Conception date calculator
Estimate a due date from a conception or ovulation date, or find your estimated conception date from a due date (or from your LMP). Runs in your browser; nothing is saved.
For IUI, use the insemination date.
How to use it
How conception dating works
A pregnancy runs about 266 days from conception, compared with 280 days from the last period — the 14-day difference being the usual gap to ovulation on a 28-day cycle. Working backwards, an estimated conception date is 266 days before the due date. The reasoning and its limits are covered in conception date and due date.
Worked example
A conception date of 15 January gives an estimated due date of about 8 October. In reverse, a due date of 8 October points to conception around mid-January, give or take a few days.
Accuracy and limitations
Conception dating is only as good as the conception date you have. Outside IVF, the exact day is rarely known — sperm survive several days, so intercourse and conception aren’t the same date. For a firmer date, an early ultrasound is best; see how accurate is your due date. For IVF, the IVF due date calculator is more precise.
Frequently asked questions
How many days after conception is the due date?
About 266 days, which is 14 days fewer than the 280 days counted from the last menstrual period.
How do I estimate my conception date from my due date?
Subtract 266 days from the due date. This tool does it for you and shows a likely window, since conception can fall a few days either side.
Is the conception date the same as the day I had sex?
Not necessarily. Sperm can survive several days, so conception can happen a few days after intercourse. That’s why conception dating is an estimate.
Can I use this for IUI?
Yes. IUI is timed to ovulation, so enter the insemination date as the conception date — the due date is about 266 days later.
Should I date my pregnancy from conception or my last period?
For most pregnancies, the last period or an early ultrasound is the standard. Conception dating is useful mainly when ovulation was tracked or as a cross-check.
Last meaningfully updated: 2026-08-25.