Enter the first day of your last period and your cycle length to see your fertile window, estimated ovulation day and next period.
Today
Ovulation result
Fertile window
—
Estimated ovulation day
—
Next period
—
Today
—
Fertile windows in the coming cycles
Cycle starts
Fertile window
Ovulation
—
—
—
—
—
—
—
—
—
Estimates only: ovulation usually happens about 14 days before the next period and can come earlier or later. Do not rely on this calculator for contraception, and see a doctor if your cycles are irregular.
Enter the first day of your last period, in Gregorian or Hijri.
Choose your cycle length: the days from the first day of one period to the first day of the next.
Press “Calculate ovulation” to see your fertile window, ovulation day and next period, plus a table of the next three cycles.
How it is calculated
Result
Method
Next period
First day of last period + cycle length
Ovulation day
Next period − 14 days
Fertile window
From 5 days before ovulation to ovulation day
Worked examples (last period started on 1 January 2026)
Cycle length
Fertile window
Estimated ovulation
Next period
28 days
10–15 January
15 January
29 January
35 days
17–22 January
22 January
5 February
Important
This calculator gives estimates only, is not medical advice and is not a contraception method. If your cycles are irregular, or you have not conceived after a year of trying (six months if you are over 35), see a doctor.
If you do conceive, use the Pregnancy Calculator for your due date and weeks pregnant.
Last updated:
Frequently asked questions
How do I know my ovulation day?
Ovulation usually happens about 14 days before the next period. With a 28-day cycle and a last period that started on 1 January 2026, the next period is due on 29 January and ovulation on about 15 January.
What is the fertile window?
The five days before ovulation plus ovulation day, because sperm can live up to 5 days while the egg lives 12 to 24 hours. In the example above, the fertile window is 10 to 15 January.
Does ovulation move if my cycle is longer than 28 days?
Yes. Ovulation stays about 14 days before the next period, so it comes later in a longer cycle. With a 35-day cycle starting on 1 January, the next period is due on 5 February, ovulation on about 22 January, and the fertile window is 17 to 22 January.
Can I use an ovulation calculator for contraception?
No. It gives estimates, and ovulation can come earlier or later because of cycle changes, stress or illness, so it is not a contraception method. Ask a doctor about suitable options.
Is it accurate if my cycle is irregular?
Less so, because it assumes a constant cycle length. Home ovulation tests and a doctor's advice help in that case.
Can I enter a Hijri date?
Yes. Choose Hijri in the date field; the results are shown in both the Hijri (Umm al-Qura) and Gregorian calendars.