Ovulation Calculator

How the ovulation calculator estimates your ovulation date

A menstrual cycle has two halves. The follicular phase runs from the first day of your period until an egg is released (ovulation). The luteal phase runs from ovulation until the next period starts. The follicular phase is the one that varies most between people and between cycles. The luteal phase is steadier, which is why calendar methods count backwards from the expected next period rather than forwards from the last one.

The calculator uses three simple steps:

• Next period = first day of last period + cycle length.
• Ovulation = next period − luteal phase length. With the default 14 days, that's last period + (cycle length − 14).
• Fertile window = the five days before ovulation, plus ovulation day itself.

Counting the first day of bleeding as cycle day 1, a 28-day cycle with a 14-day luteal phase puts estimated ovulation on day 15, which is 14 days after the period began. You'll often see this rounded to "around day 14." To keep the follicular phase at least seven days long, the calculator shortens the luteal phase on very short cycles when needed. It tells you when it does.

What the fertile window means

Sperm can survive in the reproductive tract for several days, but an egg lasts only about a day after release. That combination creates a short fertile window. In a landmark 1995 study, Wilcox, Weinberg and Baird followed 221 women trying to conceive and pinpointed ovulation with daily urine hormone tests. Every pregnancy came from intercourse during the six days that ended on ovulation day. The chance of conception rose from about 10% for intercourse five days before ovulation to about 33% on ovulation day itself.

That's the window shown above. The American College of Obstetricians and Gynecologists (ACOG) describes it slightly more broadly, from about five days before ovulation through one day after. If you're trying to conceive, treat the day after the window as a bonus day too.

Worked example

Suppose your last period started on October 1, 2026, your cycles usually last 32 days, and you don't know your luteal phase, so you leave it at 14 days:

• Next period: October 1 + 32 days = November 2
• Ovulation: October 1 + (32 − 14) = 18 days = October 19 (cycle day 19)
• Fertile window: October 14–19
• Due date if you conceive that cycle: October 19 + 266 days = July 12, 2027

With a 28-day cycle, the same period start would give ovulation on October 15. A longer cycle simply pushes everything later. If you'd tracked a 12-day luteal phase instead, ovulation would move two days later, to October 21, because the period still arrives on November 2.

Ovulation day by cycle length

Estimated cycle days for every cycle length from 21 to 35 days with a 14-day luteal phase. Day 1 is the first day of your period. The last column shows how far the estimated due date moves from the standard 280-day date.

How accurate is a calendar ovulation estimate?

It's a reasonable starting point, but real cycles are messier than the 28/14 textbook pattern. A 2019 analysis of more than 600,000 cycles logged in a fertility app found an average cycle of 29.3 days. Only about 13% of cycles were exactly 28 days long, and luteal phases averaged 12.4 days, with most falling between 7 and 17. A follow-up of the Wilcox cohort found that only about 30% of women had their fertile window entirely within days 10–17. Even women with regular cycles sometimes ovulated much later than expected.

You can sharpen the estimate with signs from your own body. Urine LH (ovulation) test strips detect the hormone surge that comes a day or so before ovulation. Cervical mucus becomes clear, slippery and stretchy as ovulation approaches. Basal body temperature rises slightly after ovulation and stays up until your period. Charting temperature over a few cycles also shows your real luteal phase length, which you can enter above.

Not for contraception. ACOG's patient guidance on fertility awareness methods reports that 12–24 out of 100 people using them in a typical way become pregnant within a year. Those methods involve daily tracking and training, not just a calendar estimate like this one. If you need to avoid pregnancy, talk to a clinician about reliable options.

If you conceive: due date and next steps

The "due date if you conceive" figure is ovulation + 266 days (38 weeks). With a 14-day luteal phase, that's the same as the standard rule of last period + 280 days, adjusted by your cycle length minus 28. Once a pregnancy is confirmed, the due date calculator gives the standard date from your last period, conception or IVF transfer. The pregnancy week-by-week calculator tracks how far along you are and your trimester. Your care provider will confirm the dating, usually with an early ultrasound.

It's normal for conception to take a while. Many healthy couples don't conceive in the first few cycles of trying. Fertility specialists generally suggest an evaluation after 12 months of trying if you're under 35, or after 6 months if you're 35 or older. Seek advice sooner if your periods are very irregular or absent, or if you know of a condition that affects fertility.

Sources

The formulas on this page come from the published research below. Last updated October 10, 2026. This calculator gives estimates for general information. It isn't medical advice, so check anything important with a qualified professional.

1. Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation — effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med. 1995;333(23):1517–1521.Used for: Six-day fertile window ending on ovulation day
2. Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259–1262.Used for: How unpredictable the fertile window is from cycle to cycle
3. Bull JR, Rowland SP, Scherwitzl EB, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med. 2019;2:83.Used for: Typical variation in cycle and luteal phase length
4. American College of Obstetricians and Gynecologists. Fertility Awareness-Based Methods of Family Planning (FAQ024).Used for: Fertile window, egg lifespan, and why calendar methods are unreliable as contraception
5. Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med. 1999;340(23):1796–1799.Used for: Implantation timing, which sets when a pregnancy test can turn positive
6. American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150–e154.Used for: 280-day due date from the last period, which the cycle-adjusted estimate builds on