What it calculates
Conception usually happens within a day of ovulation, which occurs about 14 days before the next period. Because sperm can survive for several days, the possible window is wider.
How it works
From the last period, ovulation is estimated as LMP + cycle length − 14 days. From a due date, conception is estimated as 266 days before it.
Formula
Conception ≈ LMP + (cycle − 14) days, or Due date − 266 days
- Likely window: 5 days before to 1 day after the estimate
How to use it
- Choose whether you know your last period or your due date.
- Enter the date (and cycle length if using the last period).
- Read the estimated date and window.
For a last period 60 days ago and a 28-day cycle:
Why it helps
- Gives a likely window rather than false precision.
Limitations
- Cannot establish an exact date of conception.
- Less reliable with irregular cycles.
Frequently asked questions
Can this tell who the father is?
No. Date estimates cannot reliably establish that, and should never be used for it.
Do you store what I enter?
No. This calculator runs entirely in your browser. Your measurements and dates are not sent to or stored on our servers.
Sources
These calculators provide estimates only and are not a substitute for medical advice. Dates and ranges vary between individuals; follow the guidance of your doctor or midwife. Your inputs stay in your browser and are not stored by UrbanFiling.