Conception Date Calculator
Estimate the day you conceived from your due date or your last period — with a full six-day window and the three days that carry most of the probability.
Your details
Intercourse on the two days before ovulation carries the highest probability per cycle — earlier is better than later, because sperm wait for the egg but not vice versa.
"When did I conceive?" sounds like a question with one answer, but biology gives it a range: sperm survive up to five days inside the body, the egg waits about a day, so conception can result from intercourse anywhere in a six-day window around ovulation. A single-date answer is always an estimate; an honest calculator returns the window and marks the most likely days inside it.
The arithmetic runs in reverse. If you know your due date, conception day is 266 days before it — the biological pregnancy length. If you only know your last period, we estimate your ovulation day first (last period plus cycle length minus 14) and treat that as the most likely conception day, with the six-day window around it.
People reach for this calculation for many reasons — curiosity, dating a pregnancy that was discovered late, or working out paternity windows. On that last one, be aware that a six-day biological window plus dating uncertainty of several days means calculators can narrow things substantially but cannot settle questions that only DNA testing can answer.
How the calculation works
The due-date method divides the timeline at 266 days: conception = due date minus 266. This is exact whenever the due date itself was well established (IVF, or a first-trimester ultrasound).
The last-period method estimates ovulation as last period plus cycle length minus 14 days, then treats ovulation day as the most likely conception day. The full biological window extends from five days before to one day after it, because sperm survive up to five days and the egg about one.
Because cycles vary, the window from the second method is wider than it looks: a ±2-day ovulation error shifts the whole window by two days in either direction.
Frequently asked questions
Can a conception calculator tell me exactly which day I got pregnant?
Usually not to the day, and you should be skeptical of any tool that claims otherwise. What it can do is narrow a natural six-day window and rank the days inside it. The exceptions are IVF pregnancies (fertilization day is known exactly) and pregnancies where ovulation was tracked with LH tests or ultrasound.
Can intercourse more than five days before ovulation cause pregnancy?
It is very unlikely. The longest well-documented sperm survival is about five days, and that already sits at the extreme end. Intercourse more than five days before ovulation is generally considered outside the fertile window; intercourse the day after ovulation is also effectively too late, as the egg survives only 12–24 hours.
How is this different from a due date calculator?
It is the same arithmetic run backwards — 266 days instead of 280. A due date calculator starts from a known event (last period) and projects forward; this one starts from a known projection (the due date) and reconstructs the likely event. Using both together is a good internal check: the conception window implied by your due date should overlap the one implied by your cycle.
Can this help work out who the father is?
Only very coarsely. The six-day biological window plus dating uncertainty (easily ±5 days from a last-period estimate) means multiple partners can fall inside it even when the window looks narrow. Paternity questions need DNA testing; treat any calculator window as background context, not evidence.
I conceived while breastfeeding or before my first postpartum period. Does the math still work?
If you know your due date from an ultrasound, yes — the 266-day rule does not care about cycles. Without a due date, the last-period method is unreliable here because ovulation preceded the first period and cycle length is unknown. In that situation, an ultrasound date is the anchor to use.
References & sources
- Wilcox AJ, et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995.
- ACOG Committee Opinion 700: Methods for Estimating the Due Date (2017).
- Katz DF, et al. Sperm survival in the female reproductive tract. Fertil Steril. 1997.