Ovulation Calculator
Calculate your fertile window and ovulation date based on your cycle length and last period date.
About this calculator
Ovulation is the release of an egg, and it happens about 14 days before your next period starts. This calculator takes the first day of your last period and your cycle length and shows your likely ovulation day, your fertile window and the date your next period is due.
The fertile window spans the five days before ovulation and the day itself. Sperm can survive up to five days, while the egg lives for about 12 to 24 hours, so intercourse in this window gives the best chance of conception. The estimate works best for regular cycles.
How to use it
- Enter the First Day of Your Last Period.
- Enter your average Cycle Length in days.
- Press Calculate to see your ovulation day, fertile window and next period.
Calendar method
Ovulation day = First day of last period + (Cycle length − 14). Fertile window = Ovulation day − 5 to Ovulation day. Next period = Last period + Cycle length. The luteal phase, between ovulation and the next period, is fairly constant at about 14 days, so a longer cycle means later ovulation.
Worked example
Last period 1 November 2026, 28-day cycle
| Ovulation | Sunday 15 November 2026 |
| Fertile window | 10 to 15 November 2026 |
| Next period | 29 November 2026 |
With a 32-day cycle, ovulation moves to 19 November and the fertile window to 14 to 19 November.
This tool gives general information, not medical advice. Talk to a doctor before making health decisions.
Tips and common mistakes
- Track your cycle for three months. The average is more reliable than a single cycle.
- Ovulation test kits and basal temperature give a more precise answer than the calendar.
- Cervical mucus that looks like raw egg white signals fertility.
- If you have irregular cycles, speak to a doctor rather than relying on the calendar.
- If you are not trying to conceive, a calendar method alone is not reliable contraception.
Frequently asked questions
When do I ovulate?
About 14 days before your next period, which is day 14 in a 28-day cycle.
How long is the fertile window?
About six days, ending on ovulation day.
Can I get pregnant outside the window?
It is unlikely but cycles vary.
Can stress change ovulation?
Yes, stress, illness and travel can delay it.
How long should I try before seeing a doctor?
After 12 months if under 35, and after 6 months if 35 or older.
Is this a contraception tool?
No.