Ovulation Calculator

This ovulation calculator turns the first day of your last period, your average cycle length and your luteal phase into an estimated ovulation date, a fertile window and six projected cycles. It counts the way ACOG does: day 1 is the first day of bleeding, and ovulation comes about 14 days before the next period.

Once you are pregnant, the Pregnancy Due Date Calculator and the Gestational Age Calculator take over, and the Reverse Due Date Calculator works back from a due date to the likely conception date.

How ovulation fits into your menstrual cycle

A cycle runs from the first day of one period to the first day of the next. ACOG gives 28 days as the average, NICHD calls 21 to 35 days normal, and womenshealth.gov uses 24 to 38 days for adults, adding that cycles over 38 days are common for a few years after a first period. That is why the calculator accepts 21 to 45 days.

In the first half, the follicular phase, an egg matures; this phase stretches or shrinks when a cycle runs long or short. A surge of luteinizing hormone (LH), starting about 36 hours before ovulation (womenshealth.gov), then triggers ovulation. The luteal phase that follows is the most consistent part of the cycle (ACOG); a study of 327 luteal phases averaged 14.13 days.

The egg can be fertilized for only about 12 to 24 hours (womenshealth.gov and ACOG; NICHD says usually 8 to 12), while sperm survive about 3 days and sometimes up to 5. That is why the fertile window sits mostly before ovulation.

The ovulation formula

With the default 14-day luteal phase, the first line is ACOG's rule of thumb, cycle length minus 14, and it puts ovulation exactly 14 days before your next period. ACOG Committee Opinion 700 bases the 280-day due date on ovulation on the 14th day after the period starts in a 28-day cycle, so the last line matches the site's Pregnancy Due Date Calculator, which adds 266 days to a known conception date. You will often see ovulation described as "day 14"; counted with the first day of bleeding as day 1, the 14th day after it is cycle day 15, and the calendar above labels it that way.

Worked example: a 28-day cycle starting October 1, 2026

Your last period began on Thursday, October 1, 2026, your cycles average 28 days, and you have never measured your luteal phase, so it stays at 14.

• Days to ovulation: 28 − 14 = 14 days after the period starts.
• Ovulation date: October 1 plus 14 days = Thursday, October 15, 2026, cycle day 15 and 14 days before the next period.
• Fertile window: Saturday, October 10 to Thursday, October 15.
• Most fertile days: Tuesday, October 13 to Thursday, October 15.
• Next period and first pregnancy test day: Thursday, October 29, 2026.
• Due date if you conceive this cycle: Thursday, July 8, 2027, October 15 plus 266 days, or October 1 plus 280 days.

Enter October 1, 2026 with the default 28 and 14 and the calculator reproduces every one of these dates.

Second worked example: a 33-day cycle

The last period began on Friday, September 18, 2026, cycles run 33 days, and the luteal phase is 14. Ovulation moves to 19 days after the period starts, Wednesday, October 7, 2026 (cycle day 20). The fertile window runs from Friday, October 2 to Wednesday, October 7, the most fertile days are October 5 to 7, and the next period is due on Wednesday, October 21. A pregnancy from this cycle would be due around Wednesday, June 30, 2027, 285 days after the last period instead of 280. The five extra days all sit before ovulation, which fits Fehring's finding from 1,060 cycles that the follicular phase drives most of the variation.

Why the luteal phase setting matters

Leave the luteal phase at 14 unless LH tests or temperature charts have shown you a different length. For a 26-day cycle that started on Monday, October 5, 2026, the default puts ovulation 12 days later, on Saturday, October 17 (cycle day 13). A measured 12-day luteal phase moves it to Monday, October 19 (cycle day 15) and the due date from July 10 to July 12, 2027, while the next period stays on October 31. For a 30-day cycle that started on October 1, 2026:

Lenton and colleagues classed luteal phases of 9 days or fewer as abnormal, so the calculator flags them. womenshealth.gov puts the time from ovulation to the next period anywhere from 7 to 19 days, the range the input accepts.

Ovulation day and fertile window by cycle length

Each row assumes a 14-day luteal phase and counts the first day of your period as day 1.

Each extra day of cycle length pushes everything one day later. A 21-day cycle has its fertile window on days 3 to 8, which can overlap the end of a period, while a 45-day cycle does not reach it until days 27 to 32.

