Conception Date Calculator
Estimate when conception likely occurred from your last period or your due date.
Last updated: June 20, 2026 · Medically reviewed by TG we-Calculate Editorial TeamEstimate from
First day of last period
Thu, Jan 15, 2026
How is the conception date estimated?
Conception is estimated as ovulation, about 14 days after the first day of your last menstrual period in an average 28-day cycle. From a due date, conception is roughly 266 days earlier. Because cycles vary, the true conception day usually falls within a window of about two days either side.
Formula
How this is calculated
This calculator works backward from a single date you provide. In LMP mode the input is the first day of your last menstrual period; in due-date mode it is your estimated delivery date. Pregnancy is conventionally dated from the LMP, but conception actually happens at ovulation. In an average 28-day cycle ovulation falls on cycle day 14, so the tool simply adds 14 calendar days to the LMP to get the estimated conception date.
Due-date mode reverses Naegele's rule. That rule sets the due date at LMP + 280 days (40 weeks), so conception, sitting 14 days after the LMP, is 280 − 14 = 266 days before the due date; the tool subtracts 266 days. It also fills in the complementary date — the estimated due date in LMP mode, or the approximate LMP in due-date mode.
All arithmetic is in whole calendar days at local midnight, so no rounding or time-of-day effects apply. The fertile window is shown as the conception day ±2 days. The estimate assumes a regular 28-day cycle with day-14 ovulation; shorter, longer, or irregular cycles, or a different ovulation day, shift the true conception date by several days. Only an early ultrasound or a known ovulation/IVF date is more precise.
Examples
| Input | Result |
|---|---|
| LMP 2026-01-01 | Conception ≈ 2026-01-15 (window Jan 13 – Jan 17) |
| Due date 2026-10-08 | Conception ≈ 2026-01-15 |
Frequently asked questions
It is an estimate based on an average 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, or ovulation was earlier or later, the true conception date can shift by several days.
Pregnancy is dated from the first day of the last menstrual period, but conception happens at ovulation — typically about 14 days later in a 28-day cycle. Sperm can survive a few days, so the fertile window spans roughly five days.
No. It gives a most-likely date and a short window. Only an early ultrasound or a known ovulation/IVF date can pin conception down more precisely.
Also known as
Sources & references
TG we-Calculate Editorial Team. (2026). Conception Date Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/conception-calculator
TG we-Calculate Editorial Team. "Conception Date Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/conception-calculator.
TG we-Calculate Editorial Team, "Conception Date Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/conception-calculator
@misc{wecalculate_conception_calculator, title = {Conception Date Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/conception-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?