Period Calculator — Predict Your Next Period, Ovulation Date & Safe Days Free

Use this free period calculator to instantly predict your next period date, ovulation day, and fertile window — no app download or account needed. Whether you have a regular 28-day menstrual cycle or an irregular cycle that changes every month, enter your last menstrual period (LMP) date and average cycle length to get a personalized 6-month cycle forecast in seconds.

This calculator provides estimates based on average cycle data and is intended for general planning purposes only. It is not a substitute for medical advice. If you have concerns about your menstrual health, consult a qualified healthcare provider.

Last updated: April 2026 | Reviewed for medical accuracy

How to Use This Period Calculator — 3 Steps

Enter the first day of your last period, set your average cycle length (21 to 45 days), and choose your goal — either “Just Tracking” or “Trying to Conceive.” Your next period date, ovulation day, and fertile window appear instantly. No account, no app, no data stored.

If your cycle varies month to month, add your last 3 cycle lengths together and divide by 3. Use that average as your input for the most accurate predictions.

How Your Menstrual Cycle Works — The 4 Phases

our period calculator predicts based on four biological phases that repeat every cycle.

 

Menstrual Phase (Days 1–5)

Day 1 is the first day of full bleeding — not spotting. The uterine lining sheds because the egg was not fertilized. Estrogen and progesterone are at their lowest. This phase lasts 2 to 7 days for most women.

Follicular Phase (Days 1–13)

Follicle-stimulating hormone (FSH) signals your ovaries to develop follicles. Estrogen rises, rebuilding the uterine lining. Energy and mood typically improve during this phase.

Ovulation (Around Day 14)

A surge in luteinizing hormone (LH) triggers the release of a mature egg. The egg survives only 12 to 24 hours after release. For a 28-day cycle, this happens around Day 14 — but only 13% of women ovulate exactly on Day 14. Cycles shorter than 28 days mean earlier ovulation; longer cycles mean later.

Luteal Phase (Days 15–28)

After ovulation, progesterone rises to prepare for possible pregnancy. If the egg is not fertilized, progesterone drops after 14 days and your period begins. PMS symptoms — bloating, irritability, fatigue — typically appear in the last 5 days of this phase as progesterone falls.

Ovulation Calculator — Your Fertile Window Explained

Your fertile window is 6 days long: the 5 days before ovulation and the day of ovulation itself. Sperm survive inside the body for up to 5 days, while the egg lasts only 12 to 24 hours — so the days before ovulation are often more likely to lead to conception than the day of ovulation itself.

The highest chance of conception is in the 2 days immediately before ovulation and ovulation day itself, with pregnancy rates of approximately 27–33% per cycle with timed intercourse on each of those days.

To confirm ovulation rather than just predict it, watch for these physical signs alongside your calculator results: cervical mucus becoming clear, slippery, and stretchy (like raw egg whites); a basal body temperature (BBT) rise of 0.2°C or more the morning after ovulation; or a positive result on an ovulation predictor kit (OPK), which detects the LH surge 24 to 48 hours before the egg releases.

Safe Period Calculator — When Are You Least Likely to Get Pregnant?

The safe period refers to days in your cycle when the conditions for pregnancy are not biologically met — before ovulation occurs or after the egg has passed. These are days when unprotected sex carries the lowest conception risk.

For a 28-day cycle: safe days are approximately Days 1–7 and Days 20–28. Fertile window: Days 9–15.

For a 30-day cycle: safe days are approximately Days 1–7 and Days 22–30. Fertile window: Days 11–17.

For a 35-day cycle: safe days are approximately Days 1–7 and Days 27–35. Fertile window: Days 16–22.

No calendar method is fully reliable for contraception. Ovulation can shift by 3 to 5 days due to stress, illness, or hormonal changes. Use these estimates for awareness and planning — not as a replacement for medical contraception.

Late Period Calculator — How Many Days Late Is Normal?

A period arriving 1 to 5 days after the predicted date is normal variation. Your period is medically late when it has not arrived 7 or more days past the expected date.

Common reasons for a late period when not pregnant:

1. High stress or anxiety — cortisol elevation delays ovulation
2. Significant weight change (gain or loss of 10% or more)
3. Thyroid dysfunction — both over and underactive thyroid affect cycle timing
4. Polycystic ovary syndrome (PCOS) — the most common cause of irregular cycles
5. Recent illness with fever — temporarily suppresses ovulation
6. Stopping hormonal birth control — can delay cycle return for 1 to 3 months
7. Perimenopause — can begin as early as the late 30s
8. Excessive exercise or under-eating — can suppress ovulation entirely

If your period is 7 or more days late and you have had unprotected sex, take a home pregnancy test. If it is 14+ days late with a negative test, consult a doctor.

 

 FAQs

 

How do I calculate my next period date?

Add your average cycle length to the first day of your last period. If your period started April 1 with a 28-day cycle, your next period is expected April 29. The calculator above does this automatically for 6 months at once.

Why is my period late if I am not pregnant?

The most common reasons are stress, significant weight change, thyroid dysfunction, PCOS, recent illness, stopping birth control, perimenopause, or excessive exercise. If your period is 14+ days late with a negative pregnancy test, see a doctor.

 

Can I get pregnant 2 days after my period ends?

Yes, particularly if your cycle is short (21–24 days). With a 21-day cycle, ovulation can occur around Day 7 — just 2 days after a 5-day period. Since sperm survive up to 5 days, pregnancy is biologically possible.

 

What is the safe period for a 30-day cycle?

For a 30-day cycle, the lowest-risk days are approximately Days 1–7 and Days 22–30. The fertile window spans Days 11–17 with peak fertility around Days 15–16.

 

Is a 21-day cycle normal?

Yes. Healthy menstrual cycles range from 21 to 45 days. Only 12–15% of women have exactly 28-day cycles. Consistency within your own pattern matters more than matching the average.

 

What is the 7-2-1 rule for periods?

The 7-2-1 rule is a menstrual health guideline: change your tampon or pad at least every 7 hours to reduce the risk of toxic shock syndrome (TSS); most healthy periods last between 2 and 7 days; and if you are soaking through period protection every 1 hour for 2 or more consecutive hours, seek same-day medical attention as this indicates heavy menstrual bleeding (menorrhagia) that needs evaluation.

Your age can also influence your cycle, so you can use our Chronological Age Calculator to find your exact age in years, months, and days.