Menstrual Cycle / Period Calculator

Enter September 1, 2026 as the last period start date. That is the first day of real bleeding, not spotting beforehand. Set the average cycle length to 28 and the typical period length to 5.

The results panel shows four dates. Predicted next period: Sep 29, 2026. Period estimated through: Sep 5, 2026. Estimated ovulation day: Sep 15, 2026. Estimated fertile window: Sep 10, 2026 - Sep 15, 2026. A summary card also counts the days from today's date to the predicted next period.

Count the days and September 1 is cycle day 1, so September 15 is cycle day 15. The familiar 'day 14' shorthand is a day off even here, but the bigger problem is that it ignores cycle length altogether, as the next example shows.

To find your own cycle length, count from the first day of one period to the first day of the next, not to the day bleeding stops. Do that for your last few cycles and average them. One unusual month tells you less than the average of several.

Every rule works in whole calendar days:

• Cycle length used = your entry, held between 21 and 45 days. An entry of 50 is treated as 45, and an entry of 19 as 21.
• Predicted next period = last period start + cycle length.
• Estimated ovulation = predicted next period − 14 days, which is the same as cycle day (cycle length − 13).
• Fertile window = estimated ovulation − 5 days, through estimated ovulation: six days in all.
• Period estimated through = last period start + period length − 1, so a five-day period starting September 1 ends September 5.
• Days until next period = predicted next period − today, never shown below zero.

The six-day window follows Wilcox, Weinberg and Baird's study in the New England Journal of Medicine (1995). In that study, conception happened only after intercourse during the six days ending on the day of ovulation. Sperm can survive several days before ovulation, but the egg lasts about a day after it.

The fixed 14 days is the part to hold loosely. It stands in for the luteal phase, the stretch from ovulation to the next period. Bull and colleagues (npj Digital Medicine, 2019) analysed 612,613 ovulatory cycles logged in a fertility-tracking app. They found a mean luteal phase of 12.4 days, with most values between 7 and 17, and a mean follicular phase of 16.9 days, with most between 10 and 30. The luteal phase varies less than the follicular phase, which is why counting back from the next period beats counting forward from the last one. It still varies.

Keep the start date at September 1, 2026 and change the cycle length to 35. The predicted next period moves to October 6, 2026, estimated ovulation to September 22 (cycle day 22), and the fertile window to September 17 – September 22. The period end date stays at September 5, because only the cycle length changed.

Now compare that with the day-14 rule, which would put ovulation on September 14. That is three days before this cycle's estimated fertile window even opens, and eight days before the estimated ovulation. A longer cycle mostly lengthens the follicular phase, so ovulation moves later while the gap between ovulation and the next period stays roughly the same.

Short cycles show the opposite problem. With a 21-day cycle from the same start date, the next period is predicted for September 22, ovulation for September 8, and the fertile window runs September 3 – September 8. That window overlaps the period itself, which ends September 5. It is a clear example of why 'you can't get pregnant on your period' is not a safe assumption.

ACOG Committee Opinion 651, which treats the menstrual cycle as a vital sign, gives the adolescent pattern. In the early years after the first period, cycles typically run 21 to 45 days. By the third year, 60 to 80 percent of cycles are 21 to 34 days long, as is typical in adults. The calculator's 21-to-45-day input range mirrors that span. For adults, the International Federation of Gynecology and Obstetrics (FIGO, 2018 revision) describes a normal cycle frequency as 24 to 38 days.

The same ACOG opinion lists patterns worth raising with a clinician. These include periods coming more often than every 21 days or less often than every 45 days, a gap of 90 days even once, bleeding that lasts more than 7 days, and cycles that were regular and then become markedly irregular. The calculator cannot detect any of these, because it only knows the single average you type in. If your real cycle lengths fall outside the range, that is the finding to discuss, not the predicted date.

A prediction built from an average is only as good as the next cycle's resemblance to that average. Wilcox and colleagues followed 221 women trying to conceive and published the results in the BMJ in 2000. The fertile window fell entirely within cycle days 10 to 17 for only about 30 percent of the women, and on every day from day 6 to day 21 there was at least a 10 percent chance of being in the fertile window. Their conclusion was that the timing can be highly unpredictable even in regular cycles.

Methods that do use fertility signs for pregnancy prevention involve daily tracking of temperature, cervical mucus or both, usually with training. Even then, the CDC puts typical-use failure rates for fertility awareness-based methods at 2 to 23 pregnancies per 100 women in the first year. A calendar prediction from one average cycle length has none of that daily information.

If you need to avoid pregnancy, speak to a clinician or a sexual health service about contraception. If you are trying to conceive, use the dates as a starting point only, and see the related tools below.

This calculator is not a contraceptive method, a fertility test or a diagnostic tool. It applies fixed date arithmetic to one average cycle length and cannot see hormones, temperature, cervical mucus or symptoms.

Predictions are least reliable when cycles vary. That includes the first years after periods start, polycystic ovary syndrome, thyroid disorders, the months after stopping hormonal contraception, after giving birth or while breastfeeding, and perimenopause. Illness, travel, significant weight change and intense training can also shift a single cycle.

The fixed 14-day luteal phase will be wrong by several days for many people. The predicted next period date does not move forward on its own once it has passed: if the date is behind you, enter your new start date. A period that is late when you have had unprotected sex calls for a pregnancy test, not a new estimate.

If your goal is pregnancy rather than period planning, the ovulation calculator focuses on the most fertile days, and the conception calculator works out a likely conception date and fertile window around it. Charting your waking temperature adds real evidence to a calendar estimate. The basal body temperature calculator checks for the sustained rise that shows ovulation has already happened. Once a pregnancy is confirmed, the due date calculator counts forward from the same last-period start date you entered here. For a fuller walk-through of calendar, temperature, mucus and LH-test methods, see the guide to ovulation tracking and the fertile window.