Enter the first day of your last period and your average cycle length to estimate your most fertile days.
Your Most Fertile Window
Thu, Aug 13 – Wed, Aug 19
Having intercourse every 1-2 days during this window maximizes your chances of conception.
Estimated OvulationTuesday, August 18, 2026
Next Period StartsTuesday, September 1, 2026
Take Pregnancy TestTuesday, September 1, 2026
Note: This tool provides estimates based on a regular menstrual cycle, not a diagnosis. Many factors can influence your exact ovulation date, including stress, illness, and medication. For precise tracking, consider using ovulation predictor kits (OPKs) or tracking your basal body temperature, and talk to your healthcare provider if you have concerns about your cycle or fertility.
Last updated: July 2026
Find Your Most Fertile Days
Whether you're actively trying to conceive or simply tracking your cycle, understanding your fertile window can help you plan. This calculator estimates your fertile window and likely ovulation date based on the first day of your last period and your average cycle length. It's an estimate, not a diagnosis — for medical guidance about your cycle or fertility, talk to your healthcare provider.
The Science of the Fertile Window
Pregnancy is only possible during a narrow window each cycle. That's because of two things:
Sperm lifespan: sperm can survive inside the female reproductive tract for up to 5 days under favorable conditions.
Egg lifespan: once released at ovulation, an egg survives for only about 12 to 24 hours if it isn't fertilized.
Because sperm can wait for the egg but the egg can't wait long for sperm, having intercourse in the days before ovulation — not just on the day itself — gives you the best odds.
How the Calculation Works
A menstrual cycle has two phases: the follicular phase (before ovulation), which varies in length from person to person, and the luteal phase (after ovulation), which is far more consistent — close to 14 days for most people. This calculator works backward from your predicted next period: it estimates ovulation as 14 days before your next period is due, then sets the fertile window as the 5 days before that estimated ovulation date through the day after.
A Worked Example
Say your last period started on March 1, and your average cycle is 30 days:
Next period is estimated for March 1 + 30 days = March 31
Ovulation is estimated 14 days earlier = March 17
Fertile window: 5 days before ovulation through 1 day after = March 12 – March 18
Best day to test if your period doesn't arrive: March 31
Frequently Asked Questions
Your fertile window is generally the 5 days leading up to ovulation, plus the day of ovulation and the day after. This is because sperm can survive in the female reproductive tract for up to 5 days, while an egg only survives for about 12 to 24 hours after being released.
It uses the standard 'luteal phase' method. The luteal phase — the time between ovulation and your next period — is close to 14 days for most people, regardless of total cycle length. So if your cycle is 30 days, ovulation is estimated on day 16 (30 minus 14).
For the most reliable result, wait until the first day of your missed period. By then, if you're pregnant, hCG hormone levels are generally high enough for a standard home pregnancy test to detect.
They're built for different goals. This tool is focused on trying to conceive — it centers the fertile window and estimated ovulation date. The Period Calculator is focused on general cycle tracking — predicting your next period and flagging your fertile window as a secondary data point. If conception timing is your main goal, this page is the better fit.
Less reliably. This calculator assumes a consistent cycle length and a roughly 14-day luteal phase, so if your cycles vary a lot month to month, the estimate will be less accurate. Ovulation predictor kits (OPKs) or basal body temperature tracking can give you a more direct read in that case.
Common signs include a change in cervical mucus (becoming clear, stretchy, and slippery), a slight rise in basal body temperature after ovulation, and sometimes mild one-sided abdominal discomfort. These signs vary from person to person and cycle to cycle.
They give a reasonable estimate for people with regular cycles, but they can't account for cycle-to-cycle variation, stress, illness, or medication — all of which can shift ovulation earlier or later. For time-sensitive planning, consider pairing this estimate with OPKs or temperature tracking.