Period Calculator
Enter the first day of your last period and your usual cycle length. We'll show the predicted date of your next period, the next 3 cycles, and the fertile window for the upcoming cycle.
When to test, when to call a clinician
If pregnancy is possible: NHS advises that a home pregnancy test can usually be taken from the first day of a missed period. If you do not know when your period was due, test at least 21 days after unprotected sex. NHS source.
Talk to a clinician if: your adult cycles are consistently shorter than 21 days or longer than 35 days; you have had no periods for 3 months or more and pregnancy has been ruled out (ACOG secondary amenorrhea); bleeding lasts longer than 7 days or is unusually heavy or painful; or the pattern changes suddenly without a clear reason. ACOG AUB and ACOG amenorrhea.
This is a planning aid, not medical advice, not contraception, and not a pregnancy test.
How we predict your next period
Period prediction is simple arithmetic on top of a stable biological anchor. Your cycle is the span between the first day of one period and the first day of the next. If that cycle length holds from month to month, the next period will fall on the first day of your last period plus your cycle length. We also surface the expected ovulation day and the 6-day fertile window for the coming cycle, which are useful whether you are trying to conceive, trying to avoid pregnancy, or planning around PMS and travel. For the fertility-focused view, open the Ovulation Calculator.
Calendar-based prediction is most accurate when cycles are regular within a few days between months. If cycles vary more, the predicted date becomes a planning guide rather than a fixed signal. ACOG describes the typical adult cycle as 21 to 35 days, with a broader 21 to 45 day range appropriate in the years after menarche. This calculator accepts 21 to 45 days to cover both contexts.
Read more: Due Date vs Conception Date.
Frequently asked questions
When will my next period come?
Your next period is predicted as the first day of your last period plus your cycle length. A standard 28-day cycle puts the next period 28 days after the last one started. A 32-day cycle shifts it to day 32. If your cycles are regular within a few days each month, the calculator's predicted date will typically fall within that same window.
What counts as a normal cycle length?
ACOG describes the typical adult menstrual cycle as 21 to 35 days from the first day of one period to the first day of the next (ACOG Abnormal Uterine Bleeding FAQ). The broader 21 to 45 day range appears in ACOG's Your First Period FAQ for the years after menarche, when cycles often have not settled yet. This calculator accepts 21 to 45 days so it covers both contexts. For adults on a natural cycle, consistently shorter than 21 days or longer than 35 days is worth discussing with a clinician.
How late is too late for a period?
A period can be a few days earlier or later than expected without reflecting a problem, especially if your cycles vary by several days between months. NHS notes that common reasons for a missed or late period include pregnancy, stress, sudden weight loss or being overweight, excessive exercise, PCOS, perimenopause, hormonal contraception, breastfeeding, and some medical conditions such as thyroid disease. If pregnancy is possible, NHS guidance is that a home pregnancy test can usually be taken from the first day of a missed period. If you do not know when your period is due, NHS advises testing at least 21 days after unprotected sex, and retesting if the result is negative and bleeding does not come.
Can my cycle length change over time?
Yes, cycle length can shift with age, lifestyle, hormonal contraception changes, and during perimenopause. Cycles are often longer and more variable in the first few years after the first period and again in the years leading up to menopause. Once you notice a consistent shift in cycle length, update the calculator input so future predictions reflect the new pattern.
Does hormonal contraception change when my period comes?
Yes. Combined pills, progestin-only pills, hormonal IUDs, implants, injections, and rings all alter natural ovulation and the timing and character of any bleeding you experience. Bleeding on a placebo week of a combined pill is a withdrawal bleed, not a natural period, and its timing is set by the pill schedule rather than by a natural cycle. This calculator is built for natural cycles, so predictions are not reliable on hormonal contraception.
Should I see a doctor about irregular periods?
Consider a clinician visit if your adult cycles are consistently shorter than 21 days or longer than 35 days, if you have had no periods for 3 months or more with pregnancy ruled out (how ACOG defines secondary amenorrhea), if bleeding lasts longer than 7 days or is unusually heavy or painful, or if the pattern changes suddenly without a clear reason like recent contraceptive changes. Irregular cycles alone are not automatically a sign of a problem, but persistent or sudden changes are worth reviewing with a professional.