Use this free online conception calculator to estimate when you likely conceived, based on either your due date or the first day of your last menstrual period. Select what information you know, enter the date, and get an instant estimate of your conception date along with a related pregnancy date. This tool is for general informational purposes only and is not a substitute for medical advice.
Doctors date a pregnancy from the first day of the last menstrual period (LMP), not from conception, because the LMP is a date most women know while conception is rarely known exactly. A full-term pregnancy is taken as 280 days (40 weeks) from the LMP. In a regular 28-day cycle, ovulation and conception happen about 14 days after the LMP, so the time from conception to the due date is 266 days (38 weeks).
This is also why, at the moment of conception, a pregnancy is already described as about two weeks along: the count started at the LMP.
The calculator assumes a 28-day cycle. The phase after ovulation is fairly constant at around 14 days, so with a longer or shorter cycle it is ovulation that moves. A rough rule is that ovulation falls about (cycle length − 14) days after the LMP. For an LMP of 1 January 2026, that shifts the likely conception date as follows:
| Cycle length | Ovulation after LMP | Likely conception |
|---|---|---|
| 24 days | 10 days | 11 Jan 2026 |
| 26 days | 12 days | 13 Jan 2026 |
| 28 days | 14 days | 15 Jan 2026 |
| 30 days | 16 days | 17 Jan 2026 |
| 32 days | 18 days | 19 Jan 2026 |
| 35 days | 21 days | 22 Jan 2026 |
If your cycles are irregular, the LMP-based estimate is less reliable and an early ultrasound gives a better date.
Sperm can survive in the female reproductive tract for up to about five days, while the egg lives for roughly 12 to 24 hours after ovulation. So intercourse in the five days before ovulation, or on the day itself, can lead to pregnancy. The date shown is the most likely day of fertilisation, but the intercourse that led to it may have been several days earlier.
For the same reason, the calculator cannot identify a single day of intercourse with certainty, and it should not be used to settle questions of paternity. Only a DNA test can do that.
In the first trimester, an ultrasound that measures the baby from head to bottom (crown-rump length) is the most accurate way to date a pregnancy. If the scan and the LMP disagree by more than a few days, your doctor may revise the due date, and the scan date should then be used. If you enter that revised due date here, the conception estimate will follow it.
Only a small share of babies arrive on the due date itself; birth anywhere from 37 to 42 weeks is common. These dates are for general information only. Please follow the advice of your gynaecologist or midwife for antenatal care.