Ovulation Calculator Online — Fertile Window
Calculate your ovulation day and 6-day fertile window from your last period date and cycle length. View 1 or 3 cycles ahead. Free, no signup.
The start date of your most recent menstruation — the anchor for all calculations
Days from the first day of one period to the first day of the next (typically 28)
How many days your period usually lasts (used to color the calendar)
Cycle calendar
Cycle 1 · May 2026
Ovulation: May 27, 2026 · Fertile window: May 22 – May 28 · Next period: ~June 10, 2026
Ovulation day
May 27, 2026
Best chances: May 26 – May 28
Fertile window
May 22 – May 28
7 days with the highest chance of conception
Next period
June 10, 2026
28 days after the start of this cycle
Luteal phase
14 days
From ovulation to the next period — usually stable across cycles
Planning a pregnancy?
If conception has already happened, calculate your due date and current pregnancy week.
Pregnancy & Due Date CalculatorThis calculator uses the standard 14-day luteal phase model and only works for regular cycles. It is not a contraceptive method and does not replace medical advice.
How to calculate your ovulation day
Three simple steps — you get a fertile window and a full cycle calendar.
Enter your period date
Provide the first day of your last menstruation — it's the zero point of the cycle.
Set your cycle length
Default is 28 days. If your cycle is different, slide the range between 21 and 35 days.
Get your calendar
The calculator shows the ovulation day, a 6-day fertile window, and the date of your next period.
Why use this ovulation calculator
Fertile window detection
Highlights the 6 days with the highest chance of conception and marks the 3 most fertile days around ovulation.
Visual calendar view
See it all at a glance: period, fertile days, ovulation, and next period. View one cycle by default or toggle three cycles ahead.
Link to pregnancy calculator
If conception has happened, jump straight to the pregnancy and due-date calculator in one click.
Frequently asked questions about ovulation
What is ovulation and when does it happen?
Ovulation is the release of a mature egg from the ovary into the fallopian tube, where fertilization can occur. With a standard 28-day cycle, ovulation typically happens on day 14 from the start of menstruation, but the exact date depends on your cycle length: it occurs roughly 14 days before the next period. So with a 30-day cycle ovulation falls on day 16, and with a 26-day cycle on day 12.
What is the fertile window?
The fertile window is a 6-day period with the highest chance of conception: 5 days before ovulation plus the ovulation day itself. This duration reflects how long sperm survive inside the female body (up to 5 days) and how long the egg lives after release (about 24 hours). The three most fertile days are the day before ovulation, the day of ovulation, and the day after.
How can I calculate ovulation with an irregular cycle?
The calendar method based on your last period works reliably only for regular cycles of 21–35 days with no more than 2–3 days of variation. With irregular cycles the ovulation date 'floats' and the calendar gives only a rough estimate. In that case, use ovulation predictor kits (OPKs) that detect the LH surge 24–36 hours before ovulation, basal body temperature tracking, or ultrasound follicle monitoring.
Can I use the ovulation calendar as birth control?
No. The calendar method is not reliable contraception: according to the WHO, the probability of pregnancy is about 5% per year with perfect use and 12–24% with typical use. Sperm live up to 5 days, the cycle can shift due to stress, illness, or travel, and a single 'unpredictable' ovulation can lead to an unwanted pregnancy. Use this calculator only for planning conception. For contraception, talk to a doctor — a gynecologist can recommend a suitable method. This calculator does not replace medical advice.