Ovulation Calculator

If your period started October 1 and your cycle is 28 days, your estimated ovulation day is October 15

Your estimated ovulation day is the day your next period is due, minus 14 days. October 1 plus 28 days puts your next period on October 29, so ovulation is estimated for Thursday, October 15, and your fertile window, the days when sex can lead to pregnancy, runs from 5 days before ovulation through 1 day after: October 10–16. The calculator counts back from the next period because in an average 28-day cycle ovulation happens about 14 days before the next period starts, and the stretch from ovulation to the next period is the most consistent part of the cycle (ACOG). A longer or shorter cycle therefore mostly moves the days before ovulation.

In the tool above, you can type the dates in or tap the first and last days of past periods on the calendar. Once you log two or more periods, it averages your cycle and period length, then marks your fertile window, highest-chance days, suggested days to try, the day to start ovulation tests and the earliest day for a pregnancy test. You can save the two-month calendar as an image. Every result is an estimate based on dates alone.

How the calculator works it out

1. Set your cycle length. Count from the first day of one period to the first day of the next. With several logged periods, the calculator rounds the average of the gaps between start dates (only gaps of 20 to 60 days count).
2. Next period = first day of your last period + cycle length.
3. Estimated ovulation = next period − luteal phase (14 days by default; you can set 10 to 16).
4. Fertile window = 5 days before ovulation through 1 day after (7 days). The 2 days before ovulation plus ovulation day are marked as the highest-chance days.
5. Earliest pregnancy test = the day your next period is due. If your period doesn't come, you can test from that day.

Estimated ovulation and fertile window by cycle length (last period started October 1)

The rule of thumb that you ovulate on day 14 only fits a 28-day cycle; the longer your cycle, the later you ovulate. The time from ovulation to the next period is around 10 to 16 days (NHS), so if you know your own luteal phase, change it under More settings for a closer estimate.

Why the fertile window runs from 5 days before to 1 day after ovulation

• How long sperm and eggs survive: Sperm can survive inside the body for about 3 days and sometimes up to 5 (ACOG; the NHS says up to 7), while an egg survives about 24 hours after ovulation. That is why sex any time from 5 days before ovulation until 1 day after can lead to pregnancy (ACOG).
• The highest-chance days: In a study of 221 healthy women trying to conceive, conception happened only when sex took place in the 6 days ending on ovulation day, and the chance rose from 10% for sex 5 days before ovulation to 33% on ovulation day itself (Wilcox et al., 1995). ASRM notes that the chance peaks when sex happens within 2 days before ovulation. So the calendar shades those days most strongly and keeps the suggested days to try inside the 6 days ending on ovulation day.
• Sources draw the window a little differently: ACOG counts 5 days before through 1 day after ovulation, ASRM uses the 6 days ending on ovulation day, and the Office on Women's Health says pregnancy is most likely from sex in the 3 days before and up to the day of ovulation. This calculator uses the 7-day ACOG window and adds a wider range when your logged cycles vary.

Trying to conceive: when to have sex and when to test for LH

• How often: Sex every 1 to 2 days during the 6 days ending on ovulation day gives the highest pregnancy rates, and having it more often than that doesn't lower your chances, so there's no need to hold off. ASRM leaves the exact frequency within that range to your own preference (ASRM, 2022). The ♥ marks on the calendar count back from ovulation day every other day (or every day, if you choose that setting).
• Ovulation tests (LH): Home ovulation tests detect the rise in luteinizing hormone (LH) in urine that comes before ovulation. MedlinePlus advises starting 3 to 5 days before the day you expect to ovulate and testing every day, since skipping a day can mean missing the surge; a positive result means ovulation is likely within the next 24 to 36 hours. ASRM notes that urine results match blood LH levels best with midday or evening urine. This calculator marks a start date 3 days before estimated ovulation, and if your logged cycles vary, it counts from your shortest cycle, which moves the date earlier.
• Basal body temperature: Your resting temperature rises slightly, by about 0.5–1°F, around ovulation and stays up until your next period, so a chart shows that ovulation has already happened but can't tell you in advance when it will (ACOG).
• Cervical mucus: Just before ovulation, cervical mucus increases and turns thin and slippery (ACOG), and ASRM notes that the chance of conception is highest when mucus is slippery and clear. Checking for it is a free way to see whether your body is following the calendar.

