Conception Calculator
Estimate your conception date from due date or last menstrual period. Free conception calculator with fertile window and ovulation estimates.
Conception at a glance
The estimate above is calculated from your last menstrual period (or your due date) and your cycle length, using the standard obstetric dating rules: conception is placed about 14 days after the start of your period, adjusted for how your cycle length differs from the textbook 28 days. The calculator does not guess a hidden fact: it shows the estimated conception date, the fertile window around it, and how the estimate shifts when your cycle is shorter or longer.
The calculation itself is deliberately transparent: every date shown is derived from the two or three inputs you enter, with the arithmetic spelled out so you can check each step by hand.
Estimate your conception date from due date or last menstrual period. Free conception calculator with fertile window and ovulation estimates. The intended workflow is calendar-first: the dates produced here are meant to be noted in your planner or shared with your provider; there is nothing to log in a food diary.
About the Conception Calculator
This calculator provides an evidence-based estimate of your conception date and fertile window based on standard obstetric dating conventions. The rules used are the classic Naegele's-rule family: conception placed about 14 days after the start of the last menstrual period, corrected for how your cycle length differs from 28 days, or back-computed from a due date. These conventions are described in standard clinical references on prenatal care and reproductive health.
How this calculator works
The calculator applies plain date arithmetic to the two or three inputs you enter: the first day of your last menstrual period (or your due date) and your cycle length. There are no interpretation categories and no hidden personal factors: the same inputs always produce the same dates, and the fertile window is shown around the estimated conception day. Always interpret the results in the context of your own cycle history, a single calendar estimate rarely tells the full story.
Practical guidance
Use this calculator as a starting point for understanding your fertile window when planning a pregnancy. The results are educational, not diagnostic, always discuss pregnancy planning, irregular cycles, or conception concerns with your healthcare provider. For best results, recalculate whenever a new period starts, and prefer a first-trimester ultrasound dating scan once available, as it is more accurate than any calendar estimate.
Evidence base and limitations
Calendar-based estimates assume a reasonably regular cycle and correctly remembered dates. Individual cycles vary, and ovulation can shift with stress, illness, travel, and conditions such as PCOS. This calculator does not replace professional medical advice. If your cycles are irregular, if you are undergoing fertility treatment, or if you have questions about conception or pregnancy dating, consult a physician or midwife for personalized guidance.
Formula sources, population limits, and references for this pregnancy calculator are documented in our public methodology. Read the methodology · Medical disclaimer