Wondering when your next period will show up? This period calculator predicts your next period start date from the first day of your last period and your average cycle length. It also shows your current cycle day, the estimated end of your current bleeding, and an approximate fertile window. It is built for planning your calendar — packing, travel, sports, work, and daily comfort — rather than for conception planning.
A typical menstrual cycle is counted from cycle day 1 (the first day of bleeding) all the way to the day before the next period starts. Although 28 days is the number most people quote, only about 10% of cycles are exactly 28 days. A cycle between 21 and 35 days is still considered normal, and bleeding commonly lasts 3 to 7 days. Your own average matters more than the textbook number, so tracking 3 to 6 cycles before trusting a prediction is the most reliable habit.
How to Use
- Enter the First Day of Last Period — the calendar date your most recent bleeding actually began. If you are on day 3 of bleeding today, pick today's date, not yesterday's.
- Set your Average Cycle Length. Count from day 1 of one period to day 1 of the next over your last 3 to 6 cycles, then type the average. If you are unsure, leave the 28-day default and adjust later as your own log reveals.
- Type your Typical Period Length — how many days your bleeding usually lasts, from the first day of flow to the last day you need protection.
- Read the result card: the highlighted Estimated Next Period is the headline date. Below it, the calculator shows your current cycle day, the estimated end of the current bleeding, and an approximate ovulation day and fertile window for reference.
- Treat the output as an estimate, not a guarantee. Cycles vary month to month, especially in the first few years after your first period, in perimenopause, around stress or travel, and with conditions such as PCOS or thyroid changes.
The engine simply adds days: next period start equals last period start plus your cycle length; estimated period end equals last period start plus bleeding length minus one day; ovulation is estimated as roughly 14 days before the next period. The fertile window shown is about five days before ovulation through ovulation day itself, because sperm can survive up to five days in the body while a released egg typically lives only about 24 hours.
Examples
Example 1 — Average 28-day cycle, 5-day bleed. Your last period started on January 1 and usually bleeds for 5 days. The calculator estimates your bleeding ends January 5, your next period starts January 29, your ovulation day is around January 15, and your fertile window runs January 10 through January 15.
Example 2 — Shorter 24-day cycle, 4-day bleed. If your last period began February 10 and your cycle averages 24 days, bleeding ends February 13, your next period arrives March 6, and ovulation falls around February 20. Cycles under 25 days are less common but still normal for roughly 15% of people who menstruate.
Example 3 — Longer 35-day cycle, 7-day bleed. With a March 5 start and a 35-day cycle, bleeding ends March 11, your next period is due April 9, and ovulation is estimated around March 26. Because ovulation shifts later in long cycles, the fertile window shifts later with it.
Example 4 — A 30-day cycle logged from an April 15 start. Bleeding typically lasts 6 days, so the current flow is expected to end April 20. Ovulation lands on May 1 (April 15 plus 16 days), the fertile window runs April 26 through May 1, and the next period is estimated for May 15. Move the cycle-length slider up or down by a few days and watch every date shift together.
Why Cycles Are Never Perfectly Even
Even when you feel regular, most cycles vary by a few days month to month. Variation under about 7 days across six months is typical. Cycle length commonly changes with stress, significant weight changes, intense training, travel across time zones, illness, and hormonal contraception. Adolescents and people in perimenopause often see wider swings — it can take several years for a teen's cycle to settle, and it may become irregular again years before periods stop.
Tracking on paper or in an app beats guessing, because your personal average is what this calculator relies on. Logging the start date every month for three cycles lets you replace the 28-day default with your real number, and that one change usually improves the predicted date by several days.
How This Differs From an Ovulation or Conception Calculator
This page predicts when your next period will start. An ovulation calculator and a conception calculator are built around one question — when is a person most likely to conceive — and are tuned for that purpose. Here, ovulation and fertile window appear only as reference information alongside the period date, so you can plan travel, events, and supplies without mixing up two different goals. If your aim is becoming pregnant, use a dedicated ovulation or conception tool and, ideally, discuss timing with a clinician.
FAQ
How does this calculator predict my next period?
It adds your average cycle length to the first day of your last period. If your last period started January 1 and your cycle averages 28 days, the next period is estimated for January 29. It is a calendar estimate, not a measurement — the actual date depends on when you ovulate that month.
What is a normal cycle length?
Between 21 and 35 days is considered normal for most adults who menstruate. The textbook average is 28 days, but only about 10% of cycles land exactly there. Bleeding lasting 3 to 7 days is typical. Cycles shorter than 21 days, longer than 35 days, or that change by more than 7–9 months month to month are worth discussing with a clinician.
Why is my predicted date different from last month's?
Because your cycle length is an average. If you entered 28 days but your true cycles are 26 and 30, the estimate splits the difference. Over several months of consistent logging, the predicted date will usually land within a few days of the real start.
How is the ovulation date calculated?
Ovulation is estimated as roughly 14 days before your next period: last period start plus (cycle length minus 14). With a 28-day cycle that lands on day 14; with a 32-day cycle it lands on day 18. This is an approximation — ovulation predictor kits and temperature tracking are more precise if you need them.
What does "fertile window" mean on this page?
It is the span when pregnancy is biologically possible, shown for reference only: about five days before estimated ovulation through ovulation day itself, because sperm can survive up to five days in the reproductive tract while a released egg typically lives only about 24 hours. This calculator is not a conception planner; for that goal use a dedicated ovulation or conception tool.
My period is late — should I trust this calculator?
No. A late period can happen for many reasons — stress, illness, weight change, travel, thyroid issues, PCOS, hormonal contraception, or pregnancy. If your period is more than 7 days late from the predicted date, take a home pregnancy test and, if irregularity persists, talk with a healthcare provider rather than relying on calendar math.
Disclaimer
This period calculator is provided for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Estimated dates are approximations based on the cycle length and dates you enter; individual cycles vary and the tool is not a substitute for professional care. Always consult a qualified healthcare provider about menstrual irregularity, missed periods, severe pain, heavy bleeding, or any health concern. For our editorial standards see the About page, and for data handling see the Privacy Policy; we do not store your cycle dates on this page.