How to use the ovulation calculator
Enter the first day of your last period and your average cycle length, counted from the first day of one period to the first day of the next. If your cycle varies, an average of several recent cycles gives a better picture than a single month. Results appear once a date is entered.
How the dates are worked out
Your next period is the last period plus your cycle length. Ovulation is placed 14 days before that, because the time between ovulation and the next period tends to be the most consistent part of the cycle. The fertile window runs from 5 days before ovulation through 1 day after. ACOG uses the same window, since an egg survives about 24 hours and sperm usually about 3 days, sometimes up to 5.
Worked example: a period starting April 6
If your last period began April 6, 2026, and your cycle is 30 days, your next period is estimated for May 6, 2026. Ovulation is 14 days earlier, on April 22, 2026. The fertile window is April 17, 2026 to April 23, 2026.
How firm these dates are
Treat these as your likeliest days, not certainties, because a calendar cannot confirm that ovulation happened. A study of 696 cycles found that only about 30% of women had a fertile window entirely within days 10 to 17, and concluded that timing can be highly unpredictable even when cycles are usually regular. The NHS puts the time from ovulation to a period at around 10 to 16 days, so 14 is an average, and it describes regular cycles of 21 to 35 days as normal.
Why this is not contraception
This must not be used as contraception. Even fertility awareness methods based on daily tracking lead to 12 to 24 pregnancies per 100 women in a year of typical use, according to ACOG, and this tool tracks nothing. It uses only a calendar average.
It is unreliable with irregular cycles, and ACOG notes that fertility signs are harder to read after recently having a baby, when stopping hormonal birth control, when periods have just started and when approaching menopause.