Predict your upcoming menstrual period dates, along with your ovulation and fertile window for each cycle over the next several months.
A menstrual cycle begins on the first day of your period and ends the day before your next period starts. The calculator predicts future periods by adding your average cycle length (typically 21–35 days) repeatedly to your last period date.
Most cycles range from 21 to 35 days, with an average of 28 days. Period duration typically lasts 2–7 days. Cycle length can vary month to month due to stress, illness, diet, exercise, and hormonal changes.
Note: This calculator assumes a regular cycle. If your cycle is irregular, predictions may be less accurate. Consult a healthcare provider for irregular cycles.
You have a week away starting December 5 and your last period began on November 3. Whether the next one lands before, during or after the trip comes down to date arithmetic, and this page does it for 6 to 24 cycles at a time. Enter the first day of your last period, your usual cycle length and how many days you bleed, and it prints a table of start dates, end dates, fertile windows and ovulation days.
Each predicted start date is your last period date + n × cycle length, where n is 0 for the date you entered, 1 for the next period and so on. The end date is the start plus period length minus 1, because the first day of bleeding counts as day one. Ovulation is estimated at start + (cycle length − 14) days, and the fertile window column runs from five days before that to ovulation day itself.
Worked example. The last period began Tuesday, November 3, 2026, with a 28-day cycle and a 5-day period. The next start is November 3 + 28 = Tuesday, December 1, 2026, and the end is December 1 + 4 = Saturday, December 5, which is the first day of the trip. Ovulation for that cycle is December 1 + 14 = Tuesday, December 15, with a fertile window of December 10 to December 15. The rows after that start on December 29 and January 26.
What it does not account for. Every future date assumes the same cycle length forever. Being two days off per cycle is common, for example a true 30-day cycle entered as 28, and the error stacks: after six cycles the prediction is 12 days out. The page also ignores pregnancy, hormonal contraception, breastfeeding, stress, illness, travel and perimenopause, and it cannot tell a late period from a missed one. ACOG describes typical adult cycles as 21 to 35 days, so a cycle that regularly falls outside that range, or a sudden change in your pattern, is worth raising with a doctor.
Cycles rarely repeat to the day. Stress, illness, travel, sleep changes and heavy exercise can all move ovulation, and the first half of the cycle absorbs most of that. A swing of two or three days either way is ordinary.
Use the average of your last three to six cycles. One unusually short or long cycle would skew every date that comes after it.
With a spread that wide, no single number will be reliable and the long table will drift quickly. Use a shorter list, re-enter your last-period date after every real period, and consider mentioning the pattern to a doctor.
The selector stops at 9. Bleeding that runs past about seven days, or that is very heavy, is something ACOG suggests discussing with a clinician rather than something to enter here.
No. They are estimates built on a fixed 14-day luteal phase, meant for education only, and this is not medical advice. If you want to prevent pregnancy, speak with a doctor or nurse about methods that do not depend on predicting ovulation.