Ovulation Calculator & Fertile Window

Find the days you are most likely to conceive, and when your next period is due — for any cycle length, not just 28 days.

Any Cycle, 21–60 Days

3-Month Calendar

Nothing Leaves Your Device

Your Cycle

First day of your last period

The first day you bled, not the day it ended.

Cycle length 28 days

First day of one period to the first day of the next. Leave it at 28 if you are not sure.

Luteal phase 14 days

Ovulation to the next period. Only change this if you have been told yours is different.

Ovulation is most likely on

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Fertile today?

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Cycle day

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Fertile window

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Next period due

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Key Dates

this cycle and the next three

Calendar

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Period Fertile window Ovulation Today

These dates are an estimate, not a measurement. A calculator works from averages, and ovulation moves — illness, stress, travel and poor sleep can all shift it by days. Use the fertile window as a guide rather than the single date, and speak to a doctor if you have been trying for a year (or six months if you are over 35), if your cycles are very irregular, or if they have changed suddenly.

Do not use this to avoid pregnancy. Calendar methods are among the least reliable forms of contraception, because ovulation moves and sperm survive for several days. The days outside the fertile window are not safe days. For contraception, talk to a doctor or pharmacist about a method that has a proper failure rate behind it.

Your privacy is protected! The dates you enter are used to work out your results and never leave this page.

How to use the ovulation calculator

A free ovulation calculator answers one question: which days this month are you most likely to conceive on? It needs two things — when your last period started, and how long your cycles usually run. Everything below runs in the browser, so this works as an online ovulation calculator with nothing to install and nothing to sign up for.

Enter the first day of your last period. The day bleeding started, not the day it stopped.
Set your cycle length. Count from the first day of one period to the first day of the next. This is the setting that matters most, and the one most calculators quietly assume.
Read the fertile window, not just the date. The single ovulation date is the middle of a range, and the range is what you can actually plan around.

If your cycles vary, switch to the Irregular tab and enter the start dates of your last two or three periods. The calculator averages them and tells you how much they varied, which matters more than the average itself.

Why ovulation is not always day 14

Day 14 is not a rule. It comes from a 28-day cycle, and it is the answer only for that length. The stable part of a cycle is the second half — the luteal phase between ovulation and the next period, which runs about 14 days for most people. The first half is what stretches or shortens.

So ovulation is counted backwards from the next period, not forwards from the last one:

Your cycle	Ovulation	Fertile window
21-day cycle	7 days after it starts	2–8 days after
24-day cycle	10 days after it starts	5–11 days after
28-day cycle	14 days after it starts	9–15 days after
30-day cycle	16 days after it starts	11–17 days after
35-day cycle	21 days after it starts	16–22 days after
40-day cycle	26 days after it starts	21–27 days after

This is why a 35-day cycle tracked as though it were 28 days sends people looking a full week too early. If you take one thing from this page, take that.

A note on counting, because it trips people up. These are measured from the day your period starts, so on a 28-day cycle ovulation is 14 days after that day. If you number the first day of bleeding as day 1, the same moment is cycle day 15 — which is why you will see both “day 14” and “day 15” quoted for a 28-day cycle. It is one event counted from two different starting numbers, not a disagreement about biology. The Cycle day figure in the results above uses the day-1 numbering.

Why the window is six or seven days, not one

The egg is only viable for about twelve to twenty-four hours after release, so on its own ovulation would be a single-day event. Sperm are what widen it: they can survive in the reproductive tract for up to five days waiting for an egg. That is why intercourse several days before ovulation can still result in pregnancy, and why the window opens well before the ovulation date and closes almost immediately after it.

The two best days are the day before ovulation and the day of ovulation itself, which the calculator marks separately.

Irregular cycles

Averaging works when your cycles are close together. If yours range from 26 to 30 days, an average of 28 puts ovulation within a day or two of the truth. If they range from 24 to 45, the average is arithmetic rather than information — ovulation could genuinely fall anywhere across three weeks, and no calendar can narrow that down.

In that case the useful tools are physical rather than mathematical: ovulation predictor kits, which detect the hormone surge a day or so before it happens, or basal body temperature tracking, which confirms afterwards that it did. No fertility calculator working from dates alone can beat a test that measures what your body is doing. The calculator tells you when your cycles varied by more than a week so you know which situation you are in.

When to take a pregnancy test

Wait for the date shown as your next period. Testing before that often produces a false negative even when conception happened, simply because there is not yet enough hormone to detect. If your period does not come and the test is negative, wait a few days and test again. Once you have a positive, the Pregnancy Calculator will work out a due date from the same last-period date you entered here.

What this calculator will not do

It will not tell you whether you are having a boy or a girl, and it will not offer to time anything to influence that. The idea circulates widely, but the evidence does not support it, and a calculator that gives you a confident answer is producing one out of nothing.

It is also not a contraceptive. Calendar methods fail often enough that no one should rely on them to avoid pregnancy, and the days marked as outside the window are not safe days.

Frequently Asked Questions

It counts backwards from your next period rather than forwards from your last one. Ovulation happens roughly 14 days before a period starts, and that gap is fairly stable, so the calculator adds your cycle length to the first day of your last period to find the next one, then subtracts 14 days. The fertile window is the five days before ovulation plus the day itself, because sperm can survive that long while they wait.

The two most fertile days of a cycle are the day before ovulation and the day of ovulation itself. The window is wider than that because sperm survive up to about five days, so intercourse several days before ovulation can still result in pregnancy. After ovulation the egg is only viable for around twelve to twenty-four hours, which is why the window closes quickly.

Only if your cycle is 28 days long. Day 14 comes from counting 14 days back from a 28-day cycle, not from anything fixed about day 14. On a 35-day cycle ovulation is nearer day 21, and on a 24-day cycle nearer day 10. This is the single most common mistake people make with cycle tracking, and it is why this calculator asks for your cycle length rather than assuming one.

Yes, with a caveat. Switch to the Irregular tab and enter the start dates of your last two or three periods, and the calculator averages them. But an average is only a useful guide when your cycles are reasonably close together. If they vary by more than a week, ovulation could fall anywhere across a wide span and no calendar method will pin it down. Ovulation test strips or basal body temperature tracking are more reliable in that situation.

It is an estimate based on averages, not a measurement. Even with regular cycles, ovulation can shift by a few days from month to month because of illness, stress, travel or sleep. Treat the fertile window as the useful output rather than the single ovulation date, and use ovulation predictor kits or temperature tracking if you need to know what your body is actually doing this cycle.

The luteal phase is the stretch between ovulation and your next period. It is usually about 14 days and varies less between people than the first half of the cycle does, which is what makes counting backwards work. Some people have a consistently shorter luteal phase of 10 to 12 days, and if a doctor or your own tracking has told you yours, changing the setting moves the ovulation date to match.

No. Calendar methods are among the least reliable forms of contraception, because ovulation moves and sperm survive for days. Do not treat the days outside the fertile window as safe. If you are trying to avoid pregnancy, speak to a doctor or pharmacist about a method with a proper failure rate behind it.

No, and this tool does not try. The idea that timing intercourse relative to ovulation influences the sex of a baby is often repeated online, but studies have not found a reliable effect. Any calculator that gives you an answer is inventing one.

Wait until the day your period is due, which this calculator shows you. Testing earlier often gives a false negative because there is not yet enough hormone to detect, even if conception happened. If your period does not arrive and the test is negative, test again a few days later before assuming either way.

No. Every date you type is used in your browser and nowhere else. Nothing is uploaded, nothing is saved, and the page keeps working with no connection once it has loaded.