Conception CalculatorFree online fitness & health calculator — instant results, no sign-up
Estimate when conception likely happened from due date.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Conception Calculator
The Conception Calculator estimates the approximate date of fertilization and the start of the last menstrual period based on an expected due date. It is designed for expectant parents and individuals tracking reproductive timelines who know their estimated date of delivery. The tool uses standard gestational milestones to count backward to the probable window of conception.
- Due date
- The estimated date of delivery (EDD) provided by an ultrasound or healthcare provider, entered as a calendar date.
Formula and method
Estimated Conception = Due Date − 266 days Estimated Last Menstrual Period = Due Date − 280 days
- Due DateThe projected 40-week delivery date
- 266 daysAverage human fetal development duration from fertilization to birth (38 weeks)
- 280 daysStandard full gestational duration calculated from the first day of the last menstrual period (40 weeks)
The calculator subtracts exactly 266 days (38 weeks) from the provided due date to estimate the conception date, assuming fertilization occurred at ovulation. It also subtracts 280 days (40 weeks) from the due date to approximate the first day of the last menstrual period. No adjustments are made for irregular cycle lengths or leap years beyond standard calendar date arithmetic.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Due date1970-05-31
Results
- Estimated conceptionSun, September 7, 1969
- Last period (est.)Sun, August 24, 1969
How to use the Conception Calculator
- 1Select or enter the estimated due date.
- 2Subtract 266 days from the due date to establish the estimated conception date.
- 3Subtract 280 days from the due date to find the estimated start of the last menstrual period.
- 4Review the resulting calendar dates to identify the likely timing of ovulation and intercourse.
Reading your result
The 'Estimated conception' result indicates the approximate calendar day when the egg was fertilized, typically corresponding to ovulation. The 'Last period (est.)' result shows the theoretical start of the menstrual cycle preceding pregnancy. Because sperm can survive for up to five days in the reproductive tract, sexual intercourse resulting in pregnancy may have occurred several days prior to the estimated conception date.
Limitations
The calculation assumes a standard 28-day menstrual cycle where ovulation happens exactly 14 days after the start of menstruation. It does not account for variations in follicular phase length, irregular cycles, or due dates adjusted by early ultrasound measurements. Actual delivery dates naturally vary, with only about 5% of births occurring precisely on the calculated due date.
Common questions
Why is conception calculated as 266 days before the due date instead of 280 days?+
A standard pregnancy is tracked as 280 days (40 weeks) starting from the first day of the last menstrual period. Because ovulation and conception generally happen about 14 days after the period begins, fetal development actually lasts approximately 266 days (38 weeks).
Does the conception date equal the exact day intercourse occurred?+
Not necessarily. Sperm can survive inside the female reproductive tract for up to five days, meaning intercourse that led to pregnancy could have taken place up to five days before ovulation and fertilization occurred.
How accurate is this estimate if my due date was set by an ultrasound?+
An early first-trimester ultrasound is the most accurate clinical method for establishing gestational age. If your due date was confirmed by an early scan, counting back 266 days provides a strong estimate of conception within a window of a few days.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.