Conception Calculator
Estimate your most likely conception date and fertile window using your last period, cycle length, or due date — backed by clinical dating methods used by ACOG and the NHS.
Visualizing the Fertile Window
In a textbook 28-day cycle, ovulation occurs around day 14, with conception most likely in a six-day window ending on ovulation day.
Period starts
Ovulation
Next period
How Conception Date Is Calculated
Doctors rarely know the exact moment of fertilization, so conception date is estimated using one of three clinically accepted methods, each anchored to a known reference point in your cycle.
1. From your last menstrual period (LMP)
Ovulation typically occurs about 14 days before your next period begins — not 14 days after your last one, unless your cycle is exactly 28 days. This calculator adjusts for cycles longer or shorter than 28 days by shifting the estimated ovulation date accordingly, then applies a 6-day fertile window ending on that day.
2. From your due date
If you already have a due date from an ultrasound or your provider, this calculator works backward. Full-term pregnancy is dated at 266 days (38 weeks) from conception, so subtracting 266 days from the due date gives an estimated conception date.
3. From cycle length and luteal phase
For more precision, especially with longer or shorter cycles, ovulation can be estimated by subtracting your luteal phase length (the time between ovulation and your next period, typically 12–16 days) from your total cycle length. This method is more accurate than a flat 14-day assumption for people who track their cycles closely.
Frequently Asked Questions
How accurate is a conception calculator?
It provides a probable window, typically accurate to within a few days for people with regular cycles — because conception can only happen within about a 24-hour window after ovulation, even though sperm can survive up to 5 days in fertile cervical mucus, widening the plausible conception window to roughly 6 days per cycle. Because ovulation timing can shift cycle to cycle due to stress, illness, or hormonal changes, this estimate is a starting point, not a certainty. A first-trimester ultrasound (ideally between 8–13 weeks) remains the clinical gold standard for confirming dating, and per ACOG, is accurate to within about 5–7 days — tighter than any calendar-based estimate.
Source: American College of Obstetricians and Gynecologists (ACOG)
Can I get pregnant outside the fertile window?
It’s uncommon but not impossible. Sperm can survive in the reproductive tract for up to five days, and ovulation timing can vary, which is why the fertile window spans several days rather than a single date.
Source: NHS — Pregnancy planning
Why does my due date differ from the conception-based estimate?
Standard due-date calculations (Naegele’s rule) add 280 days to your LMP, which assumes ovulation on day 14 of a 28-day cycle. If your actual cycle is longer or shorter, or if ovulation happened earlier or later than average, the conception-based estimate may land a few days apart from the LMP-based due date — both are normal variations.
Source: Mayo Clinic — Pregnancy week by week
Does this calculator work for irregular cycles?
It’s less reliable for irregular cycles, since the method depends on a predictable relationship between your period and ovulation. If your cycles vary significantly month to month, ovulation predictor kits, basal body temperature tracking, or a clinician-performed ultrasound will give a more reliable estimate.
Source: ACOG — Infertility evaluation guidance
Sources & Further Reading
This page’s clinical content is reviewed against current guidance from:
Related Calculators
Our conception calculator estimates your likely conception date based on your cycle. Once you have that date, use our Due Date Calculator to project your full pregnancy timeline, and see the NHS guide to trying to conceive for evidence-based tips on timing intercourse around ovulation.