Conception usually happens within 24 hours of ovulation. Because sperm survive up to five days, the pregnancy may have started from intercourse several days earlier, which is why the calculator shows a window.
How to use the Conception Date Calculator
- I know my…
- First day of your last period
- Average cycle length
- Due date
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose whether you know your due date or your last period, and enter it.
Inputs explained
| Field | What to enter |
|---|---|
| I know my… | Choose one: Last menstrual period, Due date |
| First day of your last period | A date |
| Average cycle length | In days |
| Due date | A date |
What each result means
- Possible intercourse dates from = [Most likely conception date] – 5
- Possible intercourse dates to = [Most likely conception date] + 1
- Due date = [Most likely conception date] + 266
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Conception ≈ LMP + (cycle length − 14) or due date − 266 days
Fertile window = ovulation − 5 days to ovulation + 1 day
Worked example
Inputs — I know my…: Last menstrual period; First day of your last period: 2026-08-26; Average cycle length: 28 days.
Results — Most likely conception date = Wed, September 9, 2026; Possible intercourse dates from = Fri, September 4, 2026; Possible intercourse dates to = Thu, September 10, 2026; Due date = Wed, June 2, 2027.
Key terms
- Ovulation
- Release of an egg from the ovary.
- Fertile window
- Days in a cycle when intercourse can lead to pregnancy.
Frequently asked questions
How accurate is a conception date estimate?
Usually within a few days; an early ultrasound helps narrow it down.
Can a due date tell me who the father is?
Not reliably – conception windows overlap, and only a DNA test is conclusive.
What is the fertile window?
The five days before ovulation plus the day of ovulation.