Conception Date Calculator

How to Use Conception Date Calculator

1. Select your reference. Choose whether you know the due date, the birth date, or want to estimate from your LMP.
2. Enter the date. For a due date, pick your estimated due date; for birth, enter the baby’s birth date; for LMP, provide the first day of your last period.
3. Click Calculate. The tool subtracts 266 days from the due date/birth date to estimate conception, or shows the fertile window (LMP+11 to LMP+21) when using LMP.
4. Note the likely window. You’ll see the most probable conception date and a window of ±3 to ±7 days, reflecting natural variation.

Benefits of Conception Date Calculator

• Works with three different inputs – due date, birth date, or LMP – making it flexible for any available information.
• Shows a realistic conception window rather than a single fixed day, accounting for sperm survival and ovulation timing.
• Useful for estimating conception when a dating scan was not performed early in pregnancy.
• Operates fully offline in your browser; no personal dates are sent over the internet.

Frequently Asked Questions

How accurate is the conception date from a due date?

It gives an approximate date with a few days of uncertainty because pregnancy length can naturally vary by up to a week. The window of ±3 days is a reasonable estimate.

Can I use birth date to find the exact conception date?

It provides a good approximation, but the variation in gestation length means the actual conception could be up to a week earlier or later. The calculator shows a ±7‑day window for this reason.

Why is there a difference between ultrasound due date and LMP due date?

Ultrasound measures the embryo’s size, which can give a more accurate due date early in pregnancy. LMP‑based calculations assume a 28‑day cycle and ovulation on day 14, which may not be true for everyone.

Is Conception Date Calculator free and private?

Yes — it is 100% free, needs no sign‑up, and everything runs in your browser; your data never leaves your device.