Conception Calculator
Work out when you conceived from your due date, last period or a known ovulation day.
What you know
The exact day cannot be known from a calendar. Sperm survive up to five days, so intercourse on any day of that six-day window could have led to this pregnancy, a fact that matters if the date is being used for anything beyond curiosity. An early dating scan is the accurate answer.
This estimates when conception occurred, working back from a due date or forward from a last menstrual period.
The result is a window of several days rather than a single date, and the page explains why.
How conception dates are estimated
Conception happens when sperm fertilises an egg, and it occurs within about 24 hours of ovulation, because that is how long the egg survives.
The complication is that sex and conception are not the same day. Sperm can survive up to five days in the reproductive tract, so intercourse on day 10 can result in conception on day 14. Any estimate is therefore a window of about six days, not a date.
Working back from a due date: subtract 280 days to reach the last menstrual period, then add about 14 to reach ovulation. That puts conception roughly 266 days before the due date, with a window of several days either side.
Conception happens within a day of ovulation, but it can result from intercourse up to five days earlier. That gap is why a precise date cannot be given.
What to enter
- Due date
- To work backwards. Subtract about 266 days to reach the likely conception date.
- First day of last period
- To work forwards. Conception is around cycle length minus 14 days from here.
- Cycle length
- Naegele's rule assumes 28 days. A longer cycle means later ovulation and a later conception date.
What this assumes
Estimates assume a regular cycle with a typical luteal phase. Irregular cycles make the window considerably wider.
A first-trimester ultrasound dates a pregnancy more accurately than any calculation from dates.
How to calculate your likely conception date
Both directions come from the same 280-day convention.
- 280
- Days from the last period to the due date
- 14
- Approximate days from the last period to ovulation on a 28-day cycle
Decide which direction you need. Backwards from a due date, or forwards from a last period.
Working backwards, subtract 266 days. That reaches the approximate ovulation and conception date.
Working forwards, subtract 14 from cycle length. On a 32-day cycle that is day 18, not day 14. Assuming 28 days is the most common error here.
Read the result as a window. Add roughly five days before the estimate to cover sperm survival. A single date overstates what can be known.
See a worked example: backwards from a due date
- Basis
- A known due date
- Cycle
- 28 days
Due date minus 280 days gives the first day of the last period.
Add about 14 days to reach ovulation, so conception is about 266 days before the due date.
The intercourse that led to it could have been on any of the five days before that.
On a 32-day cycle, ovulation is four days later, so the conception estimate shifts four days later too.
About 266 days before the due date, ± several days
Frequently asked questions
No. Conception occurs within about 24 hours of ovulation, but sperm survive up to five days, so intercourse and conception can be nearly a week apart.
The honest answer is a six-day window. Any tool giving a single date to the day is overstating what the arithmetic can support.
Subtract about 266 days. That comes from the 280-day due date convention minus the roughly 14 days between the last period and ovulation.
Adjust for cycle length: a 32-day cycle means ovulation four days later, so the conception estimate moves four days later as well.
No. Conception is fertilisation. Implantation, when the embryo attaches to the uterine lining, follows about 6-12 days later.
Pregnancy tests detect hCG, which is only produced after implantation. That is why testing too early gives a negative even when conception has occurred.
Because it is a date most people can identify, and ovulation usually is not. The whole 40-week convention is built on it.
The consequence is that you are counted as two weeks pregnant on the day you conceive, since the first two weeks of the count happen before it.
A first-trimester ultrasound is the most accurate method after IVF, typically within about five to seven days.
Accuracy falls later in pregnancy, because growth rates vary more between babies. Dating is normally established early and not revised afterwards.
Not to the day. An early scan estimates gestational age from measurements, which gives a range of several days rather than a specific date.
If the question matters for a specific reason, discuss it with a clinician. The measurement uncertainty is genuine and worth understanding properly.
Problems people actually run into
Treating the estimate as a single date
Sperm survival means the intercourse that led to conception could have been up to five days before fertilisation.
Any conception estimate covers about six days. If the answer matters, that width is the most important part of it.
Assuming a 28-day cycle
Naegele's rule and most calculators default to 28 days with ovulation on day 14. Real cycles vary considerably.
On a 32-day cycle, ovulation is day 18, so both the conception estimate and the due date shift four days later.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026