Ovulation calculator
Enter the first day of your last period and your usual cycle length to see your estimated ovulation date and fertile window for the next few cycles. It’s an estimate from averages, not a substitute for ovulation tests or advice.
Day 1 is the first day of real bleeding, not spotting.
First day of one period to the first day of the next.
Only used to shade the calendar.
Enter your shortest and longest recent cycles to see a wider window that covers both.
An estimate from averages, not medical advice. The time from ovulation to the next period averages about 14 days but usually falls between about 11 and 17 (Lenton et al., 1984). Ovulation tests, tracking your temperature or cervical mucus, or a doctor or midwife can tell you more. If you’ve been trying for a year (six months if you’re over 35), ACOG suggests seeing a doctor.
- Free, no sign-up
- Works on phone and desktop
How to find your fertile window
- 1
Enter the first day of your last period: the first day of real bleeding, not spotting.
- 2
Enter your average cycle length, and your period length if you’d like it shaded on the calendar.
- 3
If your cycles vary, switch on My cycle length varies and enter your shortest and longest recent cycles.
- 4
Read your next fertile window and estimated ovulation, then the cycles after it in the list and on the calendar.
Features
Six cycles ahead
This cycle and the next five, each with its fertile window, estimated ovulation and expected next period.
A calendar you can read
Expected periods, fertile days and ovulation marked month by month, with today outlined.
Honest about irregular cycles
Enter your shortest and longest cycles for a wider window, and a clear note when calendar estimates get weak.
Private by design
Your dates never go in the page address and are never stored or sent. There is no share link, on purpose.
How ovulation is estimated
A menstrual cycle runs from the first day of one period to the first day of the next. The calculator projects your next period from your average cycle length, then counts back 14 days to estimate ovulation, because the time from ovulation to the next period is the most consistent part of the cycle, as the American College of Obstetricians and Gynecologists explains:
- Next period = first day of the last period + cycle length.
- Ovulation ≈ next period − 14 days.
- Fertile window = from 5 days before ovulation through 1 day after it.
The window follows ACOG: sperm can survive in the body for up to 5 days and an egg for about 24 hours, so sex from 5 days before ovulation until 1 day after can lead to pregnancy. The days aren’t equal. Wilcox and colleagues’ 1995 study in the New England Journal of Medicine followed 221 women trying to conceive with daily hormone tests, and the chance of conceiving ranged from about 10% for sex five days before ovulation to about 33% for sex on the day of ovulation. Each cycle after this one is projected forward at your average length.
Worked examples
- 28-day cycle, period starting March 1, 2026: fertile Mar 10 to Mar 16, ovulation around Mar 15, next period about March 29, 2026.
- 32-day cycle, same start: everything moves four days later, with ovulation around Mar 19 and the next period about Apr 2.
- 25-day cycle: ovulation comes earlier, around Mar 12, so the window opens as soon as Mar 7, only a few days after the period ends.
- Cycles from 26 to 32 days: the window has to cover the earliest and latest ovulation, so it widens to 13 days, Mar 8 to Mar 20.
Fertile days by cycle length
For a period that started on March 1, 2026:
| Cycle | Fertile window | Ovulation | Next period |
|---|---|---|---|
| 21 days | Mar 3–9 | Mar 8 | Mar 22 |
| 24 days | Mar 6–12 | Mar 11 | Mar 25 |
| 26 days | Mar 8–14 | Mar 13 | Mar 27 |
| 28 days | Mar 10–16 | Mar 15 | Mar 29 |
| 30 days | Mar 12–18 | Mar 17 | Mar 31 |
| 32 days | Mar 14–20 | Mar 19 | Apr 2 |
| 35 days | Mar 17–23 | Mar 22 | Apr 5 |
ACOG describes a typical cycle as 21 to 35 days, and cycles as irregular when their length varies by more than 7 to 9 days. The calculator accepts 21 to 45 days and tells you when the estimate is weaker.
