Quick answer: Conception most likely happened around ovulation. Choose From due date to subtract 266 days from your due date, or From last period to add 14 days to your LMP (adjusted for cycle length). You will also get your fertile window. This is an estimate, not an exact day.
Last reviewed: October 2026
Key facts
- Conception = due date - 266 days (38 weeks).
- Conception = LMP + (cycle length - 14) days, because ovulation sits about 14 days before the next period.
- The fertile window spans 5 days before ovulation through ovulation day itself.
- Conception is always a window of days, never an exact pinpoint.
A conception calculator estimates when conception most likely happened from either your due date or your last period. Subtract 266 days from the due date, or add your ovulation day (cycle length minus 14) to your LMP, and it also shows your fertile window.
When did it happen? The honest answer is a range of days, but the math narrows it well. Conception occurs at or near ovulation, and ovulation is the stable landmark of the cycle: it lands about 14 days before the next period, whatever the cycle length. That gives two ways in. If you know your due date, count back 266 days. If you know your last period, count forward to ovulation. This free tool runs both and maps your fertile window around the estimate.
Please note: This calculator estimates conception for information only. Exact conception can only be known with tracked ovulation or IVF timing; for medical questions, ask your provider.
How to use the conception calculator
- Choose From due date if you know your estimated due date, or From last period if you know your LMP.
- Enter the date, plus your cycle length if you picked the LMP method.
- Click Calculate to see your estimated conception date and fertile window.
Conception date formulas
Conception = LMP + (cycle length - 14) days
A full-term pregnancy is 280 days from the last period to the due date, and conception happens about 14 days after the period starts, leaving 266 days from conception to birth. That is why the reverse calculation from a due date subtracts 266 days. From the LMP side, ovulation day equals cycle length minus 14: day 14 in a 28-day cycle, day 21 in a 35-day cycle. The fertile window extends about 5 days before ovulation through ovulation day itself, because sperm can survive up to 5 days while the egg is viable for 12 to 24 hours.
Worked examples
Example 1: due date of October 8, 2026
October 8, 2026 minus 266 days = January 15, 2026. That is the estimated conception date. Counting forward, the likely ovulation day was also January 15, so the fertile window ran from January 10 through January 16.
Example 2: LMP of January 1, 2026, 28-day cycle
Ovulation day = 28 - 14 = day 14, so January 1 + 14 days = January 15, 2026. The fertile window = ovulation minus 5 days to ovulation plus 1 day = January 10 to January 16, 2026. Both methods agree, as expected for a regular cycle.
Cycle length and ovulation day
| Cycle length | Ovulation day | Fertile window (cycle days) |
|---|---|---|
| 21 days | Day 7 | Days 2 - 8 |
| 26 days | Day 12 | Days 7 - 13 |
| 28 days | Day 14 | Days 9 - 15 |
| 30 days | Day 16 | Days 11 - 17 |
| 32 days | Day 18 | Days 13 - 19 |
| 35 days | Day 21 | Days 16 - 22 |
The fertile window, in plain terms
The window is 6 days wide: the 5 days before ovulation plus ovulation day itself. Pregnancy is most likely from intercourse 1 to 2 days before ovulation, because the sperm are already waiting when the egg releases. Chances drop to nearly zero more than 5 days before ovulation or more than a day after it. This is why the calculator shows the window, not just one date: conception could have started on any of those days.
Tips for a better estimate
- Use your average cycle length over the last few cycles, not just one.
- An ultrasound-set due date gives a more reliable reverse calculation than an LMP-based one for irregular cycles.
- If you tracked ovulation with test strips or temperature, use that date directly: it beats any formula.
- Remember the window, not the day: the estimate centers on ovulation, with about 6 fertile days around it.
Common mistakes
- Expecting an exact day. Conception can only ever be estimated as a window of several days.
- Assuming day 14. Ovulation shifts with cycle length: day 21 in a 35-day cycle, not day 14.
- Mixing up gestational and fetal age. Gestational age (from LMP) runs about 2 weeks ahead of the age since conception.
- Using spotting as the LMP. Only the first day of full flow starts the count.
Frequently asked questions
How is the conception date calculated from a due date?
Subtract 266 days (38 weeks) from the due date. Pregnancy from conception to birth averages 38 weeks, while the 40-week count starts from the last period, about 2 weeks earlier.
How is the conception date calculated from a last period?
Add your ovulation day to the first day of your last period. Ovulation day equals cycle length minus 14, so for a 28-day cycle, conception is estimated at LMP plus 14 days.
What is the fertile window?
The 6 days when pregnancy is possible: the 5 days before ovulation plus ovulation day itself. Sperm survive up to 5 days and the egg for 12 to 24 hours, so intercourse days before ovulation can still lead to conception.
Can the calculator give an exact conception day?
No. Without tracked ovulation or IVF timing, conception can only be estimated as a window of several days. The calculator gives the most likely date with a 6-day fertile window around it.
Does cycle length change the estimate?
Yes. Ovulation sits about 14 days before the next period, so in a 35-day cycle it falls on day 21, a full week later than in a 28-day cycle. Enter your real cycle length for a better estimate.
Is an ultrasound due date better for this?
Yes. First-trimester ultrasound dating is the most accurate method, so a reverse calculation from an ultrasound-set due date is more reliable than one from the LMP, especially with irregular cycles.
What conception date does a due date of 7 April 2027 give?
Subtract 266 days from 7 April 2027 to get 15 July 2026. Counting the fertile window as 5 days before ovulation through 1 day after it, the window runs from 10 July to 16 July 2026. As with every result here, this is an estimate centred on ovulation, not a certain day.
How does a 35-day cycle change the estimate for a last period of 1 March 2026?
Ovulation sits about 14 days before the next period, so the ovulation day is 35 - 14 = day 21 of the cycle. 1 March 2026 + 21 days = 22 March 2026. That is 7 days later than the 15 March 2026 estimate for a standard 28-day cycle, which is why entering your real cycle length matters.
Related: Ovulation Calculator: Find Your Fertile Window
Related: Period Calculator: Predict Your Next Period Date
Sources
- ACOG (American College of Obstetricians and Gynecologists): methods for estimating the due date (Committee Opinion 700)
- CDC: pregnancy and prenatal care guidance
Key takeaways
- Conception = due date minus 266 days, or LMP plus (cycle length minus 14).
- Ovulation sits about 14 days before the next period, whatever the cycle length.
- The fertile window is 5 days before ovulation through ovulation day.
- Conception is always an estimate across several days, never an exact pinpoint.
- For medical questions, confirm with your healthcare provider.
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