Conception Calculator

How to estimate your conception date

1. Choose to work from a due date or from your last period.
2. Enter that date.
3. From the last period, set your average cycle length.
4. Read your estimated conception date and the likely window.

How the conception date is estimated

Conception happens at fertilisation, around the time of ovulation. This conception calculator places it two ways. To estimate conception date from a due date, it counts back about 266 days, the usual time from conception to birth, so the conception date from due date falls near ovulation. From your last period, it adds your cycle length minus roughly 14 days, to land near ovulation. Either way the result is centred on ovulation, which is why cycle length matters when you work from a last period.

Why it is a window, not an exact day

The date this gives is a best estimate, not a certainty. Ovulation timing varies between cycles, sperm can survive several days, and the day of intercourse is not always the day of conception. So the tool shows a likely window of a few days around the estimated date rather than pretending to pinpoint one. Treat it as an approximate range.

Source: conception-to-birth interval and ovulation timing as described by the American College of Obstetricians and Gynecologists (ACOG) and the UK National Health Service (NHS).

An estimate, not medical advice

This is an estimate that varies by cycle and by person, and it cannot establish exactly when conception occurred. It is not a diagnosis or medical advice, and it should not be used to settle questions of paternity or timing. An ultrasound and your doctor or midwife confirm pregnancy dating. Always follow their guidance.

FAQ

When did I conceive?

Conception is usually around ovulation. From a due date, it is about 266 days earlier; from your last period, it is roughly your cycle length minus 14 days later. The tool shows the estimated date and a window around it.

How accurate is a conception date?

It is an estimate with a few days of uncertainty on either side, because ovulation and sperm survival vary. That is why the result is a window, not a single day.

Can this prove a date of conception?

No. It gives a probable range only and cannot establish an exact date, so it should not be used for paternity or legal questions. A doctor and an ultrasound provide dating.

Does cycle length change the estimate?

Yes, when you work from your last period. Ovulation shifts with cycle length, so a longer or shorter cycle moves the estimated conception date.

Is my information private?

Yes. Everything is calculated in your browser and nothing you enter is uploaded or saved.