volume week over week |
| Visceral fat | The metabolic-risk metric. Slowest to move, most worth watching |
| Water | Explains the scale jumps that aren't fat — creatine, salt, carbs |
| Bone mass | Baseline. Should not move over twelve weeks |
| Standing heart rate | Recovery signal. A climbing resting rate means back off |
Authorize once in the app. Tokens refresh themselves, so the link doesn't die quietly three days later.
Step on the scale. Withings notifies us the same second — nothing to open, nothing to sync.
The reading lands in your log, tagged as measured rather than typed, and the charts redraw.
Week six, you can see whether you're losing fat or losing the muscle you spent six weeks building.
Which one to buy
Any of the three feed the program. The difference is how much you get beyond weight and body fat.
Apple Watch and Apple Health
The eighteen-minute interval is a heart-rate workout, and the watch already records it better than a phone in your pocket can. The native iOS app will read workouts, heart rate, active energy and sleep from Apple Health, and write Twelve sessions back so your rings close where they should.
A watchOS app puts set logging on your wrist instead of your phone — intensity and the push set, tapped between sets rather than after them.
Worth being straight: your watch already tracks lifting and intervals today with zero setup. What the native app adds is that the training load sits in the same chart as your body composition, so you can see recovery and results together.
Apple, Apple Watch, Apple Health and HealthKit are trademarks of Apple Inc. Twelve is not affiliated with, sponsored by or endorsed by Apple.
Built on RVOS
Twelve runs as a tenant on RVOS. Ask Twelve is RVOS AIBar. Checkout is RVOS checkout. The integrations, the storefront and the AI layer are platform capabilities rather than one-off code in a fitness app.
Ask Twelve
The question bar on this site and in the app. Scoped to training and nutrition, and hard-stopped on dosing and drug interactions — those go to a pharmacist.
Checkout
Supplements, scales and hardware all transact through RVOS checkout. One cart, one payment rail.
Private label
The program, the tracker and the storefront can be branded and resold by other RVOS tenants. Twelve is the reference build.
Start measuring
The app is free and works without any hardware. Add a scale when you want the rest of the picture.