Ovulation Calculator
Find your most fertile days and estimated ovulation date from your last period and cycle length, plus a forecast of your next few cycles.
Timing intercourse around ovulation gives you the best chance of conceiving. This free ovulation date calculator estimates your ovulation day and the fertile window around it from the first day of your last period and your usual cycle length, then projects the next few cycles so you can plan ahead.
When ovulation happens
The menstrual cycle has two phases. The follicular phase (period to ovulation) varies in length between people; the luteal phase (ovulation to next period) is fairly constant at about 14 days. Because of that, ovulation is estimated by counting back 14 days from your next expected period:
Ovulation day ≈ cycle length − 14 days after your last period started.
So a 28-day cycle ovulates around day 14, and a 32-day cycle around day 18. That count-back is the whole of how to calculate ovulation from a calendar.
Your fertile window
You can only conceive in a short window each cycle, because:
- Sperm can survive in the reproductive tract for up to 5 days.
- The egg survives about 12–24 hours after it’s released.
Together that makes a fertile window of roughly six days: the five days before ovulation and ovulation day itself. The two or three days leading up to ovulation are the most fertile. This tool marks that whole window plus the peak days.
Planning ahead with the ovulation cycle calculator
If this cycle isn’t the one, it helps to see the next few. The calculator lists upcoming cycles with each one’s fertile window and ovulation day, plus the date a pregnancy test becomes reliable (about 14 days after ovulation, around when your period would be due). To follow the periods themselves month to month, use the period calculator; to work back from a due date to the likely day of conception, see the conception calculator; and once you’ve conceived, the due date calculator picks up the timeline.
How to confirm ovulation
Calendar estimates assume regular cycles, and real ovulation shifts from month to month. To pin it down:
- Ovulation predictor kits detect the luteinising-hormone (LH) surge 24–36 hours before ovulation.
- Basal body temperature rises slightly after ovulation — charting it confirms that it happened.
- Cervical mucus becomes clear and stretchy (like egg white) around your most fertile days.
When to seek advice
If you’re under 35 and haven’t conceived after a year of trying (or six months if you’re 35 or older), or your cycles are very irregular, absent, or unusually short or long, see a doctor. This calculator is for information and family planning, not a diagnosis or a contraceptive method.
Frequently asked questions
How do you calculate ovulation?
Count back about 14 days from the first day of your next expected period, because the luteal phase (ovulation to period) is fairly constant at around 14 days. So ovulation day ≈ cycle length − 14, counted from the day your last period started. The estimate works best when your cycles are regular.
When do you ovulate?
Ovulation usually happens about 14 days before your next period starts, because the luteal phase (ovulation to period) is fairly constant. In a 28-day cycle that's around day 14; in a 32-day cycle it's around day 18. The calculator uses your cycle length to estimate it.
When did I ovulate?
Work back about 14 days from the day your period actually started — that puts ovulation near the middle of a regular cycle. To confirm after the fact, a sustained rise in basal body temperature shows ovulation has happened, and an ovulation predictor kit detects the LH surge 24–36 hours beforehand.
What is the fertile window?
It's the roughly six days when intercourse can lead to pregnancy — the five days before ovulation plus ovulation day itself. Sperm can survive up to five days, while the egg lives about 24 hours, so the days just before ovulation are the most fertile.
How accurate is an ovulation calculator?
It's a good estimate for regular cycles but not a precise prediction — ovulation timing varies month to month. Ovulation predictor kits (which detect the LH surge), basal body temperature tracking, and cervical mucus changes confirm your fertile days more reliably.
Can you get pregnant outside the fertile window?
It's much less likely, but cycles vary and ovulation can shift, so no calendar method is foolproof for either conceiving or avoiding pregnancy. If you're avoiding pregnancy, use a reliable contraceptive method rather than timing alone.