Conception Calculator · Estimate Date of Conception

Conception Calculator

Estimate the date of conception from your due date, last menstrual period, ultrasound dating, or a baby's birth date

How would you like to calculate?

Method

Estimated due date

Your provider's estimated due date (EDD). Conception is estimated as 266 days before this date.

First day of last period

Average cycle length

days

Conception is estimated as LMP + (cycle length − 14) days. For a 28-day cycle that's LMP + 14 days.

Ultrasound date

Gestational age at scan

weeks

days

Gestational age is measured from the LMP. An 8-week scan means LMP was 8 weeks before the scan date.

Baby's birth date

Best estimate for full-term births (37–42 weeks). Preterm or post-term births give a less accurate conception date.

Average timings: gestation lasts 280 days (40 weeks) from LMP to birth, or 266 days (38 weeks) from conception to birth. Ovulation and conception typically occur about 14 days after the LMP in a 28-day cycle.

Quick examples

Conception Result

Estimated conception date

—

— due date — LMP —

Due date

—

LMP (estimated)

—

Gestational age today

—

Trimester

—

Your fertile window

Phase	Date	What happens

Pregnancy timeline

Milestone	Gestational age	Date	Notes

How accurate is each method?

Method	Accuracy	Best for	Limitations

Clinical notes

How conception is dated: Sperm survive up to 5 days in the reproductive tract, and the egg lives 12–24 hours after ovulation. So intercourse up to 5 days before ovulation can result in conception on the day of ovulation. The "conception date" is a best estimate — it reflects the day of fertilisation, not necessarily the day of intercourse.

Ultrasound is most accurate: A first-trimester ultrasound (before 14 weeks) is the most reliable way to date a pregnancy — accurate to within 3–5 days. Later scans are less precise. Many providers change the due date if the ultrasound differs from LMP dating by more than 7 days in the first trimester.

Limitations: A conception date is a statistical estimate, not a precise moment. Cycles vary in length, ovulation can shift by several days due to stress, illness, or travel, and sperm can survive multiple days. If you need a legally or medically precise date, a healthcare provider can help — but no method can pinpoint the exact day of fertilisation.

Important: This calculator is for educational purposes only and is not medical advice. Always rely on your healthcare provider's dating for clinical decisions.

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Creator & Maintainer

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.

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Introduction

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.

How the Conception Calculator Formula Works

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.

Worked Example

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.

Frequently Asked Questions

How is the conception date calculated?

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.

How accurate is a conception date?

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.

Can you calculate conception from an ultrasound?

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.

What is the difference between conception date and LMP?

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.

How many days after ovulation does conception happen?

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.

Can I get pregnant 5 days before ovulation?

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.

Why is my conception date different from what I expected?

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.

Can I calculate conception from a birth date?

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.

What is the most accurate way to date a pregnancy?

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.

How do I know which trimester I’m in?

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.

What if my cycle is longer than 28 days?

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.

Does conception happen on the day of intercourse?

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.

Disclaimer

This conception calculator is for educational purposes only and is not medical advice. It estimates a date using average timings and the inputs you provide. Actual conception may differ by several days. Always rely on your healthcare provider’s dating for clinical decisions, including ultrasound scheduling, genetic testing windows, and delivery planning.