How it works
Your menstrual cycle follows a predictable rhythm. Ovulation—the release of a mature egg from your ovary—happens at roughly the same point in every cycle. By knowing when your last period started and how many days your cycle typically lasts, you can estimate which day ovulation will occur and identify the days around it when pregnancy is most likely.
The timing matters because an egg survives only about 24 hours after release, but sperm can remain viable in the reproductive tract for up to 5 days. This creates a fertile window: the 5 days before ovulation plus the day of ovulation itself (sometimes called the "fertile six"). Intercourse during this window has the highest chance of resulting in conception.
Most people ovulate roughly 14 days before their next period begins. If your cycle is longer or shorter than 28 days, ovulation shifts accordingly. A 35-day cycle means ovulation happens around day 21; a 21-day cycle around day 7.
The formula
Ovulation day ≈ First day of last period + (Cycle length − 14)
Fertile window ≈ Ovulation day − 5 to Ovulation day + 1
Worked example
Assume your last period began on January 10 and your average cycle is 28 days.
Ovulation day = January 10 + (28 − 14) = January 10 + 14 = January 24
Fertile window starts = January 24 − 5 = January 19
Fertile window ends = January 24 + 1 = January 25
This means your peak fertility window runs from January 19 through January 25. Ovulation itself is predicted for January 24. If you're trying to conceive, these six days are when intercourse is most likely to result in pregnancy. January 24 is your single most fertile day, but the days immediately before are equally important since sperm need time to reach the egg.
If your cycle were 32 days instead:
Ovulation day = January 10 + (32 − 14) = January 10 + 18 = January 28
Fertile window = January 23 to January 29
Notice how a longer cycle pushes ovulation and the fertile window later in the month.
Common mistakes
Confusing cycle length with fertile window length. Your cycle is the full span from the first day of one period to the first day of the next—typically 21 to 35 days. Your fertile window is only about 6 days within that cycle.
Assuming ovulation happens on day 14 regardless of cycle length. Day 14 is only correct for a 28-day cycle. Longer cycles ovulate later; shorter cycles earlier.
Using the first day of bleeding as "day zero" instead of "day one. Always count the first day of your period as day 1, not day 0. This small shift affects all downstream calculations.
Relying on this calculator alone. Cycles vary month to month, illness and stress shift timing, and apps can't see inside your body. If conception is medically important, confirm ovulation with ovulation predictor kits (which detect the LH surge) or track basal body temperature.
This calculator is an estimate, not professional medical advice. If you have irregular cycles, endometriosis, PCOS, or other reproductive health concerns, consult a doctor or fertility specialist for personalized guidance.**