Period Calculator

Take the first day of your last period and add your average cycle length in days. For example, if your last period started on September 6 and your cycle is 28 days, your next period is expected around October 4. This calculator does the math for you and projects six periods ahead. It is an educational estimate, not medical advice.

Count from the first day of one period to the first day of the next period — that number of days is your cycle length. Track 3 to 6 cycles and take the average for the most reliable number. Enter those past cycle lengths into the cycle-history section of this calculator and it will compute your average, range, and regularity rating automatically.

For adults, a cycle of 21 to 35 days is generally considered normal, with 28 days the often-quoted average. Regular cycles of 25 or 32 days are equally healthy. What matters most is your own pattern: cycles that arrive at roughly the same interval each time are a good sign. This is general information, not medical advice.

Most periods last 2 to 8 days, with an average of about 5 days. Bleeding that consistently lasts longer than 7 days, or cycles that change suddenly, are worth discussing with a healthcare provider. This calculator lets you set your own period duration so predictions match your pattern.

Yes, but treat the result as a range rather than an exact date. Enter your last 3 to 6 cycle lengths and this calculator will show your average, your shortest and longest cycles, and a prediction window based on how much you vary. If your cycles regularly vary by more than about 9 days or fall outside 21 to 35 days, talk to a healthcare provider — a calculator cannot diagnose the cause.

For regular cycles, calendar predictions usually land within 1 to 3 days of the actual start. Accuracy drops as variation grows: if your cycle length changes by more than 7 days from month to month, predictions become rough estimates. Stress, illness, travel, intense exercise, and birth control changes can all shift a cycle, so no calculator can guarantee a date.

Cycles are generally considered irregular when their length varies by more than 7 to 9 days from month to month, or when they consistently fall outside the 21 to 35 day range. Missing three or more periods in a row (when pregnancy is ruled out) is called secondary amenorrhea and deserves a medical visit. This is educational information, not a diagnosis.

PMS symptoms commonly begin 5 to 10 days before a period, in the late luteal phase, and usually ease once bleeding starts. This calculator estimates a 7-day PMS window before each predicted period so you can plan ahead. If symptoms are severe enough to disrupt daily life, discuss it with a healthcare provider.

No. Day 1 of your cycle is the first day of full menstrual flow, not light spotting beforehand. Counting spotting as day 1 shortens your recorded cycle length and makes predictions less accurate. Enter only the first day of real bleeding into this calculator.

This calculator uses the calendar (luteal-phase) method: ovulation is estimated as 14 days before your next predicted period, because the luteal phase is the most stable part of the cycle at roughly 12 to 14 days. It is a rough estimate, not a precise fertility reading. For a full fertile-window breakdown, see our ovulation calculator.

A period calculator predicts when your next bleeding will start by counting forward from your last period using your cycle length. An ovulation calculator works backward from your predicted period to estimate your fertile window and ovulation day for family planning. They answer different questions, so we built them as separate tools that link to each other.

A missed period can be an early sign of pregnancy, but stress, weight changes, illness, travel, exercise changes, and hormonal birth control can also delay a period. This calculator only predicts dates from past patterns; it cannot detect pregnancy. If pregnancy is possible, take a test and talk to a healthcare provider.

Mark the first day of each period on any paper or digital calendar, then count the days between first days to find your cycle length. After 3 to 6 months you can average the lengths and predict future periods by hand. This web calculator works the same way with no download, no account, and no data leaving your browser.

Yes. Cycles are often irregular in the first 1 to 2 years after periods begin and again during perimenopause in the 40s, when cycles may shorten, lengthen, or skip. Between those stages most people settle into a steadier personal pattern. A sudden change at any age is worth mentioning to a healthcare provider.

Yes. This calculator runs 100% in your browser — the dates and cycle lengths you enter are never uploaded, stored on a server, or shared with anyone. There is no account, no analytics on your inputs, and the page works offline once loaded.

Consider seeing a healthcare provider if you miss 3 or more periods (and are not pregnant), bleed for longer than 7 days, have cycles consistently shorter than 21 or longer than 35 days, experience severe pain, or notice a sudden major change in your pattern. This calculator is an educational estimate and never replaces professional medical advice.