🤰 Conception Date Calculator: When Did I Conceive?
By Shihab Mia · Updated 2026-07-02
This conception date calculator gives a rough estimate only and is not a medical diagnosis. Conception cannot be pinned to a single day: ovulation timing, cycle length and sperm survival all shift the real date by several days. The result does not establish paternity, legal dates, or exact gestational age. For anything that matters, including dating a pregnancy, rely on an ultrasound and your healthcare provider, not this calculator.
This conception date calculator estimates that you most likely conceived about 14 days after the first day of your last menstrual period (LMP), or roughly 266 days (38 weeks) before your estimated due date. Pick whether you know your LMP or your due date, enter that one date, and it works backward or forward to an approximate conception date plus a likely 5 to 6 day window. Because the exact day depends on when you ovulated and how long sperm survived, the answer is a best estimate, not a precise event you can mark on a calendar.
What is the Conception Date Calculator?
Conception, when a sperm fertilizes an egg, happens around ovulation. In a textbook 28 day cycle, ovulation falls about 14 days after the first day of the last menstrual period (LMP), so conception is usually estimated as LMP plus 14 days. If your cycles run longer or shorter, ovulation shifts with them: a reliable rule is that ovulation lands about 14 days before your next period, so conception is closer to (cycle length minus 14) days after the LMP. That is why this conception date calculator lets you adjust the cycle length when you use the LMP method, instead of assuming everyone runs a perfect 28 day cycle.
When you only know your estimated due date, the math runs the other way. Pregnancy is dated as about 280 days (40 weeks) from the LMP, and since the LMP is roughly 14 days before conception, the conception date sits about 266 days (38 weeks) before the due date. So the calculator simply subtracts 266 days from the due date to land on an approximate conception date. This reverse method is handy when a clinic or ultrasound has given you a due date but you never tracked your periods.
Why a window and not a single day? An egg lives only about 12 to 24 hours after ovulation, but sperm can survive in the reproductive tract for up to about 5 days. So intercourse several days before ovulation can still lead to conception, and the actual fertilization may be a few days off from the calculated estimate. The few-day window this tool shows reflects that biological reality, which is also why a conception date calculator should never be used to judge paternity or fix a legal date.
It also helps to separate two ideas that get confused: the date of conception and gestational age. Doctors count how far along you are (your weeks pregnant) from the LMP, not from conception. Because the LMP comes about two weeks before you actually conceive, your gestational age is roughly two weeks larger than the true time since fertilization. So at the moment of conception you are already counted as about 2 weeks pregnant, which surprises many people the first time they see it.
Finally, remember that any date-based estimate is only as good as the dates you feed it. Irregular cycles, late or early ovulation, spotting mistaken for a period, and simple recall errors can all move the real conception date. An early dating ultrasound, usually done in the first trimester, measures the embryo directly and is far more accurate than any calculator. Treat this conception date calculator as a quick, private starting point, then confirm anything important with your provider.
When to use it
- Getting a rough idea of when conception likely happened after a positive pregnancy test.
- Working back from an ultrasound or clinic due date to an approximate conception date.
- Satisfying curiosity about which weekend, trip, or event might line up with conception.
- Cross-checking the conception estimate your doctor or a pregnancy app already gave you.
- Adjusting the estimate for a longer or shorter cycle when you do not have a 28 day cycle.
- Understanding why your "weeks pregnant" figure is about two weeks more than time since conception.
How to use the Conception Date Calculator
- Choose whether you know your last menstrual period (LMP) or your estimated due date.
- Enter that single date in the date field.
- If you used the LMP method, adjust the average cycle length if yours is not 28 days.
- Read the estimated conception date and the likely conception window shown below.
- Note the window, not just the single date, since exact timing cannot be known.
Formula & method
Worked examples
Your last period started on 1 March and your cycle is the usual 28 days.
- Offset from LMP = cycle length - 14 = 28 - 14 = 14 days
- Conception = 1 March + 14 days = 15 March
- Window = 15 March - 3 days to 15 March + 2 days
- Window = 12 March to 17 March
Result: Estimated conception around 15 March (likely window 12 to 17 March).
You only know your estimated due date, which is 1 December.
- Conception = due date - 266 days
- 1 December minus 266 days lands on about 10 March of the same year
- Window = 10 March - 3 days to 10 March + 2 days
- Window = 7 March to 12 March
Result: Estimated conception around 10 March (likely window 7 to 12 March).
Your last period started on 1 March but your cycle runs long at 35 days.
- Offset from LMP = cycle length - 14 = 35 - 14 = 21 days
- Conception = 1 March + 21 days = 22 March
- A longer cycle means later ovulation, so conception shifts later too
- Window = 19 March to 24 March
Result: Estimated conception around 22 March (likely window 19 to 24 March).
