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Ovulation Calculator

Estimate your ovulation day, fertile window, next period and the earliest date for a pregnancy test from your last period and cycle length, with dates for the next six cycles.

21 to 45 days, counted from the first day of one period to the first day of the next.
Optional, 10 to 16 days. Leave 14 if you do not know it.

These dates are estimates, not medical advice. Ovulation can shift from cycle to cycle, so this calculator must not be used as a method of contraception.

How to use the ovulation calculator

  1. Enter the first day of your last period. The field starts two weeks before today; change it to your own date.
  2. Enter your average cycle length: the number of days from the first day of one period to the first day of the next (21 to 45 days).
  3. If you know your luteal phase length (the days from ovulation to your next period), enter it (10 to 16 days). Otherwise leave the default of 14 days.
  4. Press Calculate.

The result shows your estimated ovulation day, your fertile window, the expected start of your next period and the date from which a home pregnancy test is most reliable. The details table repeats these dates for six cycles in a row.

How the dates are calculated

The second half of the cycle, the luteal phase, is fairly constant for each person, while the first half varies. So ovulation is counted back from the next period:

  • Ovulation day = first day of last period + (cycle length − luteal phase length)
  • Fertile window = the 5 days before ovulation plus the ovulation day itself, because sperm can survive for up to about five days and an egg lives for about one day
  • Next period = first day of last period + cycle length
  • Pregnancy test = from the day your next period is due

Later cycles are estimated by adding the cycle length again and again.

Worked example

Your last period started on March 1, 2026 and your cycles last 30 days with a 14-day luteal phase. Ovulation is estimated on March 1 + 16 days = Tuesday, March 17, 2026. The fertile window runs from March 12 to March 17, and your next period is due on Tuesday, March 31, 2026, which is also the first day a home test is likely to be reliable.

Good to know

  • Do not use this calculator as contraception. Ovulation can move by several days from one cycle to the next because of stress, illness, travel or other factors, so no calendar method can tell you when you are safe.
  • The estimate is most useful when your cycles are regular. With irregular cycles, ovulation predictor kits, basal body temperature charts or cervical mucus changes give a better picture.
  • Results are estimates, not medical advice. If you have been trying to conceive for a year (or six months if you are over 35), or your cycles are very irregular, talk to a doctor.

Frequently asked questions

How accurate is an ovulation calculator?

It gives a reasonable estimate for people with regular cycles, but it cannot detect ovulation. Even in regular cycles, the actual day can differ from the estimate by a few days. Ovulation tests, temperature tracking or a doctor's scan can confirm it more precisely.

What is the fertile window?

It is the six-day span in each cycle when sex can lead to pregnancy: the five days before ovulation and the day of ovulation. The chance of conception is highest in the two or three days leading up to and including ovulation.

Can I use this calculator to avoid pregnancy?

No. Calendar estimates are not reliable enough for contraception, because ovulation can shift without warning. Speak to a doctor, nurse or pharmacist about effective methods of birth control.

What is the luteal phase and why does it matter?

The luteal phase is the time between ovulation and the start of your next period. It usually lasts 12 to 14 days and changes little from cycle to cycle, so knowing it lets the calculator place ovulation more accurately. If you do not know yours, 14 days is a common assumption.

When should I take a pregnancy test?

Most home tests give the most reliable result from the day your next period is due. Some sensitive tests can detect pregnancy a few days earlier, but a negative result before your missed period may be wrong, so test again if your period does not start.