Track your menstrual cycle and predict your next period — get a multi-month calendar, optional fertile window, and insights on irregular cycles, late periods, and menstrual health.
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The Period Calculator is built for anyone who menstruates and wants to track their cycle — whether they're avoiding pregnancy, planning one, managing symptoms, or simply monitoring their menstrual health. It accepts your last period start date and your average cycle length (the default is 28 days; adjust to your actual average for better accuracy). The tool outputs your next expected period date, the estimated duration of your period (based on an entered or default number of days), and the pattern over the coming three to six months. It can also show your approximate fertile window for each cycle if you opt into that view, making it useful as a combined period-and-fertility tracker. Cycle lengths from 21 to 40 days are considered within the normal range by ACOG, and the tool flags cycles outside that range as worth discussing with a healthcare provider.
Enter the first day of your last period, your average cycle length, and the typical number of days your period lasts — then tap calculate. The result shows your next expected period start date, a projected calendar for the next several cycles, and optionally your estimated fertile window for each month. A period calendar that shows dates three to six months out is most useful for planning around events, travel, or athletic competitions. If your cycles vary, enter your approximate average and use the results as a rough guide rather than a precise countdown; natural cycle variation of two to three days in either direction is normal and doesn't signal a problem.
The Period Calculator uses a straightforward forward-projection formula:
Next Period Date = First Day of Last Period + Cycle Length
Subsequent Period Dates = Previous Period Date + Cycle Length (repeated)
For a 28-day cycle starting June 1: next period June 29, then July 27, then August 24, and so on. ACOG defines a normal cycle as lasting 21 to 38 days, measured from the first day of one period to the first day of the next. Cycle length, not period duration, is the primary variable in this formula.
This is one of the most searched menstrual health questions — and the answer is wider than the "28-day average" most people learned in school. ACOG confirms that a normal cycle ranges from 21 to 38 days, and the average shifts over a lifetime. Cycles tend to be longer and less predictable in the first few years after menstruation begins, gradually shortening toward a more regular rhythm in the mid-to-late 20s and early 30s, then sometimes shortening again in the late 30s and 40s as perimenopause approaches. Period duration — how long bleeding lasts — typically runs two to seven days. Flow that requires changing a pad or tampon more than once per hour for several consecutive hours, or that lasts more than seven days, is worth mentioning to a provider. The "normal" range is broader than most people realize, which is why the calculator lets you enter your actual cycle length rather than defaulting to 28.
"Irregular periods" gets searched constantly, often with anxiety attached — but the clinical definition is specific. A cycle is considered irregular when its length varies by more than seven to nine days from one cycle to the next over several months. An occasional one-or-two-day shift is just biological noise. True cycle irregularity can signal a range of things: PCOS (the most common hormonal cause of irregular cycles), thyroid dysfunction, hyperprolactinemia, perimenopause, significant stress or weight changes, or excessive exercise. ACOG recommends seeing a gynecologist if cycles are consistently shorter than 21 days or longer than 38 days, if periods are absent for three or more months (outside of pregnancy), or if cycle length varies dramatically from month to month. For irregular cycles, the Period Calculator is most useful when you enter your approximate average length and treat the result as a rough range rather than a firm date.
A late period is one of the most anxiety-inducing events for anyone who menstruates, but pregnancy is far from the only cause. Stress — including academic stress, relationship stress, and job changes — can delay the hypothalamic signal that triggers ovulation, pushing the whole cycle back. Significant weight loss or gain, intense new exercise programs, and very low body fat can all disrupt the hormonal cascade that produces a regular period. Illness, travel (especially across time zones), and even disrupted sleep schedules can temporarily shift cycle timing. Hormonal birth control, especially during the first few months after starting or stopping, frequently causes unpredictable cycles. And the beginning of perimenopause — which can start years before the final period — often shows up first as irregular or skipped cycles. If your period is more than two weeks late and a pregnancy test is negative, and you haven't recently changed medications or stress levels significantly, it's worth a call to your OB-GYN.
A period calculator is only as accurate as the data you feed it. The single most effective improvement is tracking more than just your start date — note your cycle length each month, your period duration, any mid-cycle spotting (which may indicate ovulation), and PMS symptoms in the days before your period. After three to four months of consistent tracking, you'll have an accurate average cycle length that makes the calculator's predictions much tighter. Phone apps can help with logging, but they're ultimately running the same cycle-length formula this calculator uses — their accuracy also depends on consistent data entry. Some people use BBT (basal body temperature) charting alongside cycle tracking for a clearer picture of ovulation timing; the temperature shift that follows ovulation each month becomes a reliable confirmation point that also retroactively validates the cycle-length calculation.
