Ovulation Calculator: Find Your Most Fertile Days

How your fertile days are worked out

Ovulation happens about 14 days before your next period. The calculator uses your cycle length to find that day, so with a longer cycle you ovulate later and with a shorter cycle earlier. It’s the same formula our pregnancy calculators use, so if you get pregnant, our due date calculator picks up from the same assumptions.

Your fertile days are ovulation day and the 5 days before it. Sperm can survive in the reproductive tract for several days, so sex a few days before ovulation can still lead to pregnancy. After ovulation, the chances drop quickly.

How reliable is a calendar?

A calendar assumes an average cycle. In reality, ovulation can move by a few days from month to month, even in the same person; stress, illness, travel and breastfeeding all play a part. So:

• Use the calendar as a guide to when your fertile window might open.
• For a more personal answer, start ovulation tests a few days before the calendar’s fertile days.
• This calendar is not birth control. It isn’t reliable for preventing pregnancy.

What guidelines say: time it, or just keep at it?

The UK’s national guideline (NICE) recommends regular sex every 2 to 3 days to improve your chances, rather than aiming for one specific day. That approach absorbs the uncertainty in predicting ovulation and takes away the stress of “missing the day.” The calendar still helps by showing which week matters most.

When should you see a doctor? US and UK guidance set slightly different timelines:

Don’t wait that long if your periods are very irregular, you’ve had a pelvic infection or surgery, or your partner has a known fertility problem.

Is your period late?

If the date you enter suggests your period is late, the calculator will tell you. Home pregnancy tests are most reliable from the day your period is due. If it’s positive, you can work out your due date or make sense of blood test results with our hCG calculator.