Conception Date Calculator
Conception date calculator: estimate when conception likely happened from your due date or the first day of your last period.
Editorial review: Farrow (pseudonym; identity and professional credentials are not disclosed). This page is for educational purposes, not professional advice.
Your inputs
Results update as you type. Nothing you enter leaves your device.
Results
- Likely window
- 2026-02-09 to 2026-02-15
Example with the starting values
These results use the calculator's initial inputs. Change the values above to calculate your own estimate.
Inputs
- I know my
- Due date
- Days from today (negative = past, positive = future)
- 0 days
- Average cycle length
- 28 days
Results
- Estimated conception date
- 2026-02-12
- Likely window
- 2026-02-09 to 2026-02-15
How does the Conception Date Calculator work?
A full-term pregnancy is estimated at 266 days (38 weeks) from conception to due date, or 280 days (40 weeks) from the first day of the last menstrual period (LMP), since ovulation and conception usually happen about 2 weeks after LMP.
From a due date, conception date ≈ due date − 266 days. From an LMP, conception date ≈ LMP + 14 days (adjustable for a longer or shorter cycle).
For example, with today entered as the due date and a standard 28-day cycle, conception lands about 266 days earlier, with a likely window of a few days on either side of that date.
Frequently asked questions
Is this exact?
No. Ovulation timing varies by cycle length and person, so this gives a likely window of a few days around the estimate, not a certainty.
Why isn't conception the same as my last period date?
Conception requires ovulation, which typically occurs about 2 weeks after the first day of the last period in an average 28-day cycle.
Does cycle length change the estimate?
Yes. A longer cycle shifts ovulation (and conception) later than 14 days after LMP; a shorter cycle shifts it earlier.
What if I only know my due date?
Use the due date mode: it subtracts the standard 266-day conception-to-due-date length, no cycle information needed.
Is this the same as a "when did I conceive" calculator?
Yes. Whether you search when did i conceive calculator or conception calculator from due date, this tool answers both by estimating the same ovulation conception date.