A negative pregnancy test and a missed or very late period leaves many people confused about next steps. The medical term for an absent period is amenorrhea. Primary amenorrhea refers to never having had a period by age 15–16; secondary amenorrhea refers to periods stopping for three or more months in someone who has previously menstruated. ACOG guidelines suggest evaluation if periods are absent for three consecutive months — even if pregnancy has been ruled out — to identify hormonal, structural, or nutritional causes. Common workup includes a pregnancy test (to definitively rule it out), thyroid panel, prolactin level, and FSH/LH measurement. Missing periods isn't just a fertility concern: estrogen, which depends on regular ovulation, is critical for bone density, cardiovascular health, and cognitive function. An occasional late period is not alarming; a pattern of missing periods deserves attention.
Your cycle length and the date your last period began are the two required inputs, and cycle length has the most influence on accuracy. Period duration (flow days) affects how the calendar is displayed but not the prediction of cycle timing. Factors that shift cycle length in real life include hormonal contraceptive use or discontinuation, significant weight changes, stress, illness, intense exercise (especially in competitive athletes), breastfeeding, and perimenopause. Age matters: teens often have longer, less regular cycles in the first two to three years of menstruation, while cycles tend to shorten as perimenopause approaches. Certain medications — antidepressants, antipsychotics, blood pressure medications — can affect cycle regularity. Tracking real cycle lengths rather than assuming a 28-day average is the most effective way to improve the calculator's accuracy.
Priya has a 32-day cycle. Her last period started May 3. The calculator predicts her next period on June 4, then July 6, then August 7. She has a beach vacation booked for June 8 — the calculator's heads-up gives her two weeks to plan accordingly and brings peace of mind instead of a surprise.
Rachel has noticed her cycles have been shorter recently — averaging 26 days after years at 30 days. Her last period started May 10. The calculator now predicts June 5 instead of the June 9 she'd expect on 30 days. She notes the shift over several months and mentions it at her next gynecology appointment, where her provider discusses early perimenopause and orders an FSH test — all prompted by a cycle-tracking pattern the calculator helped her notice.
Always enter the first day of actual bleeding (not spotting) as your period start date — this is the standard medical definition of day 1.
Track your real cycle length for at least three months before trusting the calculator's predictions; entering the default 28 days when your cycle is actually 33 days creates a week of error.
Use the multi-month calendar view to plan around events — knowing your period window for the next three months lets you schedule around it instead of being surprised.
If your cycles vary by more than five days regularly, note both your shortest and longest recent cycles and treat the prediction as a range.
Don't use the period calculator as a birth control method — cycle-length variation makes fertile-window prediction unreliable for contraception purposes without additional tools.
Track symptoms alongside dates (cramps, mood shifts, spotting) to build a full cycle picture for provider conversations.
Add your cycle length to the first day of your last period. For a 28-day cycle starting May 1, your next period is expected May 29. For a 30-day cycle, it's May 31. This calculator does the projection automatically for the next several months.
Count from the first day of one period to the first day of the next period — that's your cycle length. Do this for three to four consecutive months and average the numbers for your most reliable baseline. ACOG's normal range is 21 to 38 days.
The calculator predicts based on your stated average cycle length. If your cycle varies — due to stress, illness, weight changes, or hormonal fluctuations — the actual period date will differ from the prediction. Update your cycle-length input whenever you notice a consistent shift.
It's a rough guide. Use your approximate average cycle length and treat the result as a ±3–5 day window. Tracking real cycle lengths for several months and updating the input regularly improves accuracy over time.
A period that arrives a few days later than predicted is usually normal variation. If it's more than a week late, take a pregnancy test. If the test is negative and the period is more than two weeks late, consider other causes — stress, weight changes, thyroid issues — and contact your provider if the pattern persists for three or more months.
ACOG considers two to seven days of bleeding normal. Periods lasting more than seven days, or requiring pad or tampon changes more than once per hour for several consecutive hours, are worth discussing with a provider.
Yes, if you enable the fertile window option. The calculator estimates your ovulation date (~14 days before your next period) and highlights the six-day window around it. For a dedicated fertility view, the Ovulation Calculator gives a fuller picture.
Use the average of your last three to six cycle lengths. If they ranged from 26 to 34 days, enter 30 as your average. For cycles that vary widely, run the calculator at both your shortest and longest recent lengths to see the full possible range for your next period.
Brief disclaimer: This calculator provides educational period and cycle predictions based on your entered dates and average cycle length. Results are for informational and planning purposes only. This tool is not a contraceptive method and should not be used to prevent pregnancy. Cycle predictions carry inherent uncertainty — individual cycles can vary by several days even in the most regular patterns. Consult your OB-GYN if you have concerns about cycle regularity, missed periods, abnormal bleeding, or any other menstrual health questions.