If your cycles are irregular

FIGO considers cycles irregular when the shortest and longest differ by 8 days or more at ages 26 to 41, or by 10 days or more at ages 18 to 25 and 42 to 45 (FIGO 2018). Say your periods started on July 1, July 25, August 28 and October 1. The gaps are 24, 34 and 34 days, a spread of 10 days. The average 31-day cycle puts ovulation on Sunday, October 18, but a 24-day cycle would bring it earlier and a 34-day cycle later, so the wider fertile range stretches to October 6–22. With a range that wide, ovulation tests or a doctor's evaluation will tell you more than date math. If you're trying to conceive, ASRM advises starting a fertility evaluation without waiting the usual 6 or 12 months when cycles are irregular.

Each row assumes your last period started October 1, a 14-day luteal phase and three logged start dates (two gaps). When the gaps are equal, as in the first row, the wider range is simply the 7-day fertile window. As the difference between your shortest and longest cycles grows, the span that dates alone can narrow ovulation down to widens quickly.

• Periods may not come regularly when they first start (around age 9 to 14), and ovulation can be skipped during the first few years of periods and in perimenopause (ACOG). Cycles usually become more regular within 3 years of a first period (Office on Women's Health).
• ACOG notes that fertility awareness methods are harder to use after having a baby, after recently stopping hormonal birth control and when approaching menopause, and people who are breastfeeding may or may not ovulate (Office on Women's Health). Date-based estimates are less reliable at these times.

Take a pregnancy test from the day your period is due

Most pregnancy tests can be used from the first day of a missed period. If you don't know when your period is due, test at least 21 days after you last had unprotected sex (NHS). Tests detect the pregnancy hormone hCG, which starts being produced around 6 days after fertilization, so testing too early can give a negative result even if you are pregnant. If the result is negative and your period still hasn't come, wait a few days and test again, and talk to your doctor if a second test is also negative. Once a pregnancy is confirmed, the due date calculator listed below can estimate your due date.

Common mistakes

• Counting the cycle from the day your period ends: A cycle runs from the first day of one period to the first day of the next.
• Assuming ovulation is always on day 14: That only fits a 28-day cycle. With a 35-day cycle, estimated ovulation is 21 days after your period starts.
• Judging from a single cycle: Cycle length can shift by several days from month to month. In a study of more than 1,000 cycles, the gap between periods varied by more than 5 days within 6 months in 56% of women (ASRM, 2021). The more periods you log, the better the average and range.
• Treating an app's date as exact: In a study of 949 women that compared calendar apps with urine LH tests, the best app predicted the day of ovulation only 21% of the time (ASRM, 2022). Use the calendar as a starting point.
• Assuming days outside the window are safe: Ovulation can come earlier or later than predicted, so this is not a birth control method.

Limits and cautions: this calculator is for information only

• Not a diagnosis: Results are estimates from your period dates and an average luteal phase. The timing of the fertile window can vary considerably from cycle to cycle, even when cycles are regular (ASRM, 2022), and some cycles have no ovulation at all, especially in the first years of periods and in perimenopause (ACOG; Office on Women's Health).
• Not birth control: With typical use, 12 to 24 out of 100 women relying on fertility awareness methods become pregnant in the first year, and these methods don't protect against sexually transmitted infections (ACOG). If you want to avoid pregnancy, talk to an ob-gyn about a reliable method.
• Confirming ovulation takes testing: Home ovulation tests give indirect evidence of ovulation, and a doctor can check for it with a blood progesterone test or a transvaginal ultrasound (ASRM, 2021). ASRM recommends a fertility evaluation after 12 months of trying without success if you're under 35, after 6 months if you're 35 or older, and sooner if you're over 40.
• When to see a doctor first: See a doctor if your cycles are often shorter than 21 days or longer than 35 days, if bleeding lasts more than 7 days, soaks through a pad or tampon every hour, or happens between periods or after sex (ACOG), if you haven't had a period for 90 days and aren't pregnant or breastfeeding (Office on Women's Health, “Your menstrual cycle”), or if over-the-counter pain medicine doesn't help your period pain or the pain gets in the way of work or school (Office on Women's Health, “Period problems”).
• Privacy: The dates you enter are never sent to a server. They stay in this browser only if you turn on “Save records in this browser.” A copied link carries the dates after the # in the address, which isn't sent to the server, but anyone you share the link with can see them.