How the calculation depends on what date you know
| You know | Estimate method | Typical offset |
|---|---|---|
| LMP, 28 day cycle | LMP + 14 days | 14 days after LMP |
| LMP, longer cycle | LMP + (cycle - 14) days | More than 14 days after LMP |
| LMP, shorter cycle | LMP + (cycle - 14) days | Fewer than 14 days after LMP |
| Estimated due date | Due date - 266 days | 38 weeks before due date |
Key timing facts behind the estimate
| Event | Approximate timing |
|---|---|
| Pregnancy length from LMP | About 280 days (40 weeks) |
| Pregnancy length from conception | About 266 days (38 weeks) |
| Ovulation in a 28 day cycle | About day 14 |
| Egg viability after ovulation | About 12 to 24 hours |
| Sperm survival in the body | Up to about 5 days |
Conception offset from LMP by cycle length
| Cycle length | Estimated ovulation day | Days after LMP to conception |
|---|---|---|
| 24 days | About day 10 | About 10 days |
| 28 days | About day 14 | About 14 days |
| 32 days | About day 18 | About 18 days |
| 35 days | About day 21 | About 21 days |
Common mistakes to avoid
- Treating the estimate as an exact day. Conception is calculated, not observed. Because sperm can survive several days and ovulation timing varies, the real date can be a few days off. Use the window, not the single date.
- Using the LMP method with an irregular cycle. The LMP + 14 rule assumes a regular 28 day cycle. If your cycles vary or you do not track them, the estimate can be well off. An early ultrasound dates a pregnancy far more reliably.
- Confusing conception date with the LMP date. Doctors usually date a pregnancy from the LMP, which is about two weeks before conception. So your "weeks pregnant" figure is not the time since conception, it is roughly two weeks longer.
- Using it to determine paternity. A conception estimate spans several days and is not precise enough to identify a father. Only a DNA test can establish paternity, this tool cannot and should not be used for that.
- Assuming conception equals the day of intercourse. Fertilization can happen up to about 5 days after intercourse because sperm survive that long. So the conception date is not necessarily the day you had sex.
- Mistaking implantation bleeding for a period. Light spotting around implantation can be read as a period, throwing off the LMP you enter. If your last "period" was unusually light or short, the estimate may be off by weeks.
Glossary
- Conception
- The moment a sperm fertilizes an egg, which usually happens around the time of ovulation.
- LMP
- Last menstrual period, the first day of your most recent period. Pregnancies are commonly dated from this date.
- Ovulation
- The release of an egg from the ovary, typically about 14 days before the next period is due.
- Gestational age
- How far along a pregnancy is, counted from the LMP, so it is about two weeks more than the time since conception.
- Conception window
- The span of days within which conception likely occurred, used because the exact day cannot be known.
- Cycle length
- The number of days from the first day of one period to the first day of the next, commonly 21 to 35 days.
- Due date (EDD)
- The estimated date of delivery, usually about 280 days (40 weeks) after the LMP.
- Implantation
- When the fertilized egg attaches to the uterine lining, roughly 6 to 12 days after conception.
Frequently asked questions
How does the conception date calculator work?
It estimates conception as about 14 days after your last menstrual period (LMP), or 266 days before your due date. If you enter your LMP, it adds about 14 days (adjusted for your cycle length) because ovulation, and so conception, usually happens around then. If you enter a due date, it subtracts 266 days, since conception is roughly 38 weeks before the estimated due date.
How accurate is the estimated conception date?
It is only an estimate, typically accurate to within a 5 to 6 day window rather than a single day. Ovulation timing varies, and sperm can survive up to about 5 days, so the real conception date can differ from the calculated one by several days. Treat the window as the realistic answer.
Can I find out exactly when I conceived?
No method can pin conception to a single day after the fact. An early dating ultrasound is the most reliable way to date a pregnancy, but even that estimates a range rather than an exact moment of conception.
Why is conception about two weeks after my last period?
Because in a typical 28 day cycle ovulation happens around day 14, roughly two weeks after the first day of your last period. Conception occurs around ovulation, so it lands about two weeks after the LMP.
Can this calculator tell me who the father is?
No. The estimate covers a window of several days and is not precise enough to determine paternity. Only a DNA test can establish who the father is, so please do not rely on this tool for that.
My cycle is not 28 days, can I still use this?
Yes. With the LMP method you can enter your average cycle length, and the tool estimates ovulation as about 14 days before your next period, which is (cycle length minus 14) days after the LMP. For irregular cycles the estimate is less reliable, so confirm with your provider.
How do I calculate my conception date from my due date?
Subtract 266 days (38 weeks) from your estimated due date. For example, a due date of 1 December points to conception around 10 March. This calculator does that math for you when you choose the due date method.
Is the conception date the same as the day I had sex?
Not necessarily. Sperm can survive in the body for up to about 5 days, so fertilization can occur several days after intercourse. The conception date reflects when the egg was actually fertilized, which may be a few days after sex.
How many weeks pregnant am I at conception?
About 2 weeks. Because pregnancy is counted from the LMP, which is roughly two weeks before conception, you are already considered around 2 weeks pregnant on the day you actually conceive.
Does an ultrasound change the conception estimate?
It can. An early ultrasound measures the embryo directly and may adjust your due date, which in turn shifts the estimated conception date. Ultrasound dating is more accurate than any date-based calculator.
Sources
- Calculating a Due Date , Johns Hopkins Medicine
- Trying to Conceive , Office on Women's Health, U.S. Department of Health and Human Services
- How to Tell How Far Along You Are in Your Pregnancy , Cleveland Clinic