Ovulation Calculator
Enter the first day of your last period and your usual cycle length to see when you are likely to ovulate and your most fertile days over the next 3 months.
Days from the first day of one period to the first day of the next.
Likely ovulation days
Enter the first day of your last period to see your likely ovulation days and fertile days.
How the dates are worked out
The first day of your period is day 1 of your menstrual cycle [1]. The NHS says it's difficult to pinpoint exactly when ovulation happens, but in most women it happens around 10 to 16 days before the next period [1].
This calculator adds your cycle length to the first day of your last period to estimate when your next period will start. It then counts back 10 to 16 days from that date to show your likely ovulation days.
Sperm can survive in the fallopian tubes for up to 7 days after sex [1]. So the fertile days in your results start 7 days before the first likely ovulation day and end on the last one. They never start before day 1 of your cycle.
Worked example
If your last period started on October 1, 2026 and your cycle is 28 days, your next period is expected on October 29, 2026. Your likely ovulation days are October 13 to 19, and your fertile days are October 6 to 19.
With a 35-day cycle, the next period is expected on November 5, the likely ovulation days are October 20 to 26 and the fertile days are October 13 to 26. With a 21-day cycle, the next period is expected on October 22, the likely ovulation days are October 6 to 12 and the fertile days are October 1 to 12, starting on day 1 of the cycle.
Women who have a regular 28-day cycle are likely to be fertile around day 14 of their menstrual cycle, but this won't apply to women whose cycles are shorter or longer [1]. In the 28-day example, day 14 is October 14, inside both ranges.
Which cycles it works for
The average is to have periods around every 28 days, and regular cycles that are longer or shorter than this, from 21 to 35 days, are normal [1]. Periods are irregular if the gap between them is less than 21 days or more than 35 days [4]. This calculator gives dates only for regular cycles of 21 to 35 days.
If your periods are irregular, see a doctor or GP [4]. Getting pregnant can be more difficult with irregular periods, because you might not release an egg regularly [4].
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Not a method of contraception
Do not use this calculator to prevent pregnancy. It can't tell you whether or exactly when you ovulate. It's possible, although not very likely, to get pregnant soon after your period finishes if you ovulate early or have a short menstrual cycle [1].
Fertility awareness is a type of natural family planning: you track your periods and other fertility signs to see when you're most fertile, and avoid sex or use condoms on those days [5]. You need an expert, such as a fertility awareness practitioner or a midwife, to teach you how to use it [5]. When used correctly all the time, fertility awareness methods are between 91% and 99% effective at preventing pregnancy, but only 76% effective if the instructions are not followed exactly, which means 24 in 100 women will get pregnant when tracking their fertility for a year [5].
Fertility awareness may not be suitable if you have irregular periods [5]. There are apps and fertility monitoring devices designed to help you track your fertile days, but none are officially recommended by the NHS [5].
Signs around ovulation
Vaginal secretions (sometimes called vaginal discharge) change during the menstrual cycle. Around the time of ovulation, they become thinner and stretchy, a bit like raw egg white [1]. See a GP if you are concerned about a change in your vaginal discharge [1].
For more on body signs, read Signs of Ovulation: How to Know You're Fertile.
If you are trying to get pregnant
You're more likely to get pregnant around the time you ovulate [2]. The NHS says that if you're aged 39 or under and have sex without contraception every 2 to 3 days, it's likely you'll get pregnant within a year [2].
The NHS says: "It's a good idea to see a GP if you have not conceived after a year of trying. Women aged 36 and over, and anyone who's already aware they may have fertility problems, should see their GP sooner." [3]
Read more: When to See a Fertility Doctor.
What this calculator can't tell you
It can't tell you whether you have ovulated or whether you are pregnant. The NHS says that if you think you might be pregnant, you can find out by doing a pregnancy test [6].
It assumes every cycle is the same length as the one you choose. Cycles can change from month to month, so the dates further ahead are less certain.
Frequently asked questions
When am I most likely to get pregnant?
Around the time you ovulate [2]. Ovulation is usually about 14 days before the start of your next period, but this depends on how long your menstrual cycle is [2]. In most women it happens around 10 to 16 days before the next period, which is the range this calculator shows [1].
How many fertile days are there?
With this method, up to 14 days in each cycle: the 7 likely ovulation days and the 7 days before them, because sperm can survive for up to 7 days after sex [1]. With a cycle shorter than 23 days there are fewer, because the fertile days never start before day 1 of your cycle.
Is ovulation always on day 14?
No. In most women ovulation happens around 10 to 16 days before the next period, so the day depends on how long your cycle is [1]. Women who have a regular 28-day cycle are likely to be fertile around day 14 [1].
Can I use this calculator to avoid pregnancy?
No. It is not a method of contraception. Fertility awareness needs to be taught by an expert, such as a fertility awareness practitioner or a midwife [5].
What if my periods are irregular?
This calculator gives dates only for regular cycles of 21 to 35 days. The NHS says periods are irregular if the gap between them is less than 21 days or more than 35 days, and advises seeing a GP if your periods are irregular [4].
How often should we have sex when trying to get pregnant?
Every 2 to 3 days, without contraception. If you're aged 39 or under, it's likely you'll get pregnant within a year this way [2].
Sources and how we check this tool
Last checked against NHS: October 27, 2026
This calculator is based on NHS guidance on the menstrual cycle [1]: your next period is estimated as the first day of your last period plus your cycle length, the likely ovulation days are 10 to 16 days before that, and the fertile days add the 7 days before the first likely ovulation day. The 21 to 35 day range follows the NHS [1][4].
We re-check our sources every 6 months and whenever a source updates.
- NHS. Periods and fertility in the menstrual cycle. Page last reviewed: January 5, 2023; next review was due January 5, 2026. https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
- NHS. Trying to get pregnant. Page last reviewed: June 2, 2026. https://www.nhs.uk/pregnancy/trying-for-a-baby/trying-to-get-pregnant/
- NHS. Infertility. Page last reviewed: August 9, 2023; next review was due August 9, 2026. https://www.nhs.uk/conditions/infertility/
- NHS. Irregular periods. Page last reviewed: June 26, 2026. https://www.nhs.uk/symptoms/irregular-periods/
- NHS. Natural family planning (fertility awareness). Page last reviewed: February 9, 2024. https://www.nhs.uk/contraception/methods-of-contraception/natural-family-planning/
- NHS. Signs and symptoms that might mean you're pregnant. Page last reviewed: July 31, 2026. https://www.nhs.uk/pregnancy/trying-for-a-baby/signs-and-symptoms-that-might-mean-youre-pregnant/
This is an estimate for regular cycles. It is not a method of contraception, and it can't tell you whether you have ovulated. This calculator is for general information and is not medical advice, and it does not diagnose any condition.
Estimate based on NHS guidance on the menstrual cycle | med.com/ovulation-calculator | Not a method of contraception.
