Ovulation & Fertile Window Calculator
Estimate your ovulation day, fertile window, and the next three periods from your last period date and average cycle. Everything runs in your browser.
Enter your last period start date to estimate ovulation and the fertile window.
※ A calendar-method estimate that assumes regular cycles. Actual ovulation varies with stress and health, and this is not a form of contraception. Confirm ovulation with ovulation tests, basal body temperature or ultrasound.
🔒 Your dates are calculated in the browser and never sent to a server.
How ovulation and the fertile window are calculated
This tool applies the standard <strong>calendar (rhythm) method</strong> and the <strong>luteal-phase rule</strong> used in fertility awareness. First it adds your average cycle length to your last menstrual period (LMP) start date to predict your next period. Because ovulation happens about 14 days before the next period starts, it subtracts the luteal-phase length (default 14 days) from that predicted period to find your <strong>estimated ovulation day</strong>. It then builds the fertile window from 5 days before ovulation through 1 day after, reflecting that sperm can survive up to about 5 days and an egg survives roughly 12-24 hours. The calculator also lists your next three predicted periods and ovulation days so you can mark them on a calendar in advance.
Entering your inputs accurately
Your results are only as good as your inputs. Enter the <strong>first full-flow day</strong> of your most recent period as the LMP, not a day of light spotting. Your <strong>cycle length</strong> is the number of days from one period start to the next; tracking three or more cycles and averaging them gives a better estimate. Cycles range from 21 to 45 days and the default is 28. The <strong>luteal phase</strong> (ovulation to next period) is typically 12-16 days, averaging 14, which is the default. Leave it at 14 unless a clinician has told you yours is shorter or longer, in which case entering that value will refine the ovulation estimate.
How reliable is the estimate
The calendar method assumes reasonably regular cycles and produces an <strong>estimate, not an exact date</strong>. The luteal phase is fairly constant, but the follicular phase (period to ovulation) shifts with stress, sleep, weight changes, illness, and travel, so your real ovulation can land a few days earlier or later than predicted. It is least reliable when your cycle length varies by more than 7-8 days month to month, or with conditions like PCOS that make ovulation irregular. Treat the output as a likely range to focus on, not a guaranteed single day, and confirm with additional methods when timing matters.
Trying to conceive or avoid pregnancy
If you are trying to conceive, aim for the fertile window, especially the <strong>day or two before ovulation</strong>. For sharper timing, combine this estimate with <strong>basal body temperature (BBT)</strong>, <strong>ovulation (LH) test kits</strong>, cervical mucus observation, or an ultrasound. Do <strong>not</strong> rely on this calculator for contraception: the calendar method is not accurate enough to reliably prevent pregnancy, so use condoms, the pill, or another proven method if that is your goal. See a gynecologist if you have been trying for six months or more, or if your cycles are very short, very long, or suddenly irregular. Everything is computed in your browser and never sent to a server.
Frequently asked questions
How is the ovulation day calculated?
It takes your predicted next period (LMP + average cycle length) and subtracts the luteal-phase length (default 14 days). For a 28-day cycle that puts ovulation around day 14, and for a 30-day cycle around day 16. This reflects the rule that ovulation occurs about 14 days before the next period.
Why does the fertile window run from 5 days before to 1 day after ovulation?
Sperm can survive in the body for up to about 5 days, and an ovulated egg survives roughly 12-24 hours. So intercourse up to 5 days before ovulation can still lead to pregnancy, and fertility lingers into the day after ovulation. The two days just before ovulation carry the highest chance of conception.
My cycle is irregular. Can I trust this?
If your cycle length varies by more than 7-8 days between months, the calendar method alone cannot pinpoint ovulation reliably. Use your recent average as a rough guide only, and confirm timing with an ovulation (LH) test kit or basal body temperature. Remember the result is an estimate, not a certainty.
Can I use this for birth control? Is my data stored?
It is not recommended for contraception. The calendar method is an estimate and your real ovulation can shift, so it is not accurate enough to prevent pregnancy; use a proven method like condoms or the pill instead. As for privacy, the dates you enter are calculated entirely in your browser and are never sent to or stored on any server.