What a calendar can’t tell you
Ovulation doesn’t keep to a timetable. The time from ovulation to the next period averaged 14.1 days in a study of 327 cycles, with a standard deviation of 1.4 days, so it usually falls somewhere between about 11 and 17 days. A 2000 study in the BMJ found that only about 30% of women had their fertile window entirely within days 10 to 17 of the cycle, and on every day from day 6 to day 21 there was at least a 10% chance of being in it. The authors’ advice: the timing can be highly unpredictable, even when cycles are usually regular.
To narrow it down, ovulation predictor kits test urine for the hormone rise that triggers ovulation, and tracking cervical mucus or your morning temperature shows the pattern over a few cycles. An ACOG specialist suggests sex every day or every other day during the fertile window for the best chance. If you haven’t conceived after a year of trying (six months if you’re over 35), ACOG recommends an infertility evaluation.
Not a method of birth control
Please don’t use these dates to avoid pregnancy. Fertility awareness methods used as contraception rely on daily body signs, taught carefully, not a calendar projection, and even so the CDC gives a typical-use failure rate of 2 to 23%, depending on the method. A doctor, nurse or family planning clinic can help you find a reliable option.
If you’re already pregnant, the due date calculator works out your due date and how far along you are, and the conception calculator works backward to when it likely happened. For your general health, the sleep calculator and the TDEE calculator are here too.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine, 1995.
- Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle. BMJ, 2000.
- Lenton EA, Landgren BM, Sexton L. Normal variation in the length of the luteal phase of the menstrual cycle. British Journal of Obstetrics and Gynaecology, 1984.
- American College of Obstetricians and Gynecologists. Fertility awareness-based methods of family planning. Patient FAQ, 2019.
- American College of Obstetricians and Gynecologists. Abnormal uterine bleeding. Patient FAQ, 2021.
- American College of Obstetricians and Gynecologists. Evaluating infertility. Patient FAQ.
- Centers for Disease Control and Prevention. Contraception and birth control methods. 2024.
This calculator gives an estimate from averages, not medical advice, and it isn’t contraception. A doctor, midwife or nurse can help with anything about your cycle, fertility or birth control, and it’s always fine to ask.
Frequently asked questions
When do I ovulate?
About 14 days before your next period starts, whatever your cycle length. In a 28-day cycle that starts on March 1, the next period is due Mar 29 and ovulation is likely around Mar 15. In a 32-day cycle it moves to about Mar 19. The time from ovulation to the next period is the steadiest part of the cycle, but it still varies by a few days.
What is the fertile window?
The days in each cycle when sex can lead to pregnancy. Sperm can survive up to about five days and an egg about a day, so ACOG counts from five days before ovulation through one day after it, about a week in all. The chances aren’t equal across it: in a 1995 study, the chance of conceiving ranged from about 10% for sex five days before ovulation to about 33% for sex on the day of ovulation.
How accurate is an ovulation calculator?
It’s a reasonable first guess for regular cycles, not a measurement. In a study of 221 women who tracked ovulation with daily hormone tests, only about 30% had their fertile window entirely within days 10 to 17, the days textbooks often give. Ovulation tests, tracking cervical mucus or temperature, and a doctor can tell you more than a calendar can.
Can I use this to avoid getting pregnant?
No. A calendar estimate is not a method of birth control: ovulation can come earlier or later than predicted, even in regular cycles. Fertility awareness methods that are used for contraception track body signs every day, and even those fail for 12 to 24 out of 100 people in a year of typical use, according to ACOG. A doctor or nurse can help you choose reliable contraception.
What if my periods are irregular?
Switch on “My cycle length varies” and enter your shortest and longest cycles from the last few months. The calculator widens the window to cover both, which is more honest but less specific. ACOG describes cycles as irregular when their length varies by more than 7 to 9 days; then ovulation tests are a better guide, and a doctor can check for causes.
Can I get pregnant during my period?
It’s unlikely but possible, mostly with short cycles. In a 21-day cycle, ovulation can come about a week after the period starts, and sperm from sex near the end of a period can survive until then. That’s one more reason a calendar can’t be used as birth control.
When should I see a doctor about getting pregnant?
ACOG suggests an evaluation if you haven’t become pregnant after a year of regular sex without contraception, after six months if you’re over 35, and straight away if you’re over 40. See a doctor sooner if your periods are very irregular or have stopped for a few months.
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