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Period calculator

Enter the first day of your last period and your cycle length to see when your next period is due, your current cycle phase, and the next six months of dates.

This is not medical advice

Estimate only. Not medical advice. Check with a doctor or pharmacist before acting on it. Read the full note

These calculators use published population formulas. They do not know your medical history and cannot diagnose, treat or replace a doctor, pharmacist or dietitian. Do not change medication, dosing, eating or drinking decisions on the basis of a number here. If you are pregnant, under 18, over 65, have a health condition or take medication, ask a clinician first. In an emergency call your local emergency number.

days

From the first day of one period to the first day of the next.

days

Where you are in this cycle

show

Next six periods

show
Period startsBleeding daysFertile window

A period calculator projects your cycle forward from one known date. It cannot see hormones or stress, so it is a calendar estimate, not a diagnosis, but for people with fairly regular cycles it is accurate to within a day or two and is useful for planning travel, sport, or when to take a pregnancy test.

How it is calculated

  1. Your next period starts one cycle length after the first day of your last period. A 28-day cycle starting on the 1st gives a next period on the 29th.
  2. Ovulation is estimated at 14 days before the next period. The second half of the cycle (the luteal phase) is much more constant than the first, which is why counting back from the next period works better than counting forward from the last one.
  3. The fertile window is the five days before ovulation plus ovulation day: sperm survive up to five days, the egg about one.
  4. Six future cycles are laid out by repeating the same cycle length.

Example

Last period started March 1, cycle 30 days, period 5 days. Next period: March 31. Ovulation: March 17 (31 − 14). Fertile window: March 12–17. PMS symptoms, if you get them, typically begin around a week before the period, so from about March 24.

How much cycles vary

Studies of app data show the average cycle is about 29 days, with a normal range of 21 to 35 in adults. Even regular cycles vary by 2–3 days month to month, and cycles are naturally longer and less predictable in the first years after the first period and in the years before menopause. Tracking three to six cycles and entering your own average gives better predictions than the 28-day default.

Frequently asked questions

How do I calculate my next period?

Add your cycle length to the first day of your last period. With a 28-day cycle and a period that started on the 3rd, the next one is due on the 31st. The calculator above does this and projects six cycles ahead.

What is a normal cycle length?

Anything from 21 to 35 days in adults, with an average of about 28–29. Teenagers often have longer, less regular cycles for a few years.

How late is too late for a period?

A period is considered late after about a week past the expected date. If pregnancy is possible, a test from that point is reliable. If you miss three periods in a row and are not pregnant, see a doctor.

Can I get pregnant on my period?

It is unlikely but possible, especially with short cycles. Sperm can live five days, so intercourse at the end of a period can overlap with an early fertile window.

Why is my period early or late?

Stress, travel, illness, intense training, weight change, new medication and hormonal contraception all shift ovulation, and the period follows about two weeks later. One irregular cycle is normal; a pattern is worth discussing with a doctor.

Sources

  1. Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digital Medicine, 2019
  2. ACOG Committee Opinion: Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign
  3. Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. NEJM, 1995

By Calcelate Team. Formula from the sources above.

  1. 2026-09-14 · Formula and texts checked