Ovulation & Fertile Window Calculator

Written by Cynthia Obinatu, community and editorial lead. Checked against published guidance from the National Health Service, the World Health Organization, the American Society for Reproductive Medicine and peer-reviewed research. Not yet reviewed by a clinician. Educational, not medical advice.

How this ovulation calculator works

It does arithmetic with two numbers: the first day of your last period and your usual cycle length. It adds the cycle length to that date to find your next period, then counts back 14 days to estimate the day you ovulate. The fertile window it shows starts five days before that day and ends one day after it. That gives seven dates in all. Sperm can survive for up to seven days inside the body, according to the National Health Service (NHS), so sex before ovulation can still lead to pregnancy.

The count goes back from your next period. Many people learn that ovulation falls on day 14. That is only true for a 28-day cycle. The NHS says ovulation usually happens 10 to 16 days before your next period, and the 14-day rule does not hold for women whose cycles are shorter or longer. On a 35-day cycle, ovulation lands around day 22.

What your ovulation result means

Read the dates as the middle of a range. If your cycles vary by three or four days, and that is ordinary, your real fertile days can sit three or four days either side of what the calculator shows. The estimate improves when you give it a true average. Count the days from the first day of one period to the first day of the next over three or four months, and use that number.

The result is an estimate of timing. It cannot detect ovulation, confirm a pregnancy or work as birth control. If you are seeing a doctor in Nigeria about getting pregnant, take your last three or four period dates with you. They are more useful to a clinician than a single calculator result. The period tracker works out your average cycle, and the fertile window guide explains what the window is really estimating.

How many days after my period will I be fertile?

It depends on your cycle length, and on a typical 28-day cycle the fertile window opens about five days after bleeding stops. The table shows when the window starts, when ovulation is most likely and when the window ends, using the same arithmetic as the calculator. Day 1 is the first day of your period, and the table assumes bleeding lasts about five days.

A short cycle changes the dates. On a 21-day or 24-day cycle the window can start the day after bleeding ends, and on a 21-day cycle it overlaps the last days of a long period. That is why the days right after a period are not a safe time to assume you cannot get pregnant. The same applies if you are trying to avoid pregnancy, and this calculator should not be used for that.

Can I get pregnant 2 or 3 days after my period?

Yes, particularly if your cycle is short or your period is long. Sperm can survive for days, so sex at the end of a period can meet an egg released shortly afterwards. The chance is lower than around ovulation, and it is not zero.

At what age am I the most fertile?

In your 20s. The American Society for Reproductive Medicine says a woman's best reproductive years are in her 20s, with fertility declining gradually through the 30s and more quickly after 35. It gives a healthy 30-year-old about a 20 percent chance of getting pregnant in a single cycle, and under 5 percent per cycle at 40.

These are per-cycle figures. Over months they add up. The NHS says that more than 8 out of 10 couples where the woman is under 40 will conceive naturally within a year. Plan early and there is no need to panic.

At what age are 90% of your eggs gone?

Most women reach that point by about 30, on the best available model. The figure counts eggs and says nothing direct about your chance of pregnancy. A 2010 study in PLOS ONE pooled eight tissue studies and estimated that for 95% of women, only 12% of the maximum number of dormant follicles (the tiny sacs that hold immature eggs) is left by age 30, and only 3% by age 40. Put the other way, about 88% is gone by 30.

The researchers modelled follicle counts and did not measure fertility itself. A healthy 30-year-old still has about a 20 percent chance of pregnancy each cycle, according to the ASRM, so the age figures above are the better guide to your own odds.

Is 36 or 37 too old to have a baby?

No. At 36 or 37, fertility is lower than it was at 25, and it can take longer, but most couples still conceive naturally. The one change the NHS makes is about timing. It advises that women aged 36 and over should see their GP sooner instead of waiting a full year before asking for help. The World Health Organization defines infertility as not achieving pregnancy after 12 months of regular unprotected sex, so for you the advice is to book the appointment earlier.

If you are 36 or 37 and planning a baby, start tracking your cycle now, have sex every two to three days through the month and see a doctor early if nothing happens. The NHS says that for women aged 39 or under, having sex every 2 to 3 days makes pregnancy within a year likely, so you do not need to time one exact day.

How can I track ovulation if I have irregular periods?

Use a body signal. A calculator needs a steady cycle length, and with irregular cycles it will be wrong more often than it is right. Urine ovulation strips detect the luteinising hormone (LH) surge that comes just before an egg is released. A review in Bioengineering and Translational Medicine reported that a positive LH test is followed by ovulation about 20 hours later. Because you cannot predict which days to test, start earlier in your cycle and test for more days than the box suggests.

Basal body temperature (your resting temperature, taken every morning) only shows a rise after ovulation has already happened. The same review found it matched the LH surge to within a day in only 17 of 77 cycles and concluded it should not be recommended for couples trying to conceive. An ultrasound scan at a clinic can show ovulation directly.

Irregular cycles are also a reason to get checked. The NHS describes cycles anywhere from 21 to 35 days as normal. If yours swing outside that, or you go three months without a period when you are not pregnant, see a doctor. The review also noted that very few studies include women with irregular cycles, so ask a clinician before trusting any single method.

How do I know if my cycle is 28 days or 30 days?

Count from the first day of one period to the day before the first day of the next. Do it for three or four months and take the average. A cycle that comes in at 29 to 31 days each time is a 30-day cycle, and you should enter 30 in the calculator.

Can I use an app to calculate my ovulation date?

You can, and it will do the same sum this page does. Apps predict from the dates you have entered. They do not detect ovulation. If you are trying to conceive and the months are passing, pair any app with LH strips or a scan.

Can I calculate when I conceived based on my ovulation date?

Ovulation is the best clue to when you conceived, because that is when the egg is released. Pregnancy itself is dated from the first day of your last period. On a 28-day cycle that date comes about two weeks before ovulation. The due date calculator does that conversion.

Common questions

Where this comes from

• Fertility in the menstrual cycle. National Health Service.
• Infertility. National Health Service.
• Trying to get pregnant. National Health Service.
• Periods. National Health Service.
• Infertility fact sheet. World Health Organization.
• Age and fertility. American Society for Reproductive Medicine.
• Human ovarian reserve from conception to the menopause. PLOS ONE, 2010.
• Detection of ovulation, a review of currently available methods. Bioengineering and Translational Medicine, via PubMed Central, 2017.

This page has not yet been reviewed by a clinician. It stands on the published guidance above, and we say so plainly rather than implying a review that has not happened. Read it alongside our medical policy.