Ovulation calculator: your fertile days and real chances

Find your fertile days and your chance of getting pregnant on each day of your cycle, based on European research that followed almost 800 couples as they charted their cycles.

When are you fertile, and how good are your chances of getting pregnant? Fill in the first day of your last period and the calculator shows your fertility calendar: for each day, the chance that you're fertile and the chance of getting pregnant that day.

Nothing you enter is sent to us. It's all worked out in your own browser.

Why this calendar is different

Most ovulation calculators assume you ovulate on one fixed day, 14 days before your next period. Research keeps showing that it doesn't work that way. Ovulation moves from cycle to cycle and differs from woman to woman, even when your cycle is regular. A fixed fertile window can point you to days with little chance and make you miss the days that count.

So this calculator works with the real chance for each day, based on European research that followed almost 800 couples as they charted their cycles. It takes into account how long your cycle is, whether it's regular, your age and, if you know it, how fertile you are as a couple.

When are your chances highest?

Looking back from the day you ovulated, your chances are highest in the two days before it. On the day of ovulation itself the chance is already lower, and a day later it's almost gone. That's why sex in the days before you expect to ovulate matters most. Sperm can survive for several days, but the egg lives for less than a day.

Your cycle length and regularity

Because ovulation moves around, your fertile days are more spread out over your cycle than most people think. With a short cycle (under 28 days), your fertile window tends to come earlier. With a long cycle (30 days or more), it comes later and is more spread out.

With an irregular cycle, the chance per day is a bit lower and the peak comes later. That's why the calculator asks whether your cycle is regular.

How your age changes the odds

Your age is one of the biggest factors in your chance of getting pregnant. Fertility starts to drop gradually in your early 30s, and faster after about 35. If you enter your age, the calculator adjusts your chances to match.

Some couples are more fertile than others, but most don't know which group they're in until they start trying. If you're not sure, leave it on "average".

The research in charts

These are the figures the calculator works with.

When to see a doctor

Most couples get pregnant within a year. See your doctor (UK: GP) if:

  • you're under 35 and haven't conceived after 12 months of regular sex without contraception;
  • you're 35 or older and it hasn't happened after 6 months;
  • you're over 40, have very irregular or no periods, or know of a reason it may be harder (such as endometriosis): go before you start trying or soon after.

In the UK, the NHS says women aged 36 and over should see their GP sooner than a year.

FAQ

Can I get pregnant right after my period?

It's possible, but not likely. With a short cycle you can ovulate early, and sperm can survive for several days. The calendar shows that chance for your own cycle.

Can I get pregnant on the day after ovulation?

Hardly. The egg lives for less than a day, so by the day after ovulation the chance is close to zero.

Do ovulation tests help?

They can. An ovulation test picks up the LH surge that comes a day or two before you ovulate, so it shows when your best days start in this cycle. Use the calendar to know when to start testing.

Going through this right now? Talk to other parents in our Fertility forum.

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