Ovulation Calculator

Ovulation calculator at a glance#

How does an ovulation calculator work?#

It counts forward from the first day of your last period by your average cycle length to find your next period, then counts back 14 days to the day you most likely ovulate. Enter those two things in the calculator above. It returns an ovulation window, the most probable ovulation date, an intercourse window for getting pregnant, a pregnancy test date and your next period, plus estimates for six cycles.

When do I ovulate if my cycle is 28 days?#

Around 14 days after your period starts. If your last period began on April 6, the calculator puts your most probable ovulation date on April 20, which is cycle day 15 when the first day of bleeding counts as day 1. Its ovulation window runs from April 18 to April 22.

How long before your period do you ovulate?#

About 14 days. The American College of Obstetricians and Gynecologists (ACOG) says that in an average 28-day cycle, ovulation happens about 14 days before the next period starts, and that this stretch is the most consistent part of the cycle. It still varies from person to person. The Office on Women's Health puts it anywhere from 7 to 19 days.

What ovulation and the fertile window are#

Ovulation is the day an ovary releases an egg. The egg survives in the fallopian tube for about 24 hours, while sperm can live inside the body for about 3 days and sometimes up to 5. That's why the days before ovulation matter as much as the day itself. ACOG says you can get pregnant from sex anywhere from 5 days before ovulation until 1 day after it.

The odds aren't spread evenly across those days. According to the Office on Women's Health, pregnancy is most likely from sex in the three days before ovulation and on the day it happens. So the fertile window opens several days before the egg is released and closes about a day after.

How the calculator finds your ovulation date#

It works backward from your next period, not forward from your last one. The worked example on this page is a last period that started on April 6 and a 28-day cycle.

1. Next period: April 6 plus 28 days is May 4.
2. Most probable ovulation date: May 4 minus 14 days is April 20.
3. Ovulation window: 2 days either side of that, April 18 to April 22.
4. Intercourse window for pregnancy: from 3 days before the ovulation window opens to the day it closes, April 15 to April 22.
5. Pregnancy test: 7 days after the ovulation window closes, April 29.

The intercourse window is built on the same idea. Nobody knows the exact ovulation day in advance, so the calculator starts three days before the earliest likely day and runs to the latest likely one. The calendar under the result marks the ovulation window, and the table below it repeats the same steps for six cycles, starting with this one. After April 6, the cycles start on May 4, June 1, June 29, July 27 and August 24, with ovulation windows of May 16 to May 20, June 13 to June 17, and so on. If you only need period dates, the period calculator marks your bleeding days on a three-month calendar.

This gives an estimate, not medical advice. A doctor can tell you what the number means for you.

Why it counts back 14 days#

ACOG notes that the number of days between ovulation and the next period is the most consistent part of the cycle. The days before ovulation are what stretch or shrink. So when a cycle is longer, ovulation comes later, and the gap after it stays about the same.

Run the same April 6 start with three different cycle lengths and you can see it. A 24-day cycle puts ovulation on April 16, a 28-day cycle on April 20 and a 35-day cycle on April 27. In each case the next period comes 14 days after the ovulation date.

Many charts say a 28-day cycle ovulates on day 14. The calculator lands on day 15 because it counts back 14 days from day 29, the first day of the next period. That one day is well inside the uncertainty, and the five-day ovulation window covers both.

Ovulation days for common cycle lengths#

Each row comes from the calculator, with the first day of your period as day 1. Find your average cycle length and read across.

If your cycles are irregular#

The calculator takes one average cycle length, so it assumes every cycle is the same. If yours run 26 days one month and 33 the next, ovulation moves with them, and a single estimate can miss. Two things help.

• Run it twice, once with your shortest recent cycle and once with your longest. Together, the two intercourse windows cover the range where your fertile days could fall. For cycles between 26 and 33 days, that's day 8 to day 22.
• Track a body sign as well, such as cervical mucus, or use ovulation tests. Those respond to what's happening this cycle rather than to an average.

