Cycle calculator
Next period, ovulation, and fertile window in one view.
How this calculator works
The cycle calculator uses the calendar method . Given the first day of your last period and your usual cycle length, it estimates your next three periods by adding your cycle length to that start date. Ovulation is estimated as 14 days before the next period , following standard clinical guidance. Your fertile window covers the five days before ovulation plus ovulation itself.
What counts as a normal cycle
A cycle length between 21 and 35 days is considered within the normal adult range by the American College of Obstetricians and Gynecologists. Periods themselves typically last 3 to 7 days. Variation of a few days month to month is normal. Cycles consistently shorter than 21 days or longer than 35 days are worth discussing with a clinician. Stress, travel, illness, and big lifestyle changes can all delay or shorten a single cycle.
When the predictions will feel off
Predictions are estimates, not guarantees. Accuracy improves as you track more cycles — most users see predictions tighten after three or four logged periods. Irregular cycles, perimenopause, and conditions like PCOS all make the calendar method less reliable. If your cycles are irregular, consider combining the calendar method with cervical mucus or basal body temperature tracking for a fuller picture.
Frequently asked questions
Can I use this as birth control?
No. The calendar method alone is not a reliable contraceptive. Sperm can survive inside the body for up to five days, and ovulation can shift by several days even in regular cycles. If you want to prevent pregnancy, pair tracking with a barrier or hormonal method and consult a clinician.
What if my cycle length changes every month?
Enter the average of your last three cycles. If your cycles consistently vary by more than a week, the calendar method won't be very accurate. Our normal period guide covers what is and isn't worth flagging.
Why 14 days before the next period for ovulation?
The luteal phase — the time between ovulation and your next period — is the most stable part of the cycle for most people, hovering close to 14 days. That's why clinicians work backwards from the expected next period to estimate ovulation.
Do you store this information?
No. The calculation runs entirely in your browser. Nothing is sent to our servers, and nothing is saved when you leave the page.
Important disclaimer
This tool is educational and doesn't replace medical advice. If something feels off, talk to a qualified clinician.