Ovulation Calculator & Fertile Window
Estimate your ovulation day, fertile window and next periods from your last period and cycle length.
Result
How to use
- 1
Pick the first day of your last period.
- 2
Set your usual cycle length (day 1 of one period to day 1 of the next).
- 3
See your likely ovulation day, fertile window and the next three cycles.
Questions
How is ovulation estimated?
Ovulation usually happens about 14 days before the next period starts, so ovulation day ≈ cycle length − 14 days after the first day of the last period. With a 28-day cycle that is day 14.
What is the fertile window?
Sperm can survive up to 5 days and the egg about a day, so the most fertile days are the 5 days before ovulation and the day of ovulation itself.
Can I use this as birth control?
No. Cycles vary, especially with irregular periods, PCOS, after childbirth or while breastfeeding. This is only an estimate — please talk to a gynaecologist for family planning or if you are trying to conceive.
About this Ovulation tool
Ovulation Calculator & Fertile Window — Estimate your ovulation day, fertile window and next periods from your last period and cycle length. It is one of 200 free tools on namasivayam.in, made by software engineer Namasivayam Vadivelan for everyday use in India.
Everything is calculated instantly in your browser: nothing you type or upload is sent to any server, there is no sign-up, and it works on mobile and desktop. Pick the first day of your last period.
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Results are estimates to help you plan. They are not financial, tax or medical advice.