Estimate when you conceived. This free conception date calculator works from your last period or due date and shows the likely conception window around ovulation.
Conception is estimated at ovulation — about 14 days before the next period would have started, or roughly 266 days before the due date.
Estimates only and not medical advice. The exact conception date can’t be confirmed; an early ultrasound and your healthcare provider give the most accurate pregnancy dating.
How the Conception Date Calculator Works
This tool estimates when conception likely happened, working either forward from the first day of your last period or backward from an estimated due date. Because ovulation timing varies, the result is a best-estimate date plus a short likely window, not a confirmed fact.
Formula: From last period: ovulation day = cycle length − 14; estimated conception date = last period date + ovulation day; estimated due date = last period date + 280 days + (cycle length − 28). From due date: estimated conception date = due date − 266 days. The conception window shown is conception date − 2 days to conception date + 1 day, and the implantation window is conception date + 7 to + 11 days.
- Estimate from — toggle between “Last period” and “Due date” as your starting point.
- First day of your last period — used with cycle length to estimate ovulation and conception.
- Average cycle length (days, optional) — defaults to 28 days if left blank; valid range is 20–45 days.
- Estimated due date — alternate starting point when your last period date isn’t known; conception is calculated backward from it.
Example Scenarios
| Estimate From | Date Entered | Cycle Length | Estimated Conception Date |
|---|---|---|---|
| Last period | Jan 1, 2027 | 28 (default) | Jan 15, 2027 |
| Last period | Jan 1, 2027 | 24 days | Jan 11, 2027 |
| Last period | Mar 10, 2027 | 30 days | Mar 26, 2027 |
| Due date | Dec 25, 2027 | n/a | Apr 3, 2027 |
| Due date | Nov 1, 2027 | n/a | Feb 8, 2027 |
Conception Date Calculator FAQ
How accurate is an estimated conception date from my last period?
It’s a reasonable estimate, not a confirmed fact. It assumes ovulation happened 14 days before your next period would have started, based on your entered cycle length. Actual ovulation timing varies, so treat the result as a likely window rather than one exact day.What if my cycle isn’t a typical 28 days?
Enter your own average cycle length (20–45 days) and the calculator adjusts both the ovulation day and due date accordingly. If you leave it blank, it defaults to 28 days, which may shift the estimate if your actual cycle is longer or shorter.Can I estimate conception if I don’t remember my last period date?
Yes. Switch to “Due date” mode and enter your estimated due date instead. The calculator works backward, subtracting 266 days to estimate conception, which is useful if your dates are based on an ultrasound rather than your last period.Why is there a conception window instead of one exact date?
Sperm can survive a few days before ovulation and the egg is viable briefly after, so conception could reasonably fall anywhere in a short window around the calculated date. Showing a range is more honest than presenting a single day as certain.What does the implantation date range mean?
It’s the approximate window, about 7 to 11 days after estimated conception, when a fertilized egg typically implants in the uterus. Implantation timing varies between pregnancies, so this range is for general context rather than a precise prediction.Can this calculator tell me the exact day I got pregnant?
No. It gives an estimate based on typical ovulation timing relative to your entered dates, not a verified fact. An early dating ultrasound with your healthcare provider gives the most accurate assessment of pregnancy timing.Related Calculators
Once you have an estimated conception date, the due date calculator and pregnancy week calculator can help confirm how far along you are, and the ovulation calculator is useful if you’re still trying to pinpoint your fertile window ahead of conception.