Conception Calculator: When Did I Conceive?
Enter the first day of your last period, or your due date. This conception calculator estimates your conception window, ovulation day, due date and how many weeks pregnant you are today. It is filled in with an example so you can see how it works.
By Troopus · Updated: October 28, 2026
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Conception date and due date calculator
Cycle is not 28 days?
Estimates only, not medical advice. Real ovulation varies from cycle to cycle, and only a clinician's dating ultrasound is authoritative for gestational age and due date. Talk to your doctor or midwife about your pregnancy.
How does a conception calculator work?
A conception calculator counts forward from the first day of your last period to estimate ovulation, then builds a fertile window and a due date around it. With a 28 day cycle, ovulation falls about 14 days after your period starts, and conception most likely happens within a day of ovulation.
The tool uses three standard steps. First, ovulation is estimated as the first day of your last period plus your cycle length minus 14 days. Second, the conception window is the five days before ovulation up to ovulation day, which is when sex can lead to pregnancy. Third, the due date is the last period plus 280 days, adjusted if your cycle is not 28 days.
For example, if your last period started on January 1, 2026 and your cycle is 28 days, ovulation is estimated for January 15, the conception window runs from January 10 to January 15, and the due date is October 8, 2026.
How does a conception date calculator work from your due date?
Subtract 266 days (38 weeks) from the due date to get the estimated conception date. A due date of October 8, 2026 gives a conception date around January 15, 2026. Choose "Due date" in the calculator above, enter yours, and it works backward for you.
The 266 days come from the usual 280 day pregnancy count, which starts at the last period, minus the 14 days between that start and ovulation. When you start from a due date, the calculator assumes the usual timing, so the cycle length box is not used.
If your due date was set by an early ultrasound, the result is based on your baby's measured size instead of your last period, and it is usually more reliable than a date worked out from a period. Your clinician's dating is the one to trust.
How do you calculate the due date from your last period?
Add 280 days (40 weeks) to the first day of your last period, which is Naegele's rule, and adjust for your cycle length. With a 28 day cycle, a period starting January 1, 2026 gives a due date of October 8, 2026. A 32 day cycle moves it 4 days later, to October 12.
The cycle length matters because the 280 day count assumes ovulation on day 14. If you ovulate later, conception and the due date are later too. The NHS due date tool asks for your cycle length for this reason, and says pregnancy normally lasts 37 to 42 weeks from the first day of your last period, so the due date is a middle point, not a deadline.
| Cycle length | Ovulation | Due date |
|---|---|---|
| 24 days | January 11 | September 4, 2026 |
| 28 days | January 15 | October 8, 2026 |
| 32 days | January 19 | October 12, 2026 |
| 35 days | January 22 | October 15, 2026 |
Want to know your age in days or the time between two dates? Try the age calculator or the time duration calculator.
How does an ovulation date calculator find the fertile window?
The fertile window is the five days before ovulation plus ovulation day itself, six days in total. A peer-reviewed study found no fertile days after ovulation. Sperm can survive for several days, but an egg lives only about a day, so sex in the days before ovulation matters most.
That is why the calculator shows a range, not a single day. The same study found timing can be highly unpredictable, even when cycles are usually regular.
In practice the ovulation date calculator part of the tool estimates the day an egg is released from your cycle length, and the conception window is built around that day.
How accurate is a conception calculator?
A conception date from a calculator is an estimate that can be off by a week or more. It assumes ovulation exactly 14 days before your next period, and many cycles do not follow that. Only a dating ultrasound from a clinician gives an authoritative gestational age and due date.
Dating from a last period is the usual starting point, and ACOG guidance says an early ultrasound should be used to confirm or revise the due date. If the scan and your last period disagree by more than the allowed margin, the scan date is used for the rest of the pregnancy.
Do not use these dates for paternity, medical or legal decisions. With irregular cycles or any pregnancy question, speak to a doctor or midwife.
Frequently asked questions
When did I conceive?
Most likely about 14 days after your last period started, if you have a 28 day cycle. The calculator shows a fertile window of six days ending on ovulation. It cannot tell you which day within that window, and irregular cycles move the dates.
How do I calculate conception from my last period?
Add your cycle length minus 14 days to the first day of your last period to find ovulation. For a 28 day cycle that is 14 days. Conception most likely happens at or just after ovulation.
How many days after ovulation does conception happen?
An egg lives about a day after ovulation, so conception happens within roughly 24 hours of ovulation if sperm are already present. Sperm can wait for the egg for several days, which is why the window starts earlier.
Does my cycle length change the due date?
Yes, a little. The standard count assumes a 28 day cycle. Each extra day in your cycle moves the due date one day later, and each day fewer moves it one day earlier. Open the cycle option to adjust it.
Is a conception date calculator accurate?
It is a good estimate, not an exact answer. Ovulation varies between people and between cycles. A first trimester ultrasound is the most reliable way to date a pregnancy, and your clinician decides your official due date.
What if I do not know my last period date?
Use your best guess with caution, or ask your doctor or midwife about a dating scan. If you know your due date from a scan, choose Due date above and the calculator works backward to the conception window.
Sources
- NHS: Pregnancy due date calculator (28 day cycle default, 37 to 42 weeks, 12 week scan; checked October 28, 2026)
- NHS: Periods and fertility in the menstrual cycle (ovulation timing, sperm survival; checked October 28, 2026)
- Wilcox et al.: The timing of the fertile window in the menstrual cycle (PMC; checked October 28, 2026)
- Wikipedia: Naegele's rule (280 days from last period, ultrasound dating, accuracy; checked October 28, 2026)
- ACOG: Committee Opinion 700, Methods for Estimating the Due Date