Period Calculator – Next Period Date
Estimate your next period date and upcoming cycles. Enter the first day of your last period, your average cycle length, period length, and how many cycles to show.
Predict Your Next Period and Upcoming Cycles
Period Formula
Where:
- \(L\) — first day of the last period
- \(T\) — today's date
- \(C\) — average cycle length in days
- \(P\) — period length in days
- \(n\) — number of cycles advanced to reach the first upcoming predicted start
- \(i\) — upcoming cycle index, starting at 0
- \(S_i\), \(E_i\), and \(O_i\) — predicted period start, period end, and estimated ovulation date
The calculator adds whole cycle lengths to the last period's start and skips predicted starts before today. It estimates ovulation 14 days before each predicted period and the fertile window from five days before ovulation through one day after.
Period – Calculation Example
Example: On October 20, 2026, enter October 5, 2026, as the first day of your last period, a 28-day average cycle, a five-day period, and six cycles to show.
\[\begin{aligned}\text{Next start}&=\text{October 5, 2026}+28\text{ days}=\text{November 2, 2026}\\\text{Period end}&=\text{November 2}+4\text{ days}=\text{November 6, 2026}\\\text{Estimated ovulation}&=\text{November 2}-14\text{ days}=\text{October 19, 2026}\\\text{Fertile window}&=\text{October 14–20, 2026}\end{aligned}\]
Result: The next predicted period starts November 2, 2026—in 13 days—and ends November 6. The table shows six upcoming cycles, with the next starts on November 30 and December 28, 2026; and January 25, February 22, and March 22, 2027.
How to use the calculator
- Set First day of last period to the first day bleeding began, not the day it stopped. The date cannot be in the future.
- Enter Average cycle length in days. Count from the first day of one period to the first day of the next, rather than counting only the days without bleeding. The calculator accepts whole numbers from 21 to 45 days.
- Enter Period length: the number of days bleeding usually lasts, from 2 to 10 days.
- Choose Cycles to show, from 1 to 12. This controls how many upcoming cycles appear in the table, not how many months appear in the calendar.
Read Next period for the predicted start date and days until it. Upcoming cycles lists predicted start and end dates, estimated ovulation, and fertile windows. The Three-month calendar covers the current month and the next two months; it may show your most recent predicted period for context. Look at the table if a selected cycle falls beyond those three months.
What a period prediction means
A menstrual cycle begins on the first day of a period and ends the day before the next one begins. If your cycles tend to last a similar number of days, adding your average cycle length to the last start date gives a useful planning estimate. The calculator repeats that interval for each upcoming cycle. If a calculated start date has already passed, it advances to the next predicted start, so the upcoming-cycles table does not begin with a past date.
Period length determines the predicted bleeding dates. For a five-day period beginning November 2, the displayed dates are November 2 through November 6, inclusive. Period length does not change the cycle interval: your next start is calculated from one start date to another, even if bleeding lasts longer or shorter than expected.
The ovulation and fertile-window dates are rough estimates, not observations of what your body is doing. This calculator places ovulation 14 days before each predicted period and marks a window from five days before that date through one day after it. For a closer look at calendar-based estimates, see the ovulation and fertility calendar. Neither calendar can confirm that ovulation occurred.
Assumptions and practical limits
All predictions assume the same cycle length and period length repeat. Real cycles can vary from month to month, so dates farther into the future may be less useful. Stress, illness, travel, medication, breastfeeding, perimenopause, and other changes can affect timing. If your cycles vary, consider averaging several recent start-to-start intervals, and update the first-day date whenever a new period begins.
The estimated fertile window is not a reliable way to prevent pregnancy. Ovulation can shift, and a calendar prediction cannot identify days when pregnancy is impossible. If your period is late and pregnancy is possible, a pregnancy test date calculator can help you plan when to test; follow the test instructions and seek medical advice if you have concerns.
Seek medical advice if you have unusually heavy bleeding, severe pain, missed periods without a clear reason, or a substantial change in your usual pattern. This tool provides dates for planning and tracking, not a diagnosis.
Frequently Asked Questions
How do I calculate my average cycle length by hand?
Write down the first day of several consecutive periods, then count the calendar days between each pair of start dates. Add those intervals and divide by the number of intervals. For example, cycles of 27, 30, and 29 days average about 29 days. Use recent cycles that reflect your usual pattern, and update the average if that pattern changes.
What is a normal menstrual cycle length?
Adult menstrual cycles commonly range from about 21 to 35 days, while cycles in teenagers can be more variable. A cycle does not have to be exactly 28 days to be normal, and occasional differences of a few days can happen. If your cycles are consistently outside your usual range or change substantially, ask a healthcare professional for advice.
Does spotting count as the first day of my period?
Usually, count the first day of menstrual flow rather than a day of light spotting before bleeding begins. Using spotting as day one may shift every predicted date. If it is hard to tell when your period started, track how the bleeding changes over several cycles and use the same approach each time. Discuss unusual or persistent spotting with a healthcare professional.
Should I enter cycle length in days or calendar months?
Enter the number of calendar days between period starts, not the number of months. Months have different lengths, so “one month” is not a consistent cycle length. Count each date crossed, including weekends and February 29 in a leap year. There is no need to convert days into hours; daylight saving time does not change a calendar-day count.
Can I use a period prediction if I take hormonal birth control?
The dates may be less meaningful if your contraception changes or suppresses your cycle. Bleeding during a pill-free interval, for example, is often withdrawal bleeding rather than a menstrual period following ovulation. Some methods also cause irregular bleeding or no bleeding. If you are concerned about pregnancy or an unexpected change in bleeding, follow your method’s guidance and seek medical advice.
When should I take a pregnancy test if my predicted period does not arrive?
A home pregnancy test is generally most useful from the first day of a missed period, following the package instructions. If your cycle is irregular or you are unsure when your period is due, testing at least 21 days after sex that could cause pregnancy is another practical guide. If the result is negative but your period still does not arrive, test again after a few days or contact a healthcare professional.
Does ovulation always happen 14 days after a period starts?
No. “Day 14” assumes a particular cycle pattern, and actual ovulation can happen earlier or later, even when periods seem regular. The calculator uses a different calendar rule—14 days before the predicted next period—but that is still only an estimate. Do not use a predicted ovulation date or fertile window as proof that pregnancy is impossible on other days.