Conception Date Calculator: Fertile Window and Likely Day

How to use it

Choose the Start from option that matches what you know.
Enter the date for that option, and your Cycle length (28 days if you do not know it).
Read the Most likely conception day, the Window when conception could have happened and the Estimated due date.

How the dates are estimated

Ovulation usually comes about 14 days before the next period. The second half of the cycle, the luteal phase, was long thought to last about 14 days, though it varies (Wikipedia, Luteal phase). So ovulation is the first day of the last period plus the cycle length minus 14 days, and conception most often happens on or near that day.

The due date is 266 days (38 weeks) after conception. Pregnancy is counted from 14 days before conception, and a term of 280 days from that starting point gives the due date (Wikipedia, Estimated date of delivery). For a 28-day cycle this is the same as Naegele's rule: the first day of the last period, plus one year, minus three months, plus seven days. ACOG's Committee Opinion 700 (2017) sets out the methods for estimating the due date.

These dates are estimates and not medical advice. Your midwife or doctor will confirm the due date, usually with an early ultrasound.

Ovulation:       last period + cycle length - 14 days
Fertile window:  ovulation - 5 days, to the day of ovulation
Due date:        conception + 266 days
From due date:   conception = due date - 266 days
From conception: due date = conception + 266 days

The fertile window

In a study of 221 women trying to conceive, Wilcox and colleagues (1995) found that conception occurred only after intercourse in the six days that ended on the day of ovulation. The chance of conception was 0.10 for intercourse five days before ovulation and 0.33 on the day of ovulation itself.

The window is a range of days for intercourse, not a single day. The calculator uses the day of ovulation for the most likely conception day. For a fuller calendar of fertile days, see the ovulation calendar.

What this calculator does not do

It cannot give the exact day of conception. Ovulation is estimated from the cycle length, and it can move by several days from cycle to cycle.
It does not confirm a pregnancy or a due date. An early ultrasound does that.
It is not contraception and does not protect against pregnancy or infection.

Worked example: last period on 1 August 2026, 28-day cycle

Ovulation: 1 August 2026 + 28 - 14 days = 15 August 2026, the most likely conception day.

Window: 10 August to 15 August 2026, the six days that end on ovulation.

Due date: 15 August 2026 + 266 days = 8 May 2027, the same as the last period plus 280 days.

Start from	Conception day	Due date
Last period, 1 August 2026	15 August 2026	8 May 2027
Known due date, 8 May 2027	15 August 2026	8 May 2027
Known conception, 15 August 2026	15 August 2026	8 May 2027

Frequently asked questions

Can I work out the exact day I conceived?

Not exactly. Ovulation is estimated from your cycle length, and intercourse up to five days before ovulation can lead to conception, so the calendar can only give a window. An early ultrasound and the dates your team records are more reliable.

Why does my cycle length change the date?

Ovulation is counted back from the next period, so a longer cycle puts ovulation later, and the due date moves later by the same number of days.

Why does the due date come 266 days after conception?

Pregnancy is counted from the first day of the last period, which is about 14 days before conception. Adding the usual 280-day term to that start gives 266 days from conception (Wikipedia, Estimated date of delivery).

Is this calculator the same as Naegele's rule?

For a 28-day cycle, yes: the first day of the last period plus 280 days gives the same due date. For a longer or shorter cycle, this calculator moves the due date by the difference from 28 days.

Does the calculator store the dates I enter?

No. The calculation runs in your browser, and nothing you enter is sent to us.

Sources

Figures and text last checked on Tuesday, November 10, 2026.

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