Ovulation calculator

Ovulation calculator: find your most fertile days

Enter two details and instantly see your fertile window, your expected ovulation and what the coming months look like.

  • Based on medical guidelines
  • Calculated on your device
  • Done in 30 seconds

Your cycle

Not sure? Leave "28 days (average)" as it is.
Your details never leave your device and are not stored.
Your fertile window
● ovulation
Fertile window
Expected ovulation
Expected next period

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What these dates mean

Your fertile days are the five days before ovulation and the day of ovulation itself. Sperm can survive for a few days, while an egg can only be fertilised for about a day. That's why sex in the days before ovulation gives just as much chance as on the day itself.

This calculation assumes a regular cycle. Your ovulation can shift by a few days from cycle to cycle, so treat the window as your best chance period rather than an exact moment. Not sure whether your cycle is regular? Having sex every 2 to 3 days throughout your cycle works best.

SOURCES1. NHS: Trying to get pregnant  ·  2. ASRM: Optimizing natural fertility  ·  3. Office on Women's Health: Trying to conceive
This result is a guide. It never replaces advice from your GP or gynaecologist. Not suitable as contraception.

How the ovulation calculator works

Ovulation happens on average 14 days before your next period. The ovulation calculator therefore adds your cycle length to the first day of your last period, which gives the expected start of your next cycle. Fourteen days before that is your expected ovulation. Your fertile days are the five days before it plus the day itself: six days per cycle in which you can get pregnant.

With a 28-day cycle, ovulation falls around day 14. With a 32-day cycle, around day 18. That's why the calculator asks for your average cycle length: it shapes your result more than most people think.

When are you most fertile?

Your fertile window falls in the days before and on ovulation. Sperm survives for up to a few days in your body, while an egg can only be fertilised for around a day after ovulation. Sex before ovulation therefore gives just as much chance as on the day itself. Trying for a baby? The two to three days before ovulation, together with the day itself, are your best moment.

Ovulation calculator for irregular cycles

Does your cycle length vary a lot, or is it shorter than 21 or longer than 35 days? Then a calendar calculation isn't reliable enough to plan on, because your ovulation shifts with every cycle. What works best then: sex every 2 to 3 days throughout your cycle, or an ovulation test. It measures the hormone that surges just before ovulation, which makes it more precise than any calculation.

Signs of ovulation

Many women notice ovulation in their body: clear, stretchy discharge that looks like raw egg white, a mild stabbing pain low in the belly, tender breasts or a little more desire. Your basal body temperature rises slightly after ovulation. These signs differ per woman and per cycle. Treat them as extra clues alongside the calculation.

Frequently asked questions

When is your fertile window?
Six days per cycle: the five days before ovulation and the day itself. Enter your last period and cycle length above and you'll see exactly which dates that means for you.
How accurate is an ovulation calculator?
An ovulation calculator gives a good indication, because it works with the fixed pattern of your cycle. Your ovulation can still shift by a few days per cycle, even when your cycle is regular. So treat the result as your best chance period. Want more certainty? An ovulation test measures the hormone that surges just before ovulation and is more accurate than any calculation.
What if my periods are irregular?
With a strongly varying cycle, or one shorter than 21 or longer than 35 days, a calendar calculation isn't reliable enough to plan on. Sex every 2 to 3 days throughout your cycle then gives the best chance. An ovulation test helps you spot ovulation anyway.
What are the signs of ovulation?
The best-known signs are clear, stretchy discharge, a mild stabbing pain low in the belly and tender breasts. Note: your body temperature only rises after ovulation, so it confirms afterwards rather than predicting. Want to be sure? An ovulation test is the most reliable check.
How long does ovulation last?
Ovulation itself is a brief moment: the egg is released and can be fertilised for around 12 to 24 hours. Your fertile period is longer, because sperm survives for up to a few days in your body. That's why the days before ovulation count fully.
When can I take a pregnancy test?
Test from the day your period was due at the earliest. The calculator shows that date with your result. Testing earlier increases the chance of a false negative, because the pregnancy hormone may still be too low.
Can you get pregnant outside your fertile days?
The chance is by far the biggest within your fertile window. But because ovulation can shift each cycle, days outside the calculated window are never guaranteed infertile. Never use this calculator as contraception.
What if getting pregnant isn't happening?
Give it time: more than 8 in 10 couples under 40 who have sex regularly conceive within a year. Trying for 6 months and aged 35 or over? Or 12 months and younger? Then discuss it with your GP.
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