Menstrual Cycle Calculator

The menstrual cycle calculator shows which phase you are in right now, when your next period is due, when ovulation is expected and which days are fertile — the days when the chance of getting pregnant is highest. Enter the first day of your last period, your cycle length and your period length: the calculator works out your cycle day and phase, the dates of ovulation and the fertile window, and builds a calendar of your next four cycles. All calculations run on your device.

How to use the calculator

Fill in three fields. The first day of your last period is day one of the previous cycle — the day the bleeding started. Cycle length is the number of days from the first day of one period to the first day of the next: for most women it is 21–35 days, with 28 the average. Period length is how many days the bleeding usually lasts, typically 3–7. You will see: your current phase and cycle day, the date of your next period, the expected ovulation date, the fertile window and a list of the next four cycles with dates. If your cycle floats, count by the average length of your last 3–6 cycles.

Phases of the menstrual cycle

The cycle is divided into four phases, and each has its own processes:

• Menstruation — the first days of the cycle: the lining of the uterus is shed. Usually 3–7 days.
• Follicular phase — after the period ends the ovaries prepare a new egg, and the uterine lining rebuilds. Lasts until ovulation.
• Ovulation — a mature egg leaves the follicle. The chance of getting pregnant is highest around this day and the day after.
• Luteal phase — from ovulation to the next period, usually a stable 14 days. If pregnancy does not occur, a new cycle begins.

When ovulation happens

For most women the luteal phase lasts about 14 days regardless of cycle length, so ovulation is counted from the end of the cycle: ovulation day = cycle length − 14. With the classic 28-day cycle that is day 14, with a 35-day cycle day 21. The fertile window is wider than ovulation itself: sperm can fertilize for up to 5 days, and the egg lives for about a day, so the chance of getting pregnant is high from 5 days before ovulation to 1 day after — 7 days in total. That is the range the calculator shows.

What affects cycle regularity

The cycle is sensitive to almost everything: stress and poor sleep, sudden weight change, intense workouts, travel and time-zone shifts, hormonal contraception, breastfeeding, and age — teenage and pre-menopause cycles are often irregular. A one-off irregular cycle or two is usually a normal reaction of the body. But if your cycle is constantly shorter than 21 or longer than 35 days, bleeding lasts more than 7–8 days, the pain is severe, or your period is missing for more than 3 months (and you are not pregnant), it is a reason to see a gynecologist.

Limitations

The calculation uses an averaged model: ovulation exactly 14 days before the next period, and a cycle of stable length. In real life ovulation can shift by several days either way even with a regular cycle, so the dates are approximate. The calendar method is an unreliable way to prevent pregnancy: its typical effectiveness is about 75–80%, meaning up to a quarter of women relying on the calendar get pregnant within a year. For reliable contraception use methods recommended by a doctor.

The calculation is for information only and is not a medical consultation.