Conception Date Calculator
Conception date calculator: work out when you conceived from your last period, due date or ultrasound, with due date. Free, no sign-up, and it runs in your browser.
Dedicated page: /en/conception-date-calculator
Estimate only, not medical advice. The exact day of conception is unknowable from a calendar alone unless you tracked ovulation or used IVF. For anything that matters legally or medically, use a first-trimester dating scan.
Ovulation is counted as 14 days before the next period, so cycle length moves the date.
Quick answer
A conception date calculator estimates when you conceived from the first day of your last period, your due date, your baby's birth date, or an ultrasound date. It counts ovulation as 14 days before the next period, so a 28-day cycle puts conception 14 days after the last period, or 266 days before the due date.
About this Conception Date Calculator
Conception date calculator: work out when you conceived from your last period, due date or ultrasound, with due date. It is one of the Health and Family on ToolUstad, and like every tool here it runs entirely in your browser: nothing you type, paste or open is uploaded to a server, and there is no sign-up or usage limit.
What is a conception date calculator?
A conception date calculator works backwards from what you know to the day you most likely conceived. Pregnancy is dated from the first day of the last period, not from sex, a convention that predates anyone being able to see an ovary, and one that puts you two weeks pregnant before conception happened. This calculator undoes that convention. Give it a due date, a last period, a birth date, or an ultrasound date, and it works back to the day the egg was most likely fertilised, plus the window of days that could have produced it.
The core maths is short. Conception is dated to ovulation day. Ovulation is assumed to happen 14 days before the next period, so it falls on day 14 of a 28-day cycle, day 21 of a 35-day cycle, and day 7 of a 21-day cycle. From a last period, conception is the last period plus cycle length minus 14 days. From a due date, conception is the due date minus 266 days, because due dates sit 280 days after the last period.
The fertile window and the plausible range
Conception is not a single night. Sperm survive up to five days in the reproductive tract, and the egg lives 12 to 24 hours after release. The fertile window is the six days ending on ovulation day: the five days before it plus ovulation day itself. That comes from Wilcox, Weinberg and Baird, 1995, who studied 625 cycles and found no pregnancies from intercourse more than five days before ovulation. Conception odds peak in the two days before ovulation, at about 26%, and drop to about 10% on ovulation day itself.
What each starting point is worth
Not all starting points are equal. A due date from a first-trimester scan is the most reliable, because an ultrasound before 14 weeks dates a pregnancy to within about five to seven days. A last period is next, and it is accurate within a few days for regular cycles.
Best practices
- Use your real cycle length: a 32-day cycle ovulates about four days later than a 28-day one, and everything downstream shifts with it.
- Start from a scan date when you have one: it beats a period date by days, and it is the number a clinician will stand behind.
Common mistakes to avoid
- Treating the conception date as exact: unless you tracked ovulation or used IVF, it is a most-likely day inside a window of about six days.
- Counting ovulation as day 14 for every cycle: it is 14 days before the next period, so long cycles ovulate later.
Who is the conception date calculator for?
Anyone who wants to know when they conceived, couples planning a pregnancy and tracking fertile windows, and parents working out a timeline after a birth. It is not evidence for paternity, a workplace timeline, or any clinical decision: for those, ask for the dating scan report.
Related terms: when did I conceive, conception date from due date, ovulation date calculator, fertile window, implantation date, Naegele's rule, due date from last period.
Estimate only, not medical advice. For anything that matters legally or medically, use a first-trimester dating scan.
Learn: how to use the Conception Date Calculator
- Pick what you know: last period, due date, birth date, or ultrasound date with gestational age.
- Enter the date and your average cycle length, 28 days by default.
- Read the most likely conception date, the fertile window, the plausible range, and the due date.
Features
- Four ways in: last period, due date, birth date or ultrasound
- Cycle-length aware: ovulation counted as 14 days before the next period
- Fertile window, plausible range and implantation window
- Estimated due date by Naegele's rule, adjusted for cycle length
- Current week and trimester when the pregnancy is under way
- Date-by-date timeline of every event behind the estimate
- Honest accuracy note: calendar dating is an estimate, not a measurement
Worked example
Last period 1 March 2026 with a 28-day cycle: ovulation falls on 15 March, so the most likely conception date is 15 March 2026. The fertile window runs from 10 March to 16 March, implantation is expected between 21 March and 27 March, and the estimated due date is 6 December 2026.
Frequently asked questions
How do I calculate my conception date from my due date?
Subtract 266 days, or 38 weeks, from the due date. Due dates are set 280 days after the last period, and conception is dated about two weeks after that period begins, so the two numbers are 266 days apart.
Can a conception date be exact?
Only with IVF, or with ovulation confirmed by hormone testing. Otherwise the honest answer is a window of about six days, because sperm survive up to five days and ovulation day moves between cycles, even in regular ones.
Does a longer cycle change the conception date?
Yes. Ovulation happens roughly 14 days before your next period, not 14 days after the last one. A 35-day cycle ovulates around day 21, so conception and the due date both shift about a week later than a 28-day cycle.
Which is more accurate, my period date or an ultrasound?
An ultrasound before 14 weeks, easily. It dates a pregnancy to within about five to seven days, and it is what clinicians use when the two dates disagree. A calendar estimate can be off by up to two weeks.
Can I work out conception from my baby's birth date?
Approximately. Subtract 266 days for the mid-point, but expect several weeks of uncertainty: in a study of 125 pregnancies with confirmed ovulation dates, the time from conception to birth varied by 37 days.
What is the fertile window?
The six days ending on ovulation day: the five days before it plus ovulation day itself. That comes from Wilcox and colleagues, 1995, who found no pregnancies from intercourse more than five days before ovulation.
When will a pregnancy test turn positive?
Implantation happens 6 to 12 days after fertilisation, and a test cannot turn positive before it. Testing on the day the period is due is reliable; testing days earlier misses many pregnancies.
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