Paternity Calculator

Estimate the probable conception window from a birth or due date.

About Paternity Calculator

Gestational age is counted from the last period while conception occurs about two weeks later, which is why the two differ by a fortnight. Sperm survival widens the probable conception window to about six days. This arithmetic can only ever indicate a window, and DNA testing is the only reliable means of establishing paternity.

Formula

Paternity

Conception occurs about two weeks after the last period, so gestational age less 14 days

gestational age: Measured from the last menstrual period, not from conception
fertile window: Sperm survive up to five days, widening the probable window

Worked example

With the default values below, Paternity Calculator returns Conception Occurred This Many Days Before Birth of 266. Change any input above and every figure here is recalculated in your browser.

Inputs used

Gestational age at birthaccepts 20–45 weeks: 40 weeks
Days since birthminimum 0 days: 30 days
Typical cycle lengthaccepts 20–45 days: 28 days

Results

Conception Occurred This Many Days Before Birth: 266 days
Window Opened This Many Days Ago: 299 days
Window Closed This Many Days Ago: 294 days
Length of the Probable Window: 6 days
Estimated Ovulation Day of Cycle: 14.0

Step by step

1Values used
2Paternity
3Conception Occurred This Many Days Before Birth= 266 days
4Window Opened This Many Days Ago= 299 days
5Window Closed This Many Days Ago= 294 days
6Length of the Probable Window= 6 days
7Estimated Ovulation Day of Cycle= 14.0

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
This calculation indicates an approximate conception window only and cannot establish or exclude paternity. Only accredited DNA testing can determine paternity, and legal paternity is a separate question governed by local law. Seek qualified legal and medical advice.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

This calculation indicates an approximate conception window only and cannot establish or exclude paternity. Only accredited DNA testing can determine paternity, and legal paternity is a separate question governed by local law. Seek qualified legal and medical advice.

Frequently Asked Questions

How is Paternity calculated?

Conception occurs about two weeks after the last period, so gestational age less 14 days Gestational age is counted from the last period while conception occurs about two weeks later, which is why the two differ by a fortnight. Sperm survival widens the probable conception window to about six days.

Why does Paternity matter?

This arithmetic can only ever indicate a window, and DNA testing is the only reliable means of establishing paternity.

What values do I need to enter?

This calculator takes 3 inputs: Gestational age at birth, Days since birth, Typical cycle length. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.