Ovulation Calculator

How to use this calculator

Enter three things:

• the first day of your last period (the first day of full bleeding, not spotting)
• your average cycle length, counted from the first day of one period to the first day of the next
• your luteal phase, the days from ovulation to your next period; leave it at 14 if you don’t know it

The calculator shows your next estimated ovulation date, your fertile window, where today falls in your cycle, when your next period is due, and suggested dates for ovulation tests and a pregnancy test. The table lists the same dates for your next six cycles, and you can download it as a CSV file. Under Options you can change the date the calendar starts from, which defaults to today.

If you get a positive pregnancy test, the due date calculator uses the same period date to estimate when your baby is due.

How ovulation is estimated

The calculator uses the calendar method most clinicians and apps start from. Your next period is your last period plus your cycle length. Ovulation is placed one luteal phase before that:

The fertile window is the 6 days ending on ovulation day. That comes from a landmark 1995 study in the New England Journal of Medicine, in which Allen Wilcox and colleagues followed 221 women who were trying to conceive and estimated the day of ovulation from hormone levels in daily urine samples. Every pregnancy came from intercourse during the 5 days before ovulation or on ovulation day itself. The chance of conceiving was about 10% for intercourse five days before ovulation and about 33% on the day of ovulation.

Counting back from your next period, not forward from your last one, matters because most of the variation between women sits in the first half of the cycle. A woman with a 32-day cycle doesn’t ovulate on day 14; she ovulates around day 19.

Worked example

Say your last period started on October 30, 2026 and your cycles are 28 days long, with the default 14-day luteal phase:

1. Next period: October 30 + 28 days = November 27, 2026
2. Ovulation: 14 days before that = November 13, 2026 (cycle day 15)
3. Fertile window: the 6 days ending on ovulation = November 8 to 13

The calculator suggests starting ovulation (LH) tests on November 9, four days before the estimated ovulation, because the hormone surge they detect comes only about 1 to 1½ days before you ovulate. If your period hasn’t come by November 27 you can take a pregnancy test that day, and a test on December 4, a week later, is more reliable.

Ovulation day by cycle length

With a 14-day luteal phase, here is where ovulation and the fertile window fall for common cycle lengths. Day 1 is the first day of your period.

The Office on Women’s Health describes a normal adult cycle as anywhere from 24 to 38 days, with 28 days as typical. If your cycles are regularly shorter or longer than that, the calculator flags it, because it can be a sign that you don’t ovulate every month.

How accurate is the calendar method?

Less accurate than most people assume, even with regular periods. In a 2000 follow-up study in the BMJ, Wilcox and colleagues found that only about 30% of women had their fertile window entirely within days 10 to 17 of the cycle. On every day from day 6 to day 21, women had at least a 10% chance of being in their fertile window.

Real-world cycles also differ from the textbook. A 2019 analysis of more than 612,000 ovulatory cycles from about 125,000 app users found an average cycle length of 29.3 days, an average follicular phase (period to ovulation) of 16.9 days and an average luteal phase of 12.4 days. The luteal phase ranged from about 7 to 17 days, so the standard 14-day assumption is only a starting point.

If your cycles are irregular

The calculator assumes every cycle matches your average. If your cycle length changes by a week or more from month to month, a single predicted date can miss your real fertile window. In that case:

• Use your shortest recent cycle for the earliest likely window and your longest for the latest, and treat everything in between as possibly fertile.
• Confirm ovulation with your body. Urine LH tests, changes in cervical mucus and a rise in basal body temperature after ovulation all give direct evidence, and over a few cycles they show you your own luteal phase.
• Talk to a clinician if your cycles are often shorter than 24 days or longer than 38, if your periods stop, or if you’ve been trying to conceive for a year (six months if you’re 35 or older).

Tips for trying to conceive

• Focus on the whole window, not one day. Intercourse every day or two through the fertile window covers the days with the highest chance without needing to pinpoint ovulation.
• Track a few cycles. Recording your period start dates for three to six months gives you a real average and shows how much your cycle varies.
• Update the calculator each month. Enter the first day of your newest period so the projections start from real data rather than a long chain of estimates.

Assumptions and limitations

• Cycle day 1 is the first day of full menstrual bleeding.
• Every future cycle is assumed to be the same length, with ovulation one luteal phase before the next period.
• The calculator doesn’t account for hormonal birth control, breastfeeding, recent pregnancy, polycystic ovary syndrome, perimenopause or other conditions that change ovulation.
• It isn’t a contraceptive method and shouldn’t be used to avoid pregnancy.

Sources

• 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.
• 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.
• 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.
• Office on Women’s Health, US Department of Health and Human Services. Your menstrual cycle.
• US Food and Drug Administration. Ovulation (urine test) and Pregnancy (home use test).

This calculator gives estimates for general information only. It isn’t medical advice or a method of birth control; talk to your doctor or nurse about your fertility.

Updated November 2026. Results are estimates for educational purposes and depend on the inputs you provide.