Conception Date Calculator
Estimate the date of conception from a due date or your last menstrual period.
Results update as you type.
About this calculator
A conception calculator estimates when conception likely occurred. From a due date it counts back 266 days (38 weeks), because fertilization happens about two weeks after the last period on a standard 28-day cycle and pregnancy runs 280 days from that period. From a last-period date it counts forward to ovulation (cycle length minus 14 days). Because sperm survive several days, a likely conception range is shown around the central date.
Working from a due date, subtract 266 days: a due date of, say, 40 weeks out places conception 38 weeks earlier, with the last period roughly two weeks before that (due date minus 280 days). Working from your last period, it adds cycle length minus 14 to find ovulation β day 14 on a 28-day cycle, day 16 on a 30-day cycle. It then shows todayβs gestational age, measured from the last period rather than conception.
Use it to trace back roughly when a pregnancy began, either out of curiosity or to line up dates with a due date. The several-day range reflects that sperm can survive up to about five days, so conception may follow intercourse by a few days. Actual ovulation varies, so an early dating ultrasound is the most reliable check; this tool is for information, not medical advice.
Frequently asked questions
How is conception estimated from a due date?
A due date is 280 days (40 weeks) after the last menstrual period, and conception occurs about 14 days after that period. So conception is roughly the due date minus 266 days (38 weeks).
Why is a conception range shown, not one date?
Sperm can survive up to about five days in the reproductive tract, so intercourse a few days before ovulation can still cause pregnancy. The range spans that fertile window around the estimated ovulation day.
Is the conception date the same as the intercourse date?
Not necessarily. Conception (fertilization) happens when a sperm meets the egg at ovulation, which may be a few days after intercourse. The estimate targets ovulation, the biological moment of conception.
When did I conceive if I know my due date?
Subtract 266 days (38 weeks) from the due date. Because pregnancy is dated as 280 days from the last period and conception is about 14 days after it, conception lands roughly two weeks after that period.
Can a conception calculator tell me who the father is?
Only loosely. It gives a likely conception date and a several-day window around ovulation, but because sperm survive several days, paternity cannot be settled from dates alone and needs a DNA test.
Why is conception about two weeks after my last period?
Pregnancy is dated from the first day of the last period for convenience, yet fertilisation happens at ovulation about two weeks later. That is why the gestational age this tool shows is roughly two weeks more than the time since conception.
API β use this calculator from code
Call this calculator as a free JSON endpoint β no key required. Send the field values below as query parameters or JSON. Read the full API docs β
Endpoint
GET https://calculator.free/api/v1/conception/curl
curl "https://calculator.free/api/v1/conception/?mode=due"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/conception/?" + new URLSearchParams({
"mode": "due"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.