Pregnancy Conception Calculator
Estimate the most likely conception date and window from either your last menstrual period or your due date.
Reviewed by the CalcCafe editorial team · Last updated 1 July 2026 · How we test our tools
Example
If your last menstrual period (LMP) began on March 1, 2026 with a typical 28-day cycle:
Conception offset = cycle - 14 = 28 - 14 = 14 days
Estimated conception = Mar 1 + 14 days = Mar 15, 2026
Conception window = Mar 13 - Mar 18, 2026
Estimated due date = Mar 1 + 280 days = Dec 6, 2026
Working from a due date instead: conception = due date - 266 days.
How it works
Conception is estimated as LMP + 14 days, or due date - 266 days. A 5-day window around that date is shown to reflect normal ovulation and sperm-survival variability.
Good to know
The Pregnancy Conception Calculator works backward from a known date to estimate when conception most likely occurred. You can start from the first day of your last menstrual period (LMP) or, if you already have a due date from a clinic or another tool, switch to "From due date" mode and have it reverse-calculate everything. It then returns an estimated conception date, a conception window, and the matching due date and LMP, making it useful for anyone curious about timing, planning, or simply cross-checking dates given by a provider.
People typically reach for this after a positive test, when an ultrasound gave a due date that does not match what they expected, or when trying to reconcile a possible conception date with a particular event or partner. Because it adjusts the standard "LMP plus 14 days" rule using your average cycle length, it is more accurate for people whose cycles run shorter or longer than 28 days; setting the cycle field correctly (it accepts 20 to 45 days) is the single biggest factor in getting a sensible result.
Read the result as a range, not a fixed day. The big date is the single most likely conception day, but the window (about two days before to three days after) accounts for the fact that ovulation timing shifts and sperm can survive several days inside the body. If you are using this to narrow down a specific date, treat any day inside that window as plausible rather than ruling days out.
One practical caveat: this is a calendar-math estimate built on cycle averages, not a measurement of your actual ovulation. Irregular cycles, late ovulation, and the fact that early-pregnancy ultrasound dating can be off by several days all mean the real conception date may fall outside the displayed window. Everything runs in your browser, so nothing you enter is uploaded, but for anything that affects a medical or personal decision, confirm the dating with a clinician.
Frequently asked questions
Why is conception estimated as LMP plus 14 days?
Pregnancy dating starts from the first day of your last period, but conception only happens at ovulation, which is roughly 14 days later in a 28-day cycle. The calculator adjusts this offset (cycle length minus 14) if your cycle differs from 28 days.
Why does the tool show a window instead of one exact date?
Ovulation timing varies and sperm can survive up to about 5 days, so conception can occur over several days. The shown window (about 2 days before to 3 days after the estimated date) reflects this normal biological variability.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser; nothing is uploaded.
Is this a substitute for medical advice?
No. These are educational estimates — consult a qualified health professional for medical decisions.
People also ask
How accurate is a conception date calculated from LMP?
It is a reasonable estimate but not exact, because it assumes you ovulated a set number of days after your period started. Real ovulation can occur earlier or later, so the true conception date may differ by several days, especially with irregular cycles.
Can a conception calculator tell me exactly who the father is?
No. The tool only estimates a likely date range based on cycle math, and the window spans several days. It cannot determine paternity; a DNA test is the only reliable way to establish that.
What is the difference between conception date and due date?
The due date is roughly 280 days after the first day of your last period, while conception is estimated around 14 days after that period in a 28-day cycle. Conception is therefore about 266 days before the due date.
Why does my ultrasound conception date differ from this calculator?
Ultrasound dating measures the size of the developing pregnancy rather than counting from your period, and early scans are often used to adjust dates. If your ovulation was earlier or later than average, the two estimates will not match exactly.
How does cycle length change the conception estimate?
The calculator estimates ovulation as your cycle length minus 14 days after your period starts. A longer cycle pushes the estimated conception date later, and a shorter cycle moves it earlier, which is why entering your average cycle length matters.
Can I work out my last period date if I only know my due date?
Yes. Switching to the due date mode reverse-calculates an estimated LMP, which is about 280 days before the due date, along with the corresponding conception date and window.
Does conception happen on the same day as intercourse?
Not necessarily. Sperm can survive in the reproductive tract for up to about five days, so conception can occur days after intercourse whenever ovulation actually happens, which is why the tool shows a window.
Does 4 weeks pregnant mean you conceived 4 weeks ago?
No; four weeks pregnant means conception was about two weeks ago. Pregnancy is dated from the first day of your last menstrual period, and in a 28-day cycle conception happens about 14 days later, so the first two weeks of pregnancy pass before conception occurs. That is also why this calculator estimates conception as LMP + 14 days and the due date as LMP + 280 days but conception + 266 days.
Are you already 2 weeks pregnant at conception?
By standard dating, yes: gestational age starts on the first day of your last period, so on the day of conception you are already about two weeks pregnant even though no embryo existed before that day. It is a counting convention, not a biological fact, adopted because the LMP is a known date while the conception date is usually an estimate. With a longer cycle the offset is larger; with a 32-day cycle conception falls around 18 days after the LMP.
How far pregnant am I if Clearblue says 2-3 weeks?
A weeks indicator on a digital test estimates weeks since conception, so 2-3 weeks since conception is about 4-5 weeks pregnant in the LMP-based dating that doctors use, because the two weeks before conception are added on. Use the From due date or LMP mode here to line the two up: an LMP two weeks before the estimated conception date matches the test's reading. Confirm the exact dating with a prenatal provider or ultrasound.
How early can you test positive after conception?
A home test detects the hormone hCG, which starts rising only after the embryo implants, about 6 to 12 days after ovulation and conception, so the earliest positives usually appear around 8 to 10 days after conception and results are most reliable from the day of the missed period, roughly 14 days after conception. In the example (LMP March 1, conception around March 15), that means a first reliable test around March 29. Testing earlier risks a false negative because hCG may still be too low.
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Sources & references
These tools follow our methodology and provide educational estimates only — verify important figures with a qualified professional.