Conception Calculator – Estimate Your Conception Date
Estimate when conception likely happened from your due date.
About the Pregnancy Conception Calculator
What is the Pregnancy Conception Calculator?
A pregnancy conception calculator works backwards from a due date to estimate when conception probably happened.
Why use this calculator?
It is useful for dating early pregnancy events, understanding your cycle and reconciling an ultrasound date with your memory.
How to use it
- Enter your estimated due date.
- The calculator shows conception and a plausible window around it.
- Also see the approximate first day of your last period.
Formula
Conception ≈ due date − 266 days.
Outcomes vary; the true conception date can sit a few days either side of the estimate.
Frequently Asked Questions
Why 266 days from due date to conception?
Pregnancy is counted as 280 days from the LMP, but ovulation and conception happen about 14 days after the LMP, leaving about 266 days.
Can sex on the exact day of ovulation get me pregnant?
Yes, and the day before ovulation is your single most fertile day because sperm can survive several days inside the body.
Does it account for leap years?
Yes. The date logic follows the Gregorian calendar, including leap years and differing month lengths.
Are weekends and holidays excluded?
Only if the calculator says so. By default, results count every calendar day; use a business-day option where available.
Which time zone is used?
Calculations use whole calendar dates and are time-zone independent unless a time-of-day is involved.
Is this a medical diagnosis?
No. This tool provides general wellness information only. It is not a substitute for evaluation by a qualified healthcare professional.
Why do results differ from my doctor's measurements?
Clinical measurements use calibrated equipment and professional technique. Home estimates rely on formulas and self-reported values, so small differences are normal.
How often should I recalculate?
Recheck after meaningful changes such as weight gain or loss, a new workout routine, or every few months to track trends over time.
Do these formulas apply to everyone?
Most population formulas are less accurate for athletes, pregnant people, very young children and the elderly. Use them as a general guide.
Is this calculator free to use?
Yes. Every calculator on this site is completely free, with no hidden fees, no subscriptions and no limits on how many times you can use it.