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  2. Ovulation calculator

Ovulation calculator

Find your fertile window, ovulation day and next period from one date.

  • Free, no sign-up
  • Method from ACOG and the NHS

This calculator needs JavaScript in your browser. The method and every date it uses are explained below.

The first day of bleeding, not spotting.
From the first day of one period to the day before the next. Not sure? Keep 28.

Your best days to conceive are

5 days before ovulation to 1 day after

Ovulation (estimated)
Next period due
Pregnancy test from
Due date if pregnant280 days from your last period (ACOG). This one does not change with cycle length.

An estimate from the calendar method. Not medical advice, and not a form of birth control.

Pick the first day of your last period

At a glance

Your next 3 cycles
Period startsFertile windowOvulation

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How this calculator works

  1. Your next period is your last period’s first day plus your cycle length.
  2. Ovulation is about 14 days before your next period. The days after ovulation are the most stable part of the cycle.
  3. Your fertile window runs from 5 days before ovulation to 1 day after, because sperm can live up to 5 days and an egg about 24 hours.
  4. A due date is 280 days after the first day of your last period. That is ACOG’s convention, and it does not change with your cycle length, so this one date stays the same whichever cycle you choose.

If your cycle length changes a lot from month to month, a calendar estimate is less reliable. Ovulation tests or a doctor can tell you more.

Sources

  1. ACOG, Fertility Awareness-Based Methods of Family Planning, FAQ024, last reviewed February 2025.
  2. ACOG, Evaluating Infertility, FAQ136, last reviewed May 2026.
  3. ACOG Committee Opinion 700, Methods for Estimating the Due Date, 2017.
  4. NHS, Periods and fertility in the menstrual cycle, reviewed 5 January 2023.
  5. NHS, Doing a pregnancy test.

Method from ACOG and the NHS, with the sources below. Last updated October 14, 2026.

Questions people ask

How many days after my period will I ovulate?

It depends on your cycle, not your period. Ovulation is about 14 days before your next period, so with a 28-day cycle that is about 14 days after your period starts, and with a 32-day cycle about 18.

What are my most fertile days?

The 5 days before ovulation, the day of ovulation, and the day after. ACOG puts the window at 5 days before ovulation until 1 day after.

How do I know if I'm ovulating?

Before ovulation, cervical mucus becomes clear, thin and slippery. After it, your resting temperature rises slightly. Urine ovulation tests detect the hormone surge that comes first.

If my cycle is 40 days, when will I ovulate?

About 26 days after your period starts, because ovulation is 14 days before the next one and 40 minus 14 is 26. Choose 40 days in the cycle field and the fertile dates, the ovulation day, the next period and the test date all move with it. The due date does not: it is 280 days from your last period, which is ACOG's convention.

Can I get pregnant right after my period?

Yes, with a short cycle. Sperm can live up to 5 days, so sex at the end of a period can meet an early ovulation.

When should I take a pregnancy test?

From the first day of your missed period. The NHS says tests are most reliable from that day.