ACOG notes that the Standard Days method, a fixed calendar rule, works best when cycles are regular and consistently between 26 and 32 days long. The calculator accepts averages from 22 to 44 days.

Other signs of ovulation#

A calendar estimate is a prediction. These signs come from your body during the cycle you're in.

• Cervical mucus. ACOG says that just before ovulation the amount of mucus noticeably increases and it becomes thin and slippery. Just after, there's less of it and it turns thicker.
• Basal body temperature. This is your temperature at rest, taken every morning before you get out of bed. In most women it rises slightly around ovulation, by 0.5 to 1°F (about 0.3 to 0.6°C), and stays up until the period. ACOG puts the most fertile days at the 2 to 3 days before the rise, so a temperature chart confirms ovulation after the fact rather than predicting it.
• LH tests. An ovulation test kit checks your urine for luteinizing hormone (LH). LH starts to surge about 36 hours before ovulation and peaks about 12 hours before it, per the Office on Women's Health. MedlinePlus says a positive result means you should ovulate in the next 24 to 36 hours, and suggests starting to test 3 to 5 days before you expect to ovulate. For a 28-day cycle, that's day 11.
• One-sided twinges. Some women feel minor cramping on one side of the pelvis when they ovulate.

Due dates from your ovulation date#

The six-cycle table also shows a due date for each cycle, in case you conceive that month. It adds 280 days to the cycle's start date, then shifts the result by however many days your cycle is longer or shorter than 28. For the April 6 start and a 28-day cycle, the due date is January 11. A 32-day cycle moves it 4 days later, to January 15, because ovulation comes 4 days later.

The 280 days are the standard 40 weeks, counted from the first day of the last period. If you're already pregnant, the pregnancy calculator shows how far along you are from your last period, a conception date or an ultrasound.

When to take a pregnancy test#

The calculator's pregnancy test date is 7 days after the ovulation window closes, which is 5 days before your next period is due. In the example, that's April 29. Treat it as early. MedlinePlus says most home pregnancy tests won't show a pregnancy until your period is late, and testing before then often gives an inaccurate result. If an April 29 test is negative, test again once your period is late, after May 4.

You can count these gaps for any start date with the date calculator, which adds days to a date or counts the days between two.

Sources#

• ACOG, Fertility Awareness-Based Methods of Family Planning
• Office on Women's Health, Your menstrual cycle
• MedlinePlus, Ovulation home test

Frequently asked questions#

How long does ovulation last?#

The egg survives about 24 hours after it's released, and the Office on Women's Health puts it at 12 to 24 hours. Sperm last longer, about 3 days and sometimes up to 5, which is why the fertile window starts days before ovulation.

What is the most fertile day of my cycle?#

The days just before ovulation and the day itself. The Office on Women's Health says pregnancy is most likely from sex in the three days before ovulation and on the day it happens. If the calculator's most probable date of April 20 is right, that's April 17 to April 20.

Can I use an ovulation calculator with irregular periods?#

Yes, but treat the result as rough. It works from one average cycle length. Run it with your shortest and your longest recent cycle, and add a body sign such as an LH test or cervical mucus to narrow it down.

What if my cycle is shorter than 22 days or longer than 44?#

The calculator only accepts 22 to 44 days. The Office on Women's Health says to talk to a doctor or nurse if your cycles are shorter than 24 days or longer than 38, so a cycle outside the calculator's range is worth a visit too.

How many days after my period do I ovulate?#

Count from the first day of your period. Ovulation comes about your cycle length minus 14 days later. For a 28-day cycle that's 14 days after your period started, and for a 32-day cycle it's 18 days after.

Can an ovulation calculator prevent pregnancy?#

It isn't built for birth control. ACOG says that even with fertility awareness methods, which track signs like temperature and mucus as well as dates, 12 to 24 out of 100 women become pregnant in the first year of typical use. A doctor can help you choose a reliable method.