How to estimate your fertile window by hand
Most ovulation calculators — free or paid — are running one simple rule on the back end: ovulation tends to happen about 14 days before the next period starts, not 14 days after the last one started. The two only line up when a cycle is exactly 28 days. Once you know the rule, you can do the whole estimate on paper.
The method
You need two numbers: the first day of your last period (LMP) and your average cycle length — the number of days from the start of one period to the start of the next.
- Add the cycle length to the LMP date to get the expected start of the next period.
- Subtract 14 days from that date. That's the estimated ovulation day.
- The fertile window is roughly the five days before ovulation plus ovulation day itself — six days total — because sperm can survive a few days in the body while the egg itself is only viable for about 24 hours.
In one line: ovulation day = LMP + cycle length − 14.
A worked example
Last period started September 10, 2026, average cycle length 30 days.
- Expected next period: September 10 + 30 days = October 10, 2026.
- Ovulation day: October 10 − 14 days = September 26, 2026.
- Fertile window: five days before September 26 through September 26 itself, i.e. September 21 to September 26, 2026.
Check the day-count by hand if you want to be sure: from September 10 to September 26 is 16 days, and 30 − 14 = 16, so the arithmetic holds.
Why the 28-day assumption trips people up
A lot of informal advice says "ovulation is day 14." That's only true for a 28-day cycle, because 28 − 14 = 14 − the subtraction happens to land back on the same number as the cycle midpoint. For a 24-day cycle, ovulation is around day 10 (24 − 14); for a 35-day cycle, it's around day 21 (35 − 14). Counting from the wrong end — forward from the last period by a fixed 14 days regardless of cycle length — is the single most common mistake in doing this by hand.
What this can't tell you
This is a population-average estimate from cycle length alone. Real cycles vary month to month, ovulation can shift earlier or later than the 14-day-back rule predicts, and nothing here accounts for irregular cycles, hormonal conditions, or recent changes in medication, stress, or travel. It is not a method of contraception and not a fertility diagnosis — for either of those, talk to a clinician who can look at your actual cycle history.
The free ovulation window calculator runs this same subtraction and formats the dates for you, which is handy once you're doing it for several months at once, but the rule above is the entire calculation.