Zerdly

Conception Date Calculator

Estimate your likely conception date from a due date or last period using this free conception date calculator that runs in your browser.

100% FreeRuns in your browserBasic
How to use this calculator

These dates stay in this browser tab. Nothing is saved, put in the page address or sent anywhere.

Estimate

Choose a source and enter its date.

Estimate only — not medical advice. Cycles, ovulation and due dates vary between people and from cycle to cycle. Confirm with a healthcare professional.

How the method works

Three source dates, one due-date arithmetic
Illustrative method
Three source dates, one due-date arithmeticA day axis from the last menstrual period (day 0) to the estimated due date (day 280). Arrows show LMP plus 280 days, fertilisation (day 14) plus 266 days, and a day-5 embryo transfer (day 19) plus 261 days all reaching the same due date.Day 0 · LMPDay 280 · due date10 wk20 wk30 wkLMP + 280 daysfrom the last period start (day 0)Fertilisation + 266 daysfrom the conception estimate (day 14)Day-5 transfer + 261 days (266 − 5)from the IVF transfer (day 19)IVF: EDD = transfer + 266 − embryo age
Estimated due date = LMP + 280 days = fertilisation + 266 days = IVF transfer + 266 − embryo age in days (day 5 → + 261). Example: a day-5 transfer on 2026-03-31 gives 2026-12-17. Weeks are counted from the LMP, so fertilisation sits at about 2 weeks 0 days.

A clinician's dating scan or established due date takes precedence; this tool never re-dates it. Fixed example values for illustration — not your result.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Choose what to work from: a due date, your last period (with cycle and luteal lengths), a known ovulation date, or an IVF transfer.
  2. Enter the date.
  3. Read the estimated fertilisation date and, separately, a broad interval of possible intercourse dates.

How it works

From a due date: fertilisation ≈ due date − 266 days. From the last period: last period + cycle length − luteal phase. From an IVF transfer: transfer date − embryo age, which describes the embryo, not intercourse.

Fertilisation and intercourse are different events. Because sperm can survive several days, the possible intercourse interval is the few days up to estimated ovulation — broad, not a probability, and never evidence of paternity.

The precision of the source date is unknown, so no confidence interval is invented, and a clinician's dates are used as given.

Worked example

Due 8 Oct 2026. Estimated fertilisation 15 Jan 2026 (8 Oct − 266 days), with possible intercourse dates 10–15 Jan. A last period of 1 Jan with a 28-day cycle and 14-day luteal phase gives the same 15 Jan; a day-5 embryo transferred on 31 Mar has a fertilisation-equivalent date of 26 Mar.

Common mistakes

  • Taking the estimate as the day of intercourse.
  • Assuming a 14-day offset from the last period when cycles are longer or shorter.
  • Reading the interval as a probability or as proof of paternity.

Tips

  • Estimate only — not medical advice. Dates and fertile windows vary between people and cycles; confirm with a healthcare professional.

Frequently asked questions

Can it pinpoint the exact day?

No — conception is rarely known exactly; this gives an estimate and a broad interval, not a certainty.

Privacy?

Runs entirely in your browser. Your inputs are not saved or transmitted.

Related tools

Explore more in Family, Parenting & Custody.

Cite this tool

Zerdly. (2026). Conception Date Calculator. Retrieved from https://www.zerdly.com/tools/conception-date-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.