Health / Browser tools
Conception Calculator
Estimate an ovulation or conception date and the fertile intercourse window from a due date or last period.
Conception Calculator: Estimate an ovulation or conception date and the fertile intercourse window from a due date or last period. Runs 100% locally in your browser with zero server file uploads.
- Category
- Health & sport tools
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
The estimate subtracts 266 days from the due date. The fertile intercourse window extends from five days before estimated ovulation to one day after. It cannot identify the exact conception date or establish paternity, and must not be used as contraception. Irregular cycles and changes to pregnancy dating can shift the estimate. ACOG guidance
Pregnancy dating
For a due-date input, estimated conception is 266 days earlier. For a last-period input, the due date is 280 days later plus the difference from a 28-day cycle. ACOG, Methods for Estimating the Due Date: https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
Fertile intercourse window
ACOG describes ovulation as approximately 14 days before the next period and pregnancy as possible from intercourse five days before to one day after ovulation. The tool applies that window to its estimated date. https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning
How to use it
- Choose due date or first day of the last menstrual period.
- For a last-period estimate, enter your usual cycle length.
- Read the estimated date and the window of intercourse that could lead to conception.
Privacy & limitations
Your inputs stay in this browser. Nothing you enter is uploaded or saved.
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Frequently asked questions
Can this identify an exact conception date?
No. Ovulation varies and sperm can survive before it occurs. The window refers to intercourse that could result in pregnancy, not a confirmed fertilisation date.
Can I use this for contraception or paternity?
No. Calendar estimates are too uncertain for either. Clinical dating, including ultrasound, may change the due date.
Free tool · runs in your browser · no account required