Conception Calculator
Work backward from a due date or birth date to estimate when conception most likely happened.
How to use the Conception Calculator
- Choose the calculate from.
- Pick the date.
- Enter the gestational age at birth.
- Enter the plus days.
- Press Calculate. The result updates automatically as you type.
About the Conception Calculator
Conception usually happens within a day of ovulation, about two weeks after the first day of the last period in a typical 28-day cycle. Working backward 266 days from a due date (38 weeks) therefore gives a reasonable estimate of when the egg was fertilized.
The estimate is approximate. Ovulation can shift by several days between cycles, and due dates themselves have an uncertainty of about a week even when set by early ultrasound. If your due date was set or changed by ultrasound, use that date for the most accurate result. A calculation like this cannot establish paternity.
Results are estimates for general information only and are not medical advice. Talk to a doctor or registered dietitian before making major changes.
Formula
LMP = due date − 280 days (or birth date − gestational age)
Conception ≈ LMP + 14 days = due date − 266 days
The conception window shown is ±3 days around that estimate. Because sperm can survive up to five days, the intercourse that led to pregnancy may have happened up to five days before conception.
Example calculation
Using the default values:
- Calculate from = Due date
- Date = 2027-06-08
the calculator returns:
- Most likely conception date: Tuesday, September 15, 2026
- Possible conception window: Saturday, September 12, 2026 – Friday, September 18, 2026
- Fertile intercourse window: Thursday, September 10, 2026 – Wednesday, September 16, 2026
- Estimated last menstrual period: Tuesday, September 1, 2026
Frequently asked questions
How many days before the due date is conception?
About 266 days, or 38 weeks, in a typical pregnancy.
Can a conception calculator tell me the exact day?
No. It gives the most likely date, but the true day could be several days earlier or later because ovulation timing varies.
What if my baby was born early or late?
Choose "Birth date" and enter the gestational age the doctor recorded at birth. The calculator subtracts that from the birth date instead of assuming 40 weeks.
Last updated: · Category: Health & Fitness calculators