periods.day
Private cycle tracker
Tap a calendar date to get started
Track and share your cycle with the people you choose. Not advertisers.
Tap every day of your most recent period. Add an earlier period to make the estimate more personal.
Cycle length
Period length
Ovulation day
Period days logged: 0 days
Tap every day you bled to add or remove it
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Tap every day you bled. Add earlier periods for a more personal estimate.
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See your estimated phase, fertile window, and next period, then share the link with someone you trust.
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Bookmark or keep the link to return and add new days later. periods.day does not save it for you.
periods.day has no accounts, cookies, analytics, or saved cycle data. Your entries live only in a link you can share. Nobody else can edit your entries.
Hello! I wanted to understand my body without sharing my data with advertisers. I also wanted to share knowledge about women's health. Feel free to share this website if you find this useful!
periods.day uses the period days you enter to estimate where you may be in your cycle. The more past periods you add, the more personal the estimate becomes. It’s still only an estimate. We base our estimations on Real-life insights on menstrual cycles and ovulation using big data (Soumpasis et al., 2020).
Your period dates, goal and duration settings are encoded in the link and read in your browser. periods.day does not save them. Each link is an independent snapshot: people you share it with can change their own copy, but they cannot edit your entries. Changes are not synced. If you add or change data, you need to share a new link to send the updated version.
Don't be silly. Calendar predictions cannot identify safe days and should not replace contraception. Cycle timing can change, and pregnancy can occur outside a predicted fertile window. Read ACOG fertility guidance
It is an estimate of the days when pregnancy is more likely. ACOG describes the fertile period as roughly the five days before ovulation through one day after; ovulation itself can shift from cycle to cycle.
Predictions are less reliable when cycle length varies. Log every day of several periods for more personal estimates, and talk to a clinician if you are worried about your cycle.
The full URL includes the dates and tracking goal you entered. Anyone you share it with sees the same estimate and can change it in their own browser. Treat the link as private: the data is encoded, not encrypted.
These estimates use pregnancy data from 221 women who were actively trying for a baby and had stopped using contraception. They recorded each day whether they had intercourse. Li, Wilcox and Dunson (2015, Table 3) used a statistical model to estimate the chance of a clinical pregnancy from one act on a given cycle day. This does not mean the women had sex once every day. The study did not record whether ejaculation occurred inside the vagina. These figures therefore do not estimate the risk with withdrawal. Our calendar shifts the study’s curve to match the calculated ovulation day or the cycle day you set. This is an approximation, not a validated personal prediction. Days before the table starts use its first value. The percentages are population estimates, not your personal odds or a contraceptive method. Li, Wilcox & Dunson (2015)