What the ovulation calculator does
This ovulation calculator estimates the days in your menstrual cycle when you are most likely to get pregnant. You enter the first day of your last period and your average cycle length, and it shows your estimated ovulation day, your fertile window, the best days to try and the day your next period is likely to start. A calendar lays out the next 3 or 6 cycles, so you can plan ahead. If your cycles change from month to month, you can add your shortest and longest cycle and get a wider window that covers both.
Everything is worked out in your browser. Health dates are private, so the calculator never puts them into a link, and it only keeps them on your device if you ask it to.
How ovulation and the fertile window are estimated
The NHS says ovulation, the release of an egg, is usually about 14 days before the start of your next period, though it depends on how long your cycle is. So the calculator adds your cycle length to the first day of your last period to find your next period, then counts back 14 days to estimate ovulation.
You can only get pregnant on a few days around ovulation. The Office on Women's Health (OWH) at the US Department of Health and Human Services says there are about 6 days in each menstrual cycle when you can get pregnant, called your fertile window. NICHD, an institute of the National Institutes of Health, adds that sperm can remain functional for 5 days, while the egg can be fertilized for only a short time, usually between 8 and 12 hours, and that conception can only happen in the days before ovulation through the day of ovulation. The calculator therefore marks the 5 days before the estimated ovulation day plus that day as your fertile window. NICHD calls the 2 days before ovulation through the day of ovulation the best time to try.
Example: a period that started on Thursday, October 1, 2026, with a 28-day cycle, puts the next period on Thursday, October 29. Ovulation is then estimated for Thursday, October 15, the fertile window runs from October 10 to October 15, and the best days are October 13 to 15. With a 32-day cycle, everything moves 4 days later: ovulation around October 19 and the next period on November 2.
Finding your cycle length
Your cycle length is the number of days from the first day of one period to the first day of the next. The OWH suggests marking the day each period starts on a calendar; after a few months, you can see whether your periods are regular or different each month. The OWH calls 28 days the typical cycle length, but each woman is different, so your own average gives a better estimate than the default.
Cycles vary from person to person. The NHS says regular cycles from 21 to 35 days are normal. The OWH gives a normal range of 24 to 38 days and advises talking to your doctor or nurse if your cycles are longer than 38 days or shorter than 24 days. If your cycles change from month to month, tick "My cycles vary" and enter your shortest and longest recent cycle. The calculator then shows ovulation as a range, from the earliest day that fits your shortest cycle to the latest day that fits your longest, and widens the fertile window to match. Later cycles are counted with your average length, so treat the dates further out as rougher.
Why this is only an estimate
A calendar method can only work with averages. The NHS notes that it is difficult to pinpoint exactly when ovulation happens and that in most women it happens around 10 to 16 days before the next period, not always exactly 14. A study by researchers at the National Institute of Environmental Health Sciences, part of the NIH, found that the timing of the fertile window is highly variable, even among women who regard their cycles as regular. In only about 30% of the women did the fertile window fall entirely within days 10 to 17 of the cycle, the days that clinical guidelines identified as fertile at the time.
Your body gives clues that a calendar cannot. The OWH says that a few days before you ovulate, your vaginal mucus becomes more slippery and clear, and that over-the-counter ovulation kits and fertility monitors can help find the best time to conceive. Used together with the calendar, these signs make the estimate more personal.
Not a method of birth control
Please do not use this calculator to avoid pregnancy. The OWH says its own ovulation calculator provides an estimate of your fertile window and is not a guarantee of pregnancy or of birth control. The NHS says fertility awareness methods are between 91% and 99% effective when used correctly all the time, but only 76% effective if you do not follow the instructions exactly, which means about 24 in 100 women get pregnant in a year. It also says that you need an expert such as a fertility awareness practitioner or a midwife to teach you, and that they do not protect against sexually transmitted infections. A calculator alone is not that method. Talk to your doctor or nurse to find birth control that works for you.
When to talk to a doctor
The OWH suggests seeing a doctor if you are younger than 35 and have not been able to conceive after one year of frequent sex without birth control, or if you are 35 or older and have not conceived after six months. It also advises talking to your doctor or nurse if you have not had a period for 90 days and are not pregnant or breastfeeding. If anything about your cycle worries you, talk to a doctor or nurse. Once you are pregnant, the due date calculator estimates your due date from the same first day of your last period.
How to use the ovulation calculator
- 1Enter the first day of your most recent period and your average cycle length. If your cycles vary, tick "My cycles vary" and add your shortest and longest cycle.
- 2Read your estimated ovulation day, fertile window, best days and next period, and choose whether the calendar shows 3 or 6 cycles.
- 3Copy the result for your own notes, or tick "Remember on this device" to find your dates here next time. Nothing is sent anywhere.
Sources
- Trying to get pregnant, NHS. Ovulation usually about 14 days before the next period.
- Periods and fertility in the menstrual cycle, NHS. Ovulation around 10 to 16 days before the next period, cycles of 21 to 35 days.
- Natural family planning (fertility awareness), NHS. Effectiveness, teaching and STI protection.
- Ovulation calculator, Office on Women's Health, US Department of Health and Human Services. About 6 fertile days per cycle; not a guarantee of pregnancy or of birth control.
- Your menstrual cycle, Office on Women's Health. Normal cycle of 24 to 38 days, tracking, mucus changes, when to talk to a doctor or nurse.
- Trying to conceive, Office on Women's Health. Ovulation kits and when to see a doctor.
- Women's health infographic: conception, NICHD, National Institutes of Health. Fertile days, best time to try, survival of sperm and egg.
- The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study, Wilcox AJ, Dunson D and Baird DD, National Institute of Environmental Health Sciences, BMJ, 2000.
Frequently asked questions
When am I most fertile?
In the days just before ovulation and on the day itself. The Office on Women's Health says there are about 6 days in each cycle when you can get pregnant, your fertile window. NICHD, part of the NIH, says the best time to try is the 2 days before ovulation through the day of ovulation. The calculator marks both on the calendar.
How accurate is an ovulation calculator?
It gives an estimate, not a guarantee. It counts back about 14 days from your expected next period, which is when the NHS says ovulation usually happens, but the NHS also says it can be anywhere from about 10 to 16 days before the next period. An NIH study found that the timing of the fertile window can be highly unpredictable, even in women whose cycles are usually regular. Signs like changes in vaginal mucus, or over-the-counter ovulation kits, can help you narrow it down.
Can I use it if my periods are irregular?
You can, but the estimate gets less reliable. If your cycles vary, turn on "My cycles vary" and enter your shortest and longest recent cycle: the calculator then shows a wider window that covers both. The Office on Women's Health advises talking to your doctor or nurse if your cycles are longer than 38 days or shorter than 24 days, or if you have not had a period for 90 days and are not pregnant or breastfeeding.
Can I use the ovulation calculator as birth control?
No. The Office on Women's Health says its own ovulation calculator is not a guarantee of pregnancy or of birth control, and the same is true here. The NHS says fertility awareness as a method of contraception has to be taught by an expert, such as a fertility awareness practitioner or a midwife, and that it does not protect against sexually transmitted infections. Talk to your doctor or nurse to find birth control that works for you.
How long should we try before seeing a doctor?
The Office on Women's Health suggests seeing a doctor if you are younger than 35 and have not been able to conceive after one year of frequent sex without birth control, or if you are 35 or older and it has not happened after six months. It also advises not to delay, as age affects the success rates of fertility treatments.
Are my dates saved or shared?
No. The calculation runs in your browser and nothing is sent to a server. Your dates are not put into any link. They are kept on this device only if you tick "Remember on this device", and unticking it deletes them again.