Period & Ovulation Calculator
See your next period, ovulation day and fertile window on a color-coded monthly calendar — calculated from your cycle in seconds.
Estimates based on averages — not medical advice and must not be used as a contraceptive method. See a doctor for irregular cycles.
How the calendar is calculated
Everything is derived from your last period and cycle length with the standard calendar method:
- Ovulation day = first day of last period + (cycle length − 14). Ovulation happens about 14 days before your next period — day 14 in a 28-day cycle, day 18 in a 32-day cycle.
- Fertile window (high chance) = the 5 days before ovulation plus ovulation day itself. Sperm can survive up to ~5 days, so these are the highest-chance days of each cycle.
- Period days = last period date through period length − 1; next period = last period + cycle length.
- Low chance = all remaining days of the cycle.
The calendar colors each day by cycle arithmetic from your last period, so it stays correct as you flip through months. It assumes regular cycles — if yours vary, treat the dates as rough guides only.
Frequently asked questions
How is the ovulation day calculated?
Ovulation is estimated as the first day of your last period plus (cycle length − 14) days — day 14 for a 28-day cycle, day 18 for a 32-day cycle. This is the standard calendar method and works best with regular cycles.
What is the fertile window?
The fertile window is the 5 days before ovulation plus ovulation day itself. Sperm can survive up to about 5 days, so these six days carry the highest chance of pregnancy in each cycle.
Can I use this calculator as birth control?
No. This calculator must not be used as a contraceptive method. Ovulation timing varies from cycle to cycle, and the calendar method alone is not reliable protection. Talk to a doctor about contraception options.
What if my cycles are irregular?
These estimates assume fairly regular cycles. If your cycle length varies by more than a few days, the predictions will be unreliable — see a doctor to check for underlying causes and get personalised guidance.