As-of date The medical information in this article was checked against the original sources below on October 7, 2026. It has not been reviewed by a clinician, so talk to a doctor about your own health.

Sources

• 01American College of Obstetricians and Gynecologists (ACOG): Fertility Awareness-Based Methods of Family PlanningLast reviewed February 2025. Ovulation about 14 days before the next period, the most consistent part of the cycle; pregnancy possible from 5 days before to 1 day after ovulation (egg about 24 hours, sperm about 3 days, sometimes up to 5); basal body temperature shows ovulation only after it happens; 12–24 of 100 pregnant in the first year of typical use; no STI protection
• 02ACOG: Abnormal Uterine BleedingLast reviewed August 2025. Normal cycle 21–35 days, period up to 7 days; abnormal bleeding includes bleeding over 7 days, cycles under 21 or over 35 days, and no period for 3 to 6 months; irregular periods are common when periods start and in perimenopause
• 03Office on Women's Health (HHS): Your menstrual cyclePage last updated January 13, 2025. Pregnancy most likely from sex in the 3 days before and up to ovulation; breastfeeding women may or may not ovulate; cycles usually regular within 3 years of a first period; see a doctor after 90 days without a period if not pregnant or breastfeeding
• 04Office on Women's Health (HHS): Period problemsPage last updated September 26, 2025. Talk to a doctor or nurse if over-the-counter pain medicine doesn't help or period pain interferes with work or school
• 05MedlinePlus (National Library of Medicine): Ovulation home testReview date May 22, 2025. Begin testing 3 to 5 days before expected ovulation; a missed day can miss the surge; a positive result means ovulation in the next 24 to 36 hours
• 06American Society for Reproductive Medicine (ASRM): Optimizing natural fertility (2022)Fertile window = the 6 days ending on ovulation day, peak within 2 days before ovulation; sex every 1 to 2 days during those 6 days; calendar apps predicted the day of ovulation at most 21% of the time
• 07ASRM: Fertility evaluation of infertile women (2021)Evaluation at 12 months under 35 and 6 months at 35 or older, sooner over 40, without delay for irregular cycles; ovulation assessed with luteal progesterone, LH kits or transvaginal ultrasound
• 08Wilcox AJ, et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-21221 women, 625 cycles; conception only from sex in the 6 days ending on ovulation day; probability 0.10 five days before to 0.33 on ovulation day
• 09Munro MG, et al. The two FIGO systems for normal and abnormal uterine bleeding symptoms: 2018 revisions. Int J Gynaecol Obstet 2018;143:393-408FIGO AUB System 1: regularity defined by the shortest-to-longest cycle difference. This page uses only FIGO's regularity criteria; the 21–35-day typical range comes from ACOG and the NHS
• 10Jain V, Munro MG, Critchley HOD. Contemporary evaluation of women and girls with abnormal uterine bleeding: FIGO Systems 1 and 2. Int J Gynaecol Obstet 2023;162(Suppl 2):29Regular cycles vary by up to 9 days at ages 18–25 and 42–45 and up to 7 days at ages 26–41
• 11NHS: Periods and fertility in the menstrual cyclePage last reviewed January 5, 2023. Egg release to the next period around 10 to 16 days; sperm can survive up to 7 days
• 12NHS: Doing a pregnancy testPage last reviewed February 9, 2022. Most tests work from the first day of a missed period, or 21 days after unprotected sex; hCG is produced from around 6 days after fertilization

Frequently asked questions

Related tools