Ovulation Calculator

Find your most fertile days and estimated ovulation date from your last period and cycle length, plus a forecast of your next few cycles.

First day of your last period
Average cycle length days
Estimated ovulation
— Enter your dates

Ovulation is estimated as 14 days before your next period; the fertile window covers the 5 days before through the day after. Timing varies — ovulation tests or tracking basal temperature confirm it more precisely.

Timing intercourse around ovulation gives you the best chance of conceiving. This calculator estimates your ovulation day and the fertile window around it from the first day of your last period and your usual cycle length, then projects the next few cycles so you can plan ahead.

When ovulation happens

The menstrual cycle has two phases. The follicular phase (period to ovulation) varies in length between people; the luteal phase (ovulation to next period) is fairly constant at about 14 days. Because of that, ovulation is estimated by counting back 14 days from your next expected period:

Ovulation day ≈ cycle length − 14 days after your last period started.

So a 28-day cycle ovulates around day 14, and a 32-day cycle around day 18.

Your fertile window

You can only conceive in a short window each cycle, because:

  • Sperm can survive in the reproductive tract for up to 5 days.
  • The egg survives about 12–24 hours after it’s released.

Together that makes a fertile window of roughly six days: the five days before ovulation through the day after. The two or three days leading up to ovulation are the most fertile. This tool marks that whole window plus the peak days.

Planning across cycles

If this cycle isn’t the one, it helps to see the next few. The calculator lists upcoming cycles with each one’s fertile window and ovulation day, plus the date a pregnancy test becomes reliable (about 14 days after ovulation, around when your period would be due).

How to confirm ovulation

Calendar estimates assume regular cycles, and real ovulation shifts from month to month. To pin it down:

  • Ovulation predictor kits detect the luteinising-hormone (LH) surge 24–36 hours before ovulation.
  • Basal body temperature rises slightly after ovulation — charting it confirms that it happened.
  • Cervical mucus becomes clear and stretchy (like egg white) around your most fertile days.

When to seek advice

If you’re under 35 and haven’t conceived after a year of trying (or six months if you’re 35 or older), or your cycles are very irregular, absent, or unusually short or long, see a doctor. This calculator is for information and family planning, not a diagnosis or a contraceptive method.

Frequently asked questions

When do you ovulate?

Ovulation usually happens about 14 days before your next period starts, because the luteal phase (ovulation to period) is fairly constant. In a 28-day cycle that's around day 14; in a 32-day cycle it's around day 18. The calculator uses your cycle length to estimate it.

What is the fertile window?

It's the roughly six days when intercourse can lead to pregnancy — the five days before ovulation plus ovulation day itself. Sperm can survive up to five days, while the egg lives about 24 hours, so the days just before ovulation are the most fertile.

How accurate is an ovulation calculator?

It's a good estimate for regular cycles but not a precise prediction — ovulation timing varies month to month. Ovulation predictor kits (which detect the LH surge), basal body temperature tracking, and cervical mucus changes confirm your fertile days more reliably.

Can you get pregnant outside the fertile window?

It's much less likely, but cycles vary and ovulation can shift, so no calendar method is foolproof for either conceiving or avoiding pregnancy. If you're avoiding pregnancy, use a reliable contraceptive method rather than timing alone.

References