periods.day
Online menstrual cycle calculator
Enter menstrual bleeding dates
Estimate upcoming menstruation dates, cycle length and cycle phases from recorded bleeding days. Entries stay in the private link.
Recorded bleeding days are the input data. Earlier menstrual episodes help estimate cycle length from start-to-start intervals.
Cycle length
Period length
Ovulation day
Bleeding days recorded: 0 days
Tap each bleeding day to add or remove it
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Each recorded bleeding day contributes to the menstrual history. Intervals between period starts inform the cycle-length estimate.
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The summary shows the estimated phase for the current date. Calendar months and date tooltips display recorded bleeding and projected cycle phases.
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The private link retains the input dates. New bleeding records update the estimates; calendar exports provide reminders for projected events.
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.
periods.day estimates menstrual cycle timing from recorded bleeding dates. The model calculates cycle length, projected menstruation and approximate cycle phases; it does not measure hormones or establish a clinical diagnosis.
The estimator derives cycle length from intervals between recorded period starts, giving more weight to recent usable intervals. Without a usable interval, it uses a 28-day baseline. Bleeding duration defaults to five days until enough completed episodes are recorded.
Results are approximate, not exact dates. Normal cycle variation, stress and hormonal contraception can shift bleeding patterns. Variable cycle lengths and projections further into the future reduce certainty. Menstrual cycle variation (NHS)
You can enter past or future period dates. Past dates inform the calculated rhythm; future entries anchor your plans and remain estimates.
No. Calendar estimates do not measure ovulation, identify safe days or establish pregnancy status. This online tool is not a contraceptive method, pregnancy test or diagnostic device.
The menstrual cycle calculator is free and requires no account. Recorded dates are encoded in the private link, with no saved account data or browser storage. Anyone with the complete link can read the entries.
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)