How ovulation is estimated
Ovulation — the release of an egg — usually happens about 14 days before the next period starts. The days before it vary from person to person, but the time after it, the luteal phase, is fairly steady. So the calculator counts back from your next expected period: with a 28-day cycle, ovulation falls around day 14; with a 32-day cycle, around day 18.
The fertile window
Sperm can survive up to five days, and an egg about a day, so the fertile window is the five days before ovulation, the day itself and the day after. Intercourse in the two or three days before ovulation gives the best chance of pregnancy.
How to use the ovulation calculator
- Enter the first day of your last period.
- Enter your usual cycle length — the days from the start of one period to the start of the next. Leave the luteal phase at 14 unless you know yours.
- Read your estimate and the table of the next six cycles.
Good to know
- This is an estimate for regular cycles. Stress, illness, travel and breastfeeding can move ovulation.
- Ovulation test kits, basal body temperature and changes in cervical mucus confirm ovulation more reliably than a calendar.
- A calendar estimate is not a reliable method of contraception.
- If your cycles are irregular, or you have been trying to conceive for a year (six months if you are over 35), talk to a doctor.
Frequently asked questions
How many days after my period do I ovulate?
In a 28-day cycle, about 14 days after the first day of your period. Subtract 14 from your cycle length to find your usual ovulation day.
Can I get pregnant outside the fertile window?
It is unlikely but possible, because ovulation can come earlier or later than predicted in any cycle.
When should I take a pregnancy test?
From the day your period is due, or about 14 days after ovulation. Testing earlier can give a false negative.