About the Conception Calculator
This conception calculator works a pregnancy backwards to estimate when you most likely conceived. Enter your due date (from your provider or an ultrasound) or the first day of your last period and your usual cycle length, and it returns the estimated conception date, the range of days when intercourse most likely resulted in the pregnancy, and the related dates for implantation and the earliest reliable pregnancy test.
Conception happens around ovulation. An egg survives only 12–24 hours after release, but sperm can survive up to five days in the reproductive tract, so the pregnancy most likely began from intercourse in the five days before ovulation or on the day after it. That seven-day window is what the calculator shows.
The estimate is only as precise as the due date it starts from. Due dates from an early ultrasound are accurate to within about a week, so treat the conception date as the centre of a range, not proof of a specific day — which matters if you are using it to answer questions about paternity.
How to use the conception calculator
- 1Choose whether to start from your due date or your last period.
- 2Enter the date, plus your cycle and luteal phase length if using your last period.
- 3Read the most likely conception date and the fertile window.
- 4Use the table to see which days carried the highest chance.
Formula and method
A pregnancy lasts about 266 days (38 weeks) from conception to the due date, so counting back 266 days from the due date gives the most likely conception date. When you start from your last period, ovulation occurs about one luteal phase (typically 14 days) before the next period, so ovulation ≈ LMP + cycle length − luteal phase.
Fertilisation happens within about a day of ovulation. Since sperm may survive up to five days and the egg only 12–24 hours, the fertile window is taken as five days before ovulation through one day after. Implantation typically occurs 6–12 days after ovulation, and hCG is usually detectable by a home test about 14 days after ovulation.
- 266
- Average days from conception to birth
- LMP
- First day of the last menstrual period
- luteal phase
- Days from ovulation to the next period (usually 12–14)
Worked examples
Due date March 8, 2027
Counting 266 days back from March 8, 2027 gives June 15, 2026 as the most likely conception date. The fertile window runs from June 10 to June 16, and a home pregnancy test would have been reliable from about June 29.
Last period June 1 with a 32-day cycle
With a 32-day cycle and a 14-day luteal phase, ovulation happens on cycle day 19: June 19, 2026. The fertile window opens June 14, and the resulting due date is 266 days after conception: March 12, 2027.
Short luteal phase
A 26-day cycle with an 11-day luteal phase places ovulation 15 days after the period started — January 20, 2026. That gives a fertile window ending January 21 and a due date of October 13, 2026.
Frequently asked questions
How accurate is a conception date calculator?+
It gives the most likely date, but due dates themselves are accurate only to about a week, and ovulation varies from cycle to cycle. Expect the real conception date to fall within several days of the estimate.
Can a conception calculator prove paternity?+
No. Because sperm can survive up to five days and dating has about a week of uncertainty, intercourse on dates up to 10 or more days apart can produce the same due date. Only a DNA test can establish paternity.
How many days after intercourse does conception happen?+
Fertilisation happens when sperm meets the egg, usually within 24 hours of ovulation. If intercourse happened a few days before ovulation, conception can occur up to about five days afterwards.
Is conception the same day as ovulation?+
Very nearly. The egg can only be fertilised for about 12–24 hours after it is released, so conception happens on ovulation day or the day after, even if intercourse was several days earlier.
When does implantation happen after conception?+
The fertilised egg travels to the uterus and implants about 6–12 days after ovulation, most often around day 8–10. Some people notice light implantation spotting at that time.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.