Women's Health tool
Conception Date Calculator
Estimate the conception date and the days it most likely happened from a due date or the first day of the last period, for regular cycles. Free, nothing stored.
Nothing you enter leaves your device.
Inputs
Results
Enter values to see your results.
Simple steps
How it works
- 01
Choose whether you know the due date or the first day of the last period.
- 02
Enter the date, and your cycle length if you use the last period.
- 03
Select Calculate to see the estimated conception date and window.
Good to know
Frequently asked questions
How is the conception date estimated?
From a due date, 266 days (38 weeks) are counted back. From the last period, ovulation is estimated 14 days before the next period would have started.
Why is there a window, not one day?
Sperm can live up to five days and an egg about one day, so conception may follow intercourse by several days.
Can it tell who the father is?
No. Dates are estimates and cannot prove when conception happened. Only a medical test can answer questions about paternity.
How accurate is it?
It is less precise than an early ultrasound, which is the best way to date a pregnancy.
My cycles are irregular. Can I use it?
Use the due date from your scan if you have one; estimates from the last period assume regular cycles.
Is anything I enter stored or sent?
No. The calculation runs in your browser. Nothing you type is saved or sent to a server.
How it is calculated
From a due date: conception = due date − 266 days. From the last period: conception = first day + (cycle length − 14) days. The window runs from 5 days before to 1 day after (Wilcox and colleagues, 1995). The due date follows ACOG Committee Opinion 700.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion 700: Methods for estimating the due date
- Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21
Last reviewed: October 14, 2026