What the conception calculator does
The conception calculator estimates when conception most likely occurred, along with the fertile window around it. You can work backward from a due date your provider gave you, or forward from the first day of your last menstrual period. Either way, it translates between the medical 40-week dating system (which counts from your last period) and the biological event of conception, which happens about two weeks later.
The two methods
- From last period: conception ≈ last period + 14 days for a 28-day cycle. Adjust for cycle length, since longer cycles ovulate later. This also gives an estimated due date (last period + 280 days).
- From due date: conception ≈ due date − 266 days. This is handy when you already have a due date but want to know roughly when conception happened.
The number 266 is simply 280 (full-term pregnancy from the last period) minus the 14 days between the last period and ovulation.
Why pregnancy dating is "two weeks ahead"
Medically, pregnancy is counted from the first day of your last period — a convention chosen because that date is far easier to identify than the exact moment of conception. The result is that at the moment of conception you are already considered "2 weeks pregnant", and a full-term pregnancy is described as 40 weeks even though gestation from conception is closer to 38 weeks (266 days). The conception calculator simply moves between these two reference points.
How exact can it be?
Not exact — and no calculator can be. Sperm can survive up to five days in the reproductive tract and the egg lives about 24 hours, so fertilisation can occur anywhere across a roughly six-day fertile window. The calculator highlights the single most likely conception date (ovulation day) but also shows that window, because the true event could fall anywhere within it. For confirmed dating, an early ultrasound is far more reliable than any date math.
Common reasons people use it
Curiosity about a pregnancy timeline is the most common, followed by wanting to understand the dates a provider is using. Some people use it for planning or for questions about timing. Whatever the reason, treat the result as a well-grounded estimate rather than a precise or provable date.
Use with the other tools
To carry the timeline forward, the pregnancy calculator tracks your current week and milestones, and the due date calculator focuses on the delivery date. To find your fertile days for a future cycle, see the ovulation calculator.