About this calculator
The Conception Calculator estimates when conception likely occurred, based on either your due date or the first day of your last menstrual period and your average cycle length.
How is conception date estimated?
Conception typically occurs around ovulation, which for an average 28-day cycle falls about 14 days after the first day of the last menstrual period. If you enter a due date instead, the calculator works backward from the standard 40-week (280-day) pregnancy estimate to find the corresponding likely conception window.
Why is this only an estimate?
Ovulation timing varies from person to person and cycle to cycle, especially for those with cycles shorter or longer than 28 days, so the actual conception date can fall several days on either side of the estimate. Sperm can also survive in the body for up to five days, meaning intercourse before ovulation can still result in conception once ovulation occurs.
What is this calculation used for?
People commonly use a conception estimate for personal reference, timeline planning, or context around prenatal milestones. For any medical decisions, a healthcare provider can give a much more accurate estimate using ultrasound measurements, which is considered more reliable than a calculation based on cycle dates alone.