How to use it, and who it is for

1. Enter the first day of real bleeding (not spotting) of your last period.
2. Enter your average cycle length, ideally averaged over your last three to six cycles.
3. Keep the luteal phase at 14 unless you have measured it.
4. Read the result card and the line about where today falls in your cycle, then use the calendar and the six-cycle table to plan ahead.

It suits people trying to conceive, anyone setting a baseline before using ovulation tests, and app users checking a prediction. It is not a tool for avoiding pregnancy.

Key terms

• Cycle day 1: the first day of real menstrual bleeding.
• Follicular phase: day 1 to ovulation, the part of the cycle that varies most.
• Luteal phase: the days after ovulation until the next period starts, about 14 on average.
• LH surge: the hormone rise that triggers ovulation and that predictor kits detect.
• Fertile window: the 5 days before ovulation plus ovulation day.
• Basal body temperature (BBT): your resting temperature, taken right after waking.

Signs that ovulation is near

• Ovulation predictor kits detect the LH surge in urine, so a positive test means your most fertile days have arrived.
• Cervical mucus turns clear, thin and slippery, like raw egg whites, and peaks 1 to 2 days before ovulation. An ACOG ob-gyn calls mucus tracking the most reliable way to self-detect ovulation.
• Basal body temperature rises 0.4 to 0.8°F with ovulation (womenshealth.gov; ACOG gives 0.5 to 1°F) and stays up until the next period, so it confirms ovulation only after it has happened.

What changes your estimate

• Cycle-to-cycle variation: a cycle four days longer usually means ovulation came about four days later.
• Your luteal phase: each day shorter moves ovulation one day later.
• Irregular cycles or PCOS: womenshealth.gov notes that people with polycystic ovary syndrome rarely or never ovulate, and ACOG says highly irregular periods can mean ovulation is not happening.

Timing sex when you are trying to conceive

ACOG advises sex every day or every other day during the six-day window and notes that more than half of healthy couples conceive within 6 months. Wilcox's NIH study measured a chance of conception of about 10% for sex five days before ovulation and 33% on ovulation day. See a doctor after 12 months of trying if you are under 35 and after 6 months at 35 or older (ACOG, womenshealth.gov), and sooner at 40 or older or with a known condition such as endometriosis or PCOS.

Limitations of a calendar estimate

In Wilcox's study of 696 cycles, women had at least a 10% chance of being in their fertile window on every day from cycle day 6 to 21, and only about 30% had the whole window inside days 10 to 17. A calendar cannot see a late ovulation this month or a condition that stops ovulation, so it is not birth control: ACOG reports 12 to 24 pregnancies per 100 women in the first year of typical use even for full fertility awareness methods, which track temperature and mucus daily.

Related calculators

• Due date, trimesters and milestones after a positive test: Pregnancy Due Date Calculator.
• Weeks and days pregnant on any date: Gestational Age Calculator.
• Likely conception date from a due date: Reverse Due Date Calculator.
• Days between two period start dates, to find your cycle length: Date Calculator.
• Days until your next period or a planned date: Countdown Calculator.

Sources

• American College of Obstetricians and Gynecologists (ACOG), Fertility Awareness-Based Methods of Family Planning (last reviewed February 2025).
• ACOG, Trying to Get Pregnant? Here's When to Have Sex (last reviewed August 2025).
• ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017, reaffirmed 2025).
• Office on Women's Health (womenshealth.gov), Your Menstrual Cycle, Trying to Conceive and Pregnancy Tests.
• NICHD (NIH), Ovulation and Conception infographics.
• Wilcox AJ, Weinberg CR, Baird DD (1995), "Timing of sexual intercourse in relation to ovulation," New England Journal of Medicine 333(23):1517-1521, PubMed.
• Wilcox AJ, Dunson D, Baird DD (2000), "The timing of the 'fertile window' in the menstrual cycle," BMJ 321(7271):1259-1262, PubMed.
• Fehring RJ, Schneider M, Raviele K (2006), "Variability in the phases of the menstrual cycle," Journal of Obstetric, Gynecologic, and Neonatal Nursing 35(3):376-384, PubMed.
• Lenton EA, Landgren BM, Sexton L (1984), "Normal variation in the length of the luteal phase of the menstrual cycle," British Journal of Obstetrics and Gynaecology 91(7):685-689, PubMed.