CalcKitHome›Family & Life›Conception
Conception Calculator
Your cycle dates
Example: last period August 3, 2026, 28-day cycle — fertile window August 12–18, 2026; due date May 10, 2027.
Behind the numbers
How conception dates are estimated
Conception can only happen around ovulation, when an egg is released. Ovulation usually happens about 14 days before the next period starts, so this calculator finds it as the last menstrual period plus the cycle length minus 14 days. Sperm can survive up to 5 days inside the body and an egg survives about 24 hours, so the fertile window runs from 5 days before ovulation through 1 day after it. The likely conception dates are that same window, shown as a range, because the exact day cannot be known from cycle dates alone. The estimated due date is the last menstrual period plus 280 days, which is 40 weeks and the standard way due dates are estimated.
Last menstrual period: August 3, 2026. Cycle length 28 days, so ovulation is August 3 + (28 − 14) days = August 3 + 14 days = August 17, 2026. Fertile window is ovulation minus 5 days through ovulation plus 1 day: August 17 − 5 days = August 12, 2026 to August 17 + 1 day = August 18, 2026. Due date is August 3, 2026 + 280 days = May 10, 2027. All dates use real calendar arithmetic, including the actual number of days in each month.
Examples
Example results
| Last period starts | Cycle length | Ovulation | Fertile window | Estimated due date |
|---|---|---|---|---|
| August 3, 2026 | 28 days | August 17, 2026 | August 12–18, 2026 | May 10, 2027 |
| February 2, 2026 | 28 days | February 16, 2026 | February 11–17, 2026 | November 9, 2026 |
| March 10, 2026 | 30 days | March 26, 2026 | March 21–27, 2026 | December 15, 2026 |
| April 6, 2026 | 26 days | April 18, 2026 | April 13–19, 2026 | January 11, 2027 |
Tips
Track at least three cycles and use the average length — one unusual cycle can shift the window by several days.
The most fertile days are the 2 days before ovulation and the day of ovulation itself.
Ovulation tests and basal body temperature can narrow the window more than dates alone.
Cycles shorter than 21 days or longer than 35 days, or very irregular cycles, make date-based estimates less reliable — talk to a doctor.
Only about 5% of babies arrive on their estimated due date, so treat it as a guide, not a fixed day.
Good to know
Frequently asked questions
Why is conception shown as a range, not one day?
Because sperm can live up to 5 days and an egg for about 1 day, conception can happen on any day in the fertile window. Cycle dates alone cannot pin it to a single day.
How is the due date calculated?
By adding 280 days (40 weeks) to the first day of the last menstrual period. This is the standard Naegele method, and it matches ovulation plus 266 days for a 28-day cycle.
What if my cycle is not 28 days?
Enter your average cycle length. Ovulation is estimated as the last period plus your cycle length minus 14 days, so a longer cycle moves ovulation and the fertile window later.
Is this medical advice?
No. These are estimates, not medical advice. If you are trying to conceive, are pregnant, or have concerns about your cycle, talk to a doctor or midwife.