Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Estimate your fertile window and ovulation date from cycle length.
This tool estimates ovulation date and the fertile window from the first day of your last period and your average cycle length, using the standard clinical rule of thumb that ovulation occurs about 14 days before the next period begins.
Enter the first day of your last period and your average cycle length in days. Estimated ovulation date, fertile window, and next period date all appear instantly.
Ovulation Day = LMP + (Cycle Length - 14) days
Fertile Window = 4 days before ovulation through ovulation day
(Reflects typical sperm viability of ~5 days and egg viability of ~24 hours.)
This estimate assumes a regular cycle -- actual ovulation timing can vary, especially with irregular cycles, and this tool is not a substitute for clinical fertility tracking methods (like basal body temperature or ovulation predictor kits) if you're actively trying to conceive or avoid pregnancy.
Take a hypothetical last period that started on 1 January 2026. With a 28 day cycle, ovulation is estimated at day 14 after the start, which is 15 January. The fertile window starts four days earlier, on 11 January, and runs through 15 January, and the next period is expected on 29 January. Now suppose the cycle is 32 days. The estimate becomes 1 January + 18 days = 19 January for ovulation, a fertile window from 15 to 19 January, and a next period on 2 February. A shorter 24 day cycle moves ovulation to 11 January, with the window opening on 7 January.
The examples show the key point: the cycle length does not change the 14 day stretch after ovulation, so the whole difference shows up before ovulation. A cycle that is 4 days longer moves ovulation 4 days later.
Ovulation = LMP + (Cycle length - 14). Window = Ovulation - 4 days through Ovulation. Next period = LMP + Cycle length
The tool lists an ovulation date, the fertile window start, and the next period date. The window ends on the estimated ovulation day, so it covers five days in total. In real life the window can shift, and the days just after the estimate are not guaranteed to be infertile. Treat the dates as the middle of a plausible range rather than a precise event, and consider adding a day or two on either side if you are planning around them.
A calendar estimate cannot detect that ovulation has actually happened. Stress, illness, travel, breastfeeding, hormonal conditions, and recent stopping of hormonal contraception can all move ovulation or cause irregular cycles. People trying to conceive often add methods that check for physical signs or hormone surges, and those who have been trying for a long time or have very irregular cycles are usually advised to see a healthcare provider. It should never be used as the sole method to avoid pregnancy. This page is educational, not medical advice.
Beyond conception planning, some people simply use the dates to anticipate when a period is due, to schedule travel, or to understand patterns before talking with a clinician.
The luteal phase (after ovulation, before the next period) is relatively consistent at about 14 days for most people, while the follicular phase (before ovulation) varies more -- so counting backward from the next period is more reliable than forward from the last one.
Less reliable -- this method assumes a consistent cycle length; irregular cycles make ovulation timing harder to predict from dates alone, and other tracking methods may be more accurate.
No -- calendar-based fertility estimates like this one are not reliable enough to use alone for contraception; consult a healthcare provider about reliable birth control methods.
The tool counts four days before ovulation through the ovulation day, five days in total. It reflects sperm survival and egg lifespan as a general rule and does not extend past the ovulation estimate.
Yes. Enter the first day of your most recent period each time, because ovulation is calculated from that date and your cycle length.
The tool only accepts 20 to 45 days and will not calculate outside that range. Cycles far outside typical ranges are worth discussing with a healthcare provider.
Not always. The calendar estimate assumes a fixed 14 day gap before the next period, while a kit detects a hormone surge in your body, so the two can differ by a few days.
Reference: Centers for Disease Control and Prevention