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How to find your fertile window and ovulation day

You know when your last period started and roughly how long your cycle is. You will find the likely ovulation day, the six-day fertile window and the next period for the next three cycles. With a 1 January start and 28 days, ovulation falls on 15 January. Nothing leaves this tab.

6-minute read

Today's result For each of the next three cycles: the fertile window, the likely ovulation day and the next period.

Open the ovulation calculator with your period dates at hand. Use the average of your last few cycles. It is an estimate, not contraception or medical advice. Nothing is uploaded.

Open Ovulation calculator
Contents

  1. Know how ovulation is timed

    The time from ovulation to the next period, the luteal phase, is about 14 days for most people. The part before ovulation is what varies, so ovulation = next period − 14 days.

    Ovulation ≈ next period − 14 days; the fertile window runs from 5 days before through ovulation day.
    Ovulation ≈ next period − 14 days; the fertile window runs from 5 days before through ovulation day.

  2. Open Ovulation calculator on cv.cm

    A sample last period and a 28-day cycle are filled. Three cycles sit on the right. Nothing is uploaded.

    Open Ovulation calculator

  3. Enter your last period

    Type the first day of your last period, the first day of bleeding. For 1 January with 28 days, the next period is 29 January.

  4. Set your cycle length

    Count from the first day of one period to the day before the next and average a few cycles. A 32-day cycle moves ovulation from 15 January to 19 January.

  5. Read the fertile window

    The window is the five days before ovulation plus ovulation day: 10–15 January. Sperm can live up to five days and the egg about one, so these six days carry the highest chance.

    Period 1 January, 28 days: fertile 10–15 January, ovulation 15 January, next period 29 January.
    Period 1 January, 28 days: fertile 10–15 January, ovulation 15 January, next period 29 January.

  6. Treat it as an estimate, then copy

    Cycles shift by a few days from month to month, and calendar estimates miss many ovulations. Ovulation test strips give a better signal. Copy puts the dates on the clipboard.

Use with these tools

FAQ

Can I calculate my ovulation day free online?

Yes. cv.cm estimates it in this tab. Nothing is uploaded or stored.

When do I ovulate with a 28-day cycle?

Around day 14, counting the first day of your period as day 1.

How long is the fertile window?

About six days: the five days before ovulation plus ovulation day.

Can I use this to avoid pregnancy?

No. Calendar estimates are not reliable contraception. Use a proper method and ask a doctor.

My cycle is irregular. What should I enter?

The average of your last six cycles. Irregular cycles make the estimate less accurate; ovulation tests help.