Fertile window calculator

The fertile window is the six days that end on ovulation. Here are your next three, so you can see the pattern.

Your details

Your next fertile window

18 Oct to 23 Oct

six days ending on ovulation day

18 Octabout 10% chance from sex that day
19 Octabout 16% chance from sex that day
20 Octabout 14% chance from sex that day
21 Octabout 27% chance from sex that day
22 Octabout 31% chance from sex that day
23 Oct, ovulation dayabout 33% chance from sex that day
Next cycle15 Nov to 20 Nov
The one after13 Dec to 18 Dec
When to see your GPafter a year of trying
Over a yearover 80% of couples conceive within a year of regular sex (NICE CG156), and about half of the rest in the second year

Sperm can live up to about 5 days, and an egg about a day, which is why the window opens before ovulation. Ovulation day is an estimate, so the chances by day are a guide. Sex every 2 to 3 days through the month covers the window without needing exact dates.

Trying for a year (6 months if 36 or over) without a pregnancy is the time to see your GP.

The working

Why six days

Sperm can survive up to about 5 days and an egg about 24 hours, so a large 1995 study (Wilcox) found pregnancy happened only from sex in the 5 days before ovulation or on the day itself. For a 28 day cycle starting 1 October, that is 10 to 15 October.

The same study gives a chance for each day: about 10% from sex 5 days before ovulation, 16% and 14% on the next two days, then 27%, 31% and 33% on the 2 days before and the day itself. The page puts those against your dates. They come from 221 couples, so they are a guide to which days matter most, not a prediction for you.

Your age sets when to see a GP: after a year of trying, or after 6 months from 36 (NHS). NICE CG156 says over 80% of couples conceive within a year of regular sex when the woman is under 40, and about half of the rest in the second year.

Best chances

Within the window

Day	Chance in Wilcox 1995
5 days before ovulation	about 10%
2 days before	about 30%
Ovulation day	about 33%
Day after	very low

When to get help

Timescales

Most couples conceive within a year. NICE suggests seeing your GP after a year of trying, or after 6 months if you are 36 or older, or sooner if you have irregular periods or a known condition.

Using ovulation tests

Your own window this cycle

Ovulation tests pick up the rise in luteinising hormone (LH) that comes 24 to 36 hours before ovulation, so the day of a positive test and the day after are the most fertile. Add the cycle day of your positive test and we move the window to match: a positive on day 16 of a cycle that started 9 October means ovulation around 25 October and a window of 20 to 25 October, 2 days later than the calendar guess of day 15 for a 28 day cycle. If you test for three cycles without a positive, Fertility Network UK suggests asking your GP.

Read with care

What this cannot tell you

Irregular cyclesWindows move when cycles vary.
Your fertilityIt cannot say whether you or your partner are fertile.
ContraceptionDo not use these dates to avoid pregnancy.

Questions

Common questions

What is the fertile window?

The six days ending on ovulation day when sex can lead to pregnancy.

What are the most fertile days?

The two days before ovulation and ovulation day.

How long does it take to get pregnant?

About 8 in 10 couples conceive within a year.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Support when you need it

Contact your midwife or GP any time you are worried, and call NHS 111 if you cannot reach them. If a date or a result brings up difficult feelings, Tommy's and the Miscarriage Association offer kind, free support.