Estimate the date of conception from your due date, last menstrual period, ultrasound dating, or a baby's birth date
| Phase | Date | What happens |
|---|
| Milestone | Gestational age | Date | Notes |
|---|
| Method | Accuracy | Best for | Limitations |
|---|
ADVERTISEMENT
Founder & CEO, Toolraxy
Faiq Ur Rahman is a web designer, digital product developer, and founder of Toolraxy, a growing platform of web-based calculators and utility tools. He specializes in building structured, user-friendly tools focused on health, finance, productivity, and everyday problem-solving.
User Ratings:
ADVERTISEMENT
ADVERTISEMENT
Conception dates don’t get stamped on a calendar. What you usually have is a due date, a positive test, or a memory of your last period. This calculator bridges that gap. It takes whichever anchor point you have and back-calculates the day fertilization most likely happened, using the standard 266-day interval from conception to birth and the 280-day interval from last menstrual period to birth.
Four input methods cover the common situations: a due date from your provider, an LMP with cycle length, an ultrasound report with gestational age, or a birth date if the baby is already here. Each method has a different accuracy window, and the tool shows that too.
How to Use the Conception Calculator
Pick a method from the dropdown: due date, LMP, ultrasound, or birth date.
Enter the date the method asks for. The input form swaps to match your choice.
Set your average cycle length if you chose LMP. The default sits at 28 days.
Adjust weeks and days separately for an ultrasound. Both fields accept whole numbers.
Read the hero result at the top. It shows the estimated conception date in large type.
Check the four tiles below for due date, LMP, gestational age today, and trimester.
Scroll the fertile window and pregnancy timeline tables for dated milestones.
Formula: Conception date = Due date − 266 days
Every method feeds into the same anchor. Once the calculator knows one date, it derives the other two using fixed offsets.
For a known due date: conception = due date − 266 days, and LMP = due date − 280 days.
For LMP: conception = LMP + (cycle length − 14) days, and due date = LMP + 280 days. A 28-day cycle puts conception at LMP + 14. A 35-day cycle pushes it to LMP + 21.
For ultrasound: gestational age at scan (weeks × 7 + days) tells you how far along you were on the scan date. The calculator subtracts that from 280 to get the due date, then subtracts 266 from the due date to get conception.
For a birth date: conception = birth date − 266 days. This assumes a term delivery. Preterm and post-term births widen the error.
Cycle length is clamped between 20 and 45 days. Gestational days at scan are clamped between 0 and 6. The tool never outputs a conception date beyond 42 weeks of gestational age without flagging it.
Suppose your provider gives you an estimated due date of September 2, 2026. You want to know the likely conception date.
Due date: September 2, 2026
Conception = September 2 − 266 days = December 10, 2025
LMP = September 2 − 280 days = November 26, 2025
The calculator would display “December 10, 2025” as the estimated conception date. Your fertile window opens around December 5 and closes around December 11. Implantation would fall roughly between December 16 and December 22. A home pregnancy test could turn positive as early as December 21.
That said, the real conception could be a day or two on either side. Sperm survive up to five days, so intercourse on December 5 could lead to fertilization on December 10. The 266-day offset is an average, not a fixed biological constant.
Conception is calculated as 266 days before your due date or 14 days after your LMP in a 28-day cycle. The calculator picks the formula that matches your input method and derives the other dates from there.
It’s accurate to within about one to two weeks for most methods. Early ultrasound narrows that to 3 to 5 days. No method can pinpoint the exact day because ovulation timing varies and sperm can survive up to five days.
Yes, and it’s the most accurate method available. Enter the scan date and the gestational age from the report. The calculator works out the due date and then the conception date from there.
The LMP is the first day of your last period, about two weeks before conception. Conception is the day of fertilization. Clinical dating starts from the LMP because it’s observable, which is why a 40-week pregnancy is really 38 weeks of fetal development.
Conception happens on the day of ovulation or within 24 hours of it. The egg lives 12 to 24 hours. Sperm can survive up to five days, so intercourse before ovulation can still lead to conception on ovulation day.
Yes. Sperm survive up to five days in the reproductive tract. Intercourse five days before ovulation can result in conception if sperm are still viable when the egg releases. That’s why the fertile window spans six days.
Cycle length, ovulation timing, and the dating method all shift the estimate. A 35-day cycle pushes conception a week later than a 28-day cycle. Stress and illness can delay ovulation further. Check your cycle length input first.
Yes, for full-term births. The calculator subtracts 266 days from the birth date. Preterm and post-term births widen the margin significantly, so treat the result as a rough estimate if the baby arrived before 37 weeks or after 42.
A first-trimester ultrasound before 14 weeks is the most accurate method, precise to within 3 to 5 days. LMP dating is reliable only for regular cycles. Later ultrasounds are less precise because fetal growth varies after 20 weeks.
The first trimester runs from 0 to 13 weeks 6 days. The second covers 14 to 27 weeks 6 days. The third starts at 28 weeks. The calculator shows your current trimester in the results tile based on today’s date.
Enter your actual average cycle length in the LMP method. The calculator adds (cycle length − 14) days to your LMP instead of the default 14. A 35-day cycle puts conception at LMP + 21.
Not always. Sperm can survive up to five days, so intercourse several days before ovulation can lead to conception when the egg releases. The conception date reflects fertilization, not the day of intercourse.
ADVERTISEMENT
ADVERTISEMENT