Ovulation Calculator
Find your ovulation day and fertile window
Free Ovulation Calculator — Your Fertile Window
Ovulation day, fertile window and next period date
Enter the first day of your last period and your cycle length; this tool estimates your ovulation day (14 days before the next period), your most fertile 6-7 day window (5 days before ovulation through 1 day after), your next period date and the right day for a pregnancy test. If the date you enter is in the past, the tool automatically finds your current cycle. Your data stays on your device. Results are calendar-based estimates — cycles vary between people and from month to month.
Features
Ovulation Day
14 days before the next period (luteal phase rule)
Fertile Window
The −5/+1 day range around ovulation
Next Period & Test
Next period date and the right day to test
100% Private
Your data never leaves your device
Cycle Timeline
How it's calculated
- Ovulation ≈ next period − 14 days
- Fertile window: ovulation −5 … +1 days
- Next period = last period + cycle length
- Sperm live 3-5 days, the egg ~24 hours
Example (28-day cycle)
- Day 1: period starts
- Days 9-15: fertile window
- Day 14: estimated ovulation
- Day 29: next period / test day
Frequently Asked Questions
How is the ovulation day calculated?
Since the luteal phase lasts ~14 days for most women, ovulation is estimated by counting 14 days back from the next expected period.
Which are my most fertile days?
The 5 days before ovulation and 1 day after — sperm can survive 3-5 days, so the window opens before ovulation itself.
How reliable is this method?
The calendar method is an estimate; stress, illness and hormones can shift ovulation. For more precision use ovulation test strips or basal body temperature tracking.
Can I use it for birth control?
No — the calendar method is not a reliable form of contraception. This tool is informational, not medical advice.
When should I take a pregnancy test?
For the most reliable result, test from the day of your expected period; earlier tests carry a higher chance of a false negative.