Next-period estimate
Automatic cycle detection and a next-period estimate with a range and how sure it is, worked out on your device from your own logged cycles. Estimates can be wrong.
Private by design
Ondi
A personal cycle diary that keeps your logs on your phone. No account, no tracking, no Ondi server — with Apple Watch temperature and sleep, smart ring support and a diary for every stage, from first periods to pregnancy and menopause.
On-device only
Apple Watch temperature
Smart ring
No account
One-time purchase
iPhone & iPad
At a glance
- Price
- Free version; 7-day trial of everything, then a one-time purchase
- Devices
- iPhone, iPad
- Requires
- iOS 17.0 or later
- Languages
- English, German, Spanish, French, Japanese, Chinese (Simplified), Hindi
- Age rating
- 12+
- Account
- Not needed
- Ads
- None
- Your data
- Stays on your device
Your cycle, at a glance
Log the day in a couple of taps, see your estimates and your own patterns, and keep a diary of every cycle. None of it needs an account, and nothing you log is sent to us.
What it does
A personal cycle diary, from the first log to pregnancy and beyond, built so your most personal data stays yours.
Daily log in seconds
Flow, mood, energy, sleep and symptoms in a couple of taps, a calendar of every cycle, and a note when your logged cycles vary a lot.
Apple Watch temperature
With your permission, Ondi reads wrist temperature and sleep from Apple Health and shows them as trends next to your log. You can also log temperature, cervical mucus and ovulation tests as notes.
Every stage
Pregnancy, postpartum and post-menopause diaries, in plain language. In pregnancy mode the due date is an estimate worked out from the date you enter; your midwife or doctor gives you your real dates.
Insights in plain language
Notes on what you logged, such as PMS patterns, headaches and sleep across your cycle, with a reminder to talk to a doctor. On supported devices, Ask Ondi answers questions about your own logs, on your iPhone.
No account required
No email, no sign-up, no Ondi server. Open the app and start. Everything is free for 7 days; after that logging, history and the calendar stay free and one purchase unlocks the rest.
Works with your smart ring
Pair an affordable Colmi-family smart ring straight from Ondi — no separate ring app and no account. Your ring's heart rate, blood oxygen and sleep appear on a dedicated Device tab, and you decide whether ring sleep is saved to Apple Health.
Everyday vitals
Heart rate, blood oxygen, stress and HRV from the ring, with a day chart and a 7-day view. Measured and kept on your phone — no Ondi server, no login.
Sleep, at a glance
Sleep efficiency, the stage breakdown and a hypnogram you can scrub with your finger, plus your last 7 nights. If you wear an Apple Watch to bed, its sleep comes first.
Temperature, kept separate
The ring's skin temperature is a readout only. Wrist temperature trends come from Apple Watch — we keep the two apart on purpose.
Private by design
Ondi talks to the ring over Bluetooth itself. Ring nights go to Apple Health only if you turn that switch on, and only when no Apple Watch sleep exists for that night. You can use the ring with PaceGrid on the same phone.
Supported rings
Tested: COLMI R09. Compatible (same family, not individually tested): COLMI R02, R03, R06, R10 and most rings that pair without their own app. Heart rate is confirmed across the family; sleep accuracy varies by model.
How to pair
1 · Charge it
Give the ring a few minutes on its charger so it has power to connect.
2 · Quit the ring's own app
If you used the official ring app, fully close it. You can keep Ondi and PaceGrid paired at the same time — just don't sync in two apps at the same moment.
3 · Turn on Bluetooth
Make sure Bluetooth is on and Ondi is allowed to use it.
4 · Open Smart Ring
In Settings, open the Smart Ring screen and tap Scan with the ring next to your phone.
5 · Tap your ring
Choose your ring from the list to connect. Ondi remembers it for next time.
6 · Wear it
Wear it overnight — sleep syncs automatically when you reopen the app.
Private, not just promised
Most trackers ask for an account before you can log a single day, then sync your most intimate data to the vendor's servers. Ondi doesn't.
- Your logs are stored on your device and encrypted by iOS when it's locked.
- No account, no Ondi server for your health records, no tracking analytics.
- Apple Health and smart-ring features are optional and run on your device; saving periods to Apple Health is off until you turn it on.
- Discreet notifications and Lock Screen widgets that hide personal details.
Who it's for
Ondi is for people who want a private period tracker, a personal cycle diary, or a period app that does not begin with an account signup. It is especially relevant if privacy is not an abstract preference for you, but the main reason you are choosing a new health app.
No account-first workflow
You do not need to create an account just to log a cycle. That changes the feel of the app immediately.
On-device privacy
Designed to keep sensitive cycle data on the device instead of sending it to a vendor's servers.
Every stage of life
From first periods to pregnancy, postpartum and menopause, Ondi has a diary for each stage, and you switch modes as your life changes.
Export without lock-in
Export your full history to a file for a doctor's appointment, so your data stays portable rather than trapped.
Built for trust
On-deviceLogs stay on your device, encrypted by iOS.
Apple WatchWrist temperature via Apple Health, if you allow it.
No accountNo sign-up, no Ondi server, no tracking.
Frequently asked questions
Is Ondi private?
Yes. There is no account, no Ondi server and no tracking analytics. Your logs are stored on your device, and device backups follow Apple's rules. The only network request is an App Store lookup for the names and icons of our other apps; it carries no health data.
What's free?
Logging in every mode, your full history, the calendar, the next-period estimate and export of your own data. Everything is free for 7 days; then one purchase unlocks insights, Ask Ondi on supported devices, smart ring insights and the pregnancy, postpartum and post-menopause dashboards. No subscription.