Conception Calculator
Estimate your conception date, ovulation day and fertile window from your last menstrual period or your due date, adjusted for your cycle length. Example: a last period starting January 10 with a 28-day cycle means ovulation around January 24 and a due date near October 17.
Show steps
How to use the Conception Calculator
- Choose whether you know your last period date or your due date.
- Enter the date and your usual cycle length (28 days if unsure).
- Press Calculate to see ovulation, the fertile window, the conception date and the due date.
Formula and method
Ovulation is estimated at cycle length minus 14 days after the first day of your last period. The fertile window is the five days before ovulation plus ovulation day and the day after. A due date is 280 days after the last period, shifted by the difference between your cycle length and 28.
Worked example: last period January 10, 28-day cycle
| Step | Date |
|---|---|
| First day of last period | January 10 |
| Ovulation (day 15) | January 24 |
| Fertile window | January 19 to January 25 |
| Conception (about) | January 24 |
| Due date (+280 days) | October 17 |
How cycle length changes ovulation
| Cycle length | Ovulation day |
|---|---|
| 24 days | Day 11 |
| 26 days | Day 13 |
| 28 days | Day 15 |
| 30 days | Day 17 |
| 32 days | Day 19 |
| 35 days | Day 22 |
Conception and the due date
Pregnancy is dated from the first day of the last period, about two weeks before conception, so a full-term pregnancy is about 266 days from conception and 280 days from the last period. An early ultrasound can adjust the date if your cycles are irregular.
Frequently asked questions
How do I calculate my conception date?
Count 14 days back from your next expected period, which is about 14 days after the first day of your last period in a 28-day cycle. Or subtract 266 days from your due date.
When am I most fertile?
In the five days before ovulation and on ovulation day. The two days before ovulation are usually the most fertile.
How long after ovulation does conception happen?
The egg survives 12 to 24 hours after release, so conception happens within about a day of ovulation.
Can I get pregnant outside the fertile window?
It is unlikely, but ovulation timing varies, so the window is an estimate.
What if my cycle is irregular?
The estimate is less reliable. Ovulation tests, basal body temperature tracking or a healthcare provider can pinpoint ovulation better.
Is the conception date the same as the date I had sex?
Not always. Sperm can live up to five days, so conception can occur days after intercourse.
Sources
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