How does the Conception Calculator estimate a conception date from a due date?

It subtracts 266 days from the due date — the standard 280-day (40-week) full term minus the typical 14-day luteal phase between ovulation and the next expected period.

How does the Conception Calculator use my last menstrual period?

It adds 14 days to the first day of your last menstrual period, the usual ovulation and likely-conception day in a standard 28-day cycle.

Why does the Conception Calculator show a window instead of one exact date?

Ovulation timing varies from person to person and cycle to cycle, so the tool centers a 7-day likely window (plus or minus 3 days) on the estimated conception date rather than implying false precision.

Is the Conception Calculator's result medical advice?

No. It is a general estimate based on a standard obstetric formula. It does not diagnose or replace guidance from a doctor, midwife, or other qualified healthcare professional.

Does the Conception Calculator work if my cycle is not 28 days?

It uses the standard 28-day-cycle assumption for both modes, so a notably longer or shorter cycle can shift your actual conception date earlier or later within the shown window.

Are the dates I enter into the Conception Calculator uploaded anywhere?

No. The Conception Calculator runs entirely in your browser. The dates you enter are used only to compute the estimate on your device and are never sent to ArrayKit or anywhere else.

Conception Calculator Online

Estimate the likely date you conceived from a due date or your last period, right in your browser. A general estimate, not medical advice.

The Conception Calculator runs entirely in your browser. The dates you enter never leave your device and nothing is uploaded to ArrayKit. This is a general estimate using a standard formula, not medical advice.

Open the Ovulation Calculator

About Conception Calculator

Conception Calculator works out the likely date you conceived, starting from either a due date or the first day of your last menstrual period (LMP). From a due date, it subtracts 266 days — a full 280-day term minus the typical 14-day luteal phase — to land on an estimated conception date. From an LMP, it adds 14 days, the usual ovulation day in a standard 28-day cycle. Either way, the tool also shows a 7-day likely conception window centered on that date, since ovulation timing varies from cycle to cycle. It is handy when you are trying to pin down a due date's origin, curious about a baby's likely conception timing, or comparing notes with an ovulation tracker. Everything runs locally on your device — nothing is uploaded. This is a general estimate using a standard obstetric formula, not medical advice.

Features

Estimate conception date from either a due date or last menstrual period
Toggle between 'From due date' and 'From last period' modes
Shows a 7-day likely conception window, not just a single date
Uses the standard 280-day term and 14-day luteal-phase assumption
Copy a plain-text summary of the conception date and window
Instant results as you change the date, with no page reloads
Clear error message for an invalid or missing date
Runs entirely in your browser — your dates never leave your device

How to use the Conception Calculator

Choose 'From due date' or 'From last period'
Enter the relevant date
Read the estimated conception date and likely window
Copy the summary or switch modes to compare

Example

Input

due date Oct 8, 2026

Output

Conception: ~Jan 15, 2026

Oct 8, 2026 minus 266 days lands on an estimated conception date of Jan 15, 2026.

Common errors & troubleshooting

Conception Calculator shows an error after typing a date. — Use the date picker or a valid YYYY-MM-DD date — partial or malformed dates are rejected so the estimate stays accurate.
Result seems off by a few days compared to another due-date tool. — The estimate assumes a standard 28-day cycle with ovulation on day 14; a longer or shorter cycle shifts the actual conception date within the shown window.
Switching between 'From due date' and 'From last period' loses the entered date. — Each mode has its own date field — re-enter the relevant date after switching, or note both dates before toggling.
Conception date falls outside the shown window when double-checking by hand. — The window is fixed at plus or minus 3 days around the estimated conception date; recompute by hand using the same 266-day or 14-day offset to confirm.

Frequently asked questions

How does the Conception Calculator estimate a conception date from a due date?: It subtracts 266 days from the due date — the standard 280-day (40-week) full term minus the typical 14-day luteal phase between ovulation and the next expected period.
How does the Conception Calculator use my last menstrual period?: It adds 14 days to the first day of your last menstrual period, the usual ovulation and likely-conception day in a standard 28-day cycle.
Why does the Conception Calculator show a window instead of one exact date?: Ovulation timing varies from person to person and cycle to cycle, so the tool centers a 7-day likely window (plus or minus 3 days) on the estimated conception date rather than implying false precision.
Is the Conception Calculator's result medical advice?: No. It is a general estimate based on a standard obstetric formula. It does not diagnose or replace guidance from a doctor, midwife, or other qualified healthcare professional.
Does the Conception Calculator work if my cycle is not 28 days?: It uses the standard 28-day-cycle assumption for both modes, so a notably longer or shorter cycle can shift your actual conception date earlier or later within the shown window.
Are the dates I enter into the Conception Calculator uploaded anywhere?: No. The Conception Calculator runs entirely in your browser. The dates you enter are used only to compute the estimate on your device and are never sent to ArrayKit or anywhere else.

Related tools

Ovulation Calculator — Estimate your fertile window and ovulation day from your cycle.
Due Date Calculator — Estimate a pregnancy due date from the last menstrual period (Naegele’s rule).
Pregnancy Conception Calculator — Estimate the conception date and fertile window from a known pregnancy.
Pregnancy Calculator — Track pregnancy progress, trimester and milestones from the due date or LMP.
Period Calculator — Predict your next several periods and fertile windows from your cycle.
Pregnancy Weight Gain Calculator — Estimate a healthy pregnancy weight-gain range from pre-pregnancy BMI.