Conception Date Estimate Calculator
Conception usually occurs around ovulation, roughly in the middle of the menstrual cycle. From the first day of your last menstrual period (LMP) and your average cycle length, you can estimate the conception date and the resulting due date. This calculator adds your cycle length minus 14 days to the LMP to estimate ovulation and conception, then adds 280 days to the LMP for the due date. It is for information only and not a clinical dating method.
Conception estimate formula
Ovulation day after LMP = cycle length - 14
Days since conception = days since LMP - ovulation day
Gestational age (weeks) = days since LMP / 7
Due date = LMP + 280 days
Ovulation is estimated as 14 days before the next period, so cycle length minus 14 gives the ovulation day after the LMP. Conception is assumed to occur near ovulation. Gestational age is measured from the LMP, so it is about two weeks more than time since conception.
Worked example
60 days since the LMP with a 28-day cycle: ovulation day = 28 - 14 = 14. Days since conception = 60 - 14 = 46. Gestational age = 60 / 7 = 8.57 weeks. Due date is at 280 days, so days remaining = 280 - 60 = 220.
Frequently asked questions
How is conception date estimated?
Conception is assumed to happen near ovulation, which this calculator estimates as 14 days before the next expected period, that is cycle length minus 14 days after the last period started. The actual fertile window spans several days, so the date is an estimate.
Why is gestational age more than time since conception?
By convention, pregnancy is dated from the first day of the last menstrual period, which is about two weeks before conception. So gestational age is roughly two weeks greater than the time elapsed since conception.
How reliable is LMP-based dating?
It depends on cycle regularity and accurate recall of the last period. For irregular cycles, an early ultrasound is more accurate. This tool is for information only; your maternity provider will confirm dating.
Sources
- U.S. Office on Women's Health: pregnancy and prenatal care.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.