Natural Cycles vs Flo: Which One Do You Need in 2026?
One is cleared as birth control and reads your temperature. The other predicts from the dates you log. Here is how to tell which one you need.
Natural Cycles vs Flo is a choice between two different kinds of app: Natural Cycles is an FDA-cleared birth control app that reads your temperature, and Flo is a period and cycle tracker that predicts from the dates you log. If you want to prevent pregnancy without hormones, only one of them is built and cleared for that job. If you want to know when your period is due and log how you feel, the other does it for free.
This guide compares the two on what they measure, how accurate they are, what they ask of you, cost and privacy, then closes with a two-minute way to choose.
Natural Cycles vs Flo: which one should you use?
Use Natural Cycles if your goal is hormone-free birth control and you are willing to wear a device overnight and pay a subscription. It is the only app in this comparison that a regulator has cleared for preventing pregnancy. Use Flo if your goal is tracking: knowing roughly when your period is due, logging symptoms, and reading about what your body is doing.
The two are often compared as if they were rivals, but Natural Cycles itself draws the line in an article titled “A Birth Control App, Not A Period Tracker.” Flo draws the same line from the other side, stating on its accuracy page that its predictions should not be used as a method of birth control. Once you know which job you need done, most of this decision is made.
Flo vs Natural Cycles at a glance
Is Natural Cycles accurate?
Natural Cycles states that it is 93% effective with typical use and 98% effective when used as intended, meaning no unprotected sex on the days it marks red. Those figures sit behind a regulatory decision: the FDA granted Natural Cycles a De Novo classification on August 10, 2018, as a software application for contraception intended for women 18 and older.
The US numbers come from a 2021 cohort study in the Journal of Women’s Health that followed 5,879 women who bought an annual subscription to prevent pregnancy. With typical use the one-year Pearl Index was 6.2, which means about 6 pregnancies per 100 women in a year, and with perfect use it was 2.0. The 13-cycle cumulative pregnancy probability was 7.2%.
Two cautions belong next to those numbers. The cohort study was written by Natural Cycles researchers using the company’s own user data. A 2026 systematic review of contraceptive apps found typical-use Pearl Indexes between 5.8 and 6.8 across six studies, in line with the company’s figures, but rated every included study as low quality and called for randomized trials.
Can you use Flo as birth control?
No. Flo’s own accuracy page says its predictions should not be used as a method of birth control, and its App Store listing repeats that Flo is not a method of birth control. Flo’s help center adds that if you have logged fewer than three cycles, or your recent cycles vary a lot, it builds a longer fertile window of up to 14 days.
The reason is the method, not the brand. An app that works from dates is guessing at ovulation, and in the largest hormone-verified test of calendar apps, none predicted ovulation day correctly more than 21% of the time. We cover that research in how accurate period tracker apps are, and Flo’s predictions specifically in is Flo accurate.
For context, the American College of Obstetricians and Gynecologists reports that with fertility awareness-based methods in general, 12 to 24 women out of 100 become pregnant in the first year of typical use. A date-based app is a convenient way to run one of those methods, and it does not improve those odds.
How does each app find ovulation?
Natural Cycles measures it and Flo estimates it. After ovulation, rising progesterone lifts your body temperature slightly, and Natural Cycles’ algorithm uses that shift to identify ovulation and set each day as green (not fertile) or red (use protection). Flo combines the cycle dates you have logged with its own algorithm to estimate when your period, ovulation and fertile window are likely.
The difference matters because cycles are not evenly split. In a study of 612,613 cycles, the phase before ovulation ranged from 10 to 30 days while the phase after it stayed much tighter, and only 13% of cycles were exactly 28 days. Dates can tell you when the total is likely to end, but only a physical signal can tell you where the split fell.
Flo does let you sync temperature from wearables such as smartwatches and rings, and says this helps make its predictions more accurate. That improves a tracker. It does not turn it into a cleared contraceptive, and Flo does not claim that it does.
What does each one cost?
Flo has a free version that covers period and ovulation predictions and symptom logging, and a paid tier called Flo Premium that adds more content and features. Natural Cycles has no free version. It offers two plans, annual or monthly, and its site says the NC Band is included free with the annual plan.
Natural Cycles explains the charge directly: it says birth control costs money to make and supply, and that charging a subscription means it does not need to profit from selling users’ data. We have left dollar amounts out because both companies change them and show different prices by region, so check each app’s current listing before you decide.
How do they compare on privacy?
Flo has a public record here. In January 2021 Flo Health settled Federal Trade Commission allegations that it had shared users’ health information with analytics providers after promising to keep it private. In August 2025 a California jury found Meta violated state privacy law in a class action over Flo data.
Flo has since added Anonymous Mode, which lets you use the app without a name, email or IP address linked to your data.
Natural Cycles describes its own setup as a privacy program called NC Secure, with pseudonymization and encryption, and notes that as a regulated medical device it must meet the FDA’s cybersecurity requirements and follow GDPR. Those are the company’s statements, and we found no regulator action against it to report. Our Flo vs Clue comparison goes deeper on Flo’s record.
Where does Otty fit?
Otty sits on Flo’s side of the line. It is a calendar-based period and cycle tracker: you log day one of your period, and Otty maps the month into five phases with plain names like Storm (Period) and Glow (Ovulation), with one specific tip for the day. Its ovulation phase is an estimated window and never a measured day, so Otty is not a contraceptive and should not be used to avoid pregnancy.
What it adds is the everyday layer. You get a daily check-in for mood and symptoms, a forecast of how today is likely to feel, and a heads-up a few days before your period. If you have a partner, you can invite them with a code so they see the same phase and a tip of their own, and you decide whether to share at all.
How to choose in two minutes
Natural Cycles vs Flo FAQ
The short version of Natural Cycles vs Flo: choose by the job, then by what you are willing to wear and pay. If you are still weighing trackers, our comparison of alternatives to Flo covers six options.
