How to Estimate Ovulation and Your Fertile Window From Your Cycle

Knowing roughly when you ovulate helps if you are trying to conceive or to understand your cycle. A calculator gives an estimate; bodies vary, so treat it as a guide.

Quick answer

Enter the first day of your last period and your usual cycle length. The tool shows the expected ovulation day, the fertile window and the next three periods.

How the estimate works

• Ovulation usually happens about 14 days before the next period, not always on day 14 of the cycle.
• For a 28-day cycle, ovulation is around day 14. For a 32-day cycle it is around day 18.
• The fertile window is about six days: the five days before ovulation and the day of ovulation. Sperm can live for several days.

Steps

1. Enter the first day of your last period.
2. Enter your average cycle length (21 to 35 days is typical).
3. Read the dates for the next three cycles.

Limits you should know

• Irregular cycles make the estimate unreliable.
• Stress, illness, travel and breastfeeding can shift ovulation.
• A calendar method is not a reliable contraceptive.
• Ovulation tests, basal body temperature and cervical mucus give more precise signals.

When to see a doctor

If you have been trying to conceive for 12 months (6 months if over 35), if cycles are very irregular, or if periods are very painful or heavy.

This tool provides general information and is not medical advice.