Conception-date math checked against the standard 14-day ovulation estimate on the editorial standards page.
In a standard 28-day cycle, conception happens around day 14 after the first day of your last period, which is also roughly 266 days before your due date. You can reach an estimate either way, from your due date backward or from your last period forward, and both methods land on the same window: a handful of days around ovulation, not one exact date.
Two ways to work it out
Working backward from a due date: since Naegele's rule adds 280 days to LMP and ovulation typically falls 14 days after LMP, conception sits about 280 minus 14, or 266 days, before the due date. Subtract 266 from your due date and you have an estimate. Working forward from LMP: add roughly 14 days to the first day of your last period to land on the likely ovulation window. Adjust for cycle length if yours runs longer or shorter than 28 days, for instance adding about 18 days instead of 14 for a 32-day cycle. The Conception Calculator and Conception Date Calculator on this site run both directions automatically.
Why sperm survival turns a date into a window
Sperm can survive in the reproductive tract for 3 to 5 days, and an egg survives roughly 12 to 24 hours after ovulation. That means intercourse anywhere from about 5 days before ovulation to around a day after it can result in a pregnancy. A "conception date" is honestly a window of several days rather than a single point, and pinning it to one exact day usually is not possible without deliberate fertility tracking, such as basal body temperature charting or ovulation predictor kits used in the relevant cycle.
Adjusting for a longer or shorter cycle
Ovulation timing shifts with cycle length: longer cycles generally mean later ovulation, shorter cycles earlier. If you know your average cycle length, both calculators on this site use it instead of assuming 28 days, which tightens the estimate somewhat, though it still describes a window rather than a single day.
How an ultrasound narrows the window
An early ultrasound gives a fetal age that can be worked backward to a narrower conception estimate than last-period math alone, since it measures development directly rather than assuming a textbook cycle. First-trimester dating is accurate to within about 5 to 7 days under ACOG's guidance (see the editorial standards page for the full redating table), which is tighter than an LMP-only guess but still not a single verified date.
When the exact date actually matters
For everyday planning, a several-day window is plenty. For a situation that needs real precision, a legal question or a specific medical determination, a calculator on this site is not the right tool; that calls for clinical dating and, where relevant, testing done through your provider.
Estimate your window
Work from a due date or last period to see the likely conception window for your cycle.