Conception Calculator
Estimate when conception most likely happened from a last period, a due date or a birth date, with the fertile window, the usual implantation days and the due date.
How to Use
- Choose what you know: the first day of the last period, a due date, or a birth date and how many weeks pregnant the birth was.
- For a last period, enter your usual cycle length. The luteal phase (ovulation to the next period) is usually left at 14 days; change it if you know yours.
- Read the most likely conception date, the six-day fertile window that ends on it, the days implantation most often happens and the due date.
- The strip shows the days around ovulation: fertile days in green, ovulation in the accent colour, implantation in purple and the expected period in red.
- Press a preset to try a short or long cycle, a due date or a birthday.
Worked Example
From a last period. The last period started on Saturday 1 August 2026 and cycles are 28 days, so the next period would start on 29 August. Ovulation is about 14 days before that: Saturday 15 August 2026, the most likely conception date. The fertile window is the six days ending on it, 10 to 15 August; implantation usually follows 6 to 12 days later (21 to 27 August), most often on 23 to 25 August; and the due date is 15 August + 266 days = Saturday 8 May 2027.
The common mistake: assuming day 14 in a long cycle. With a 32-day cycle the next period is due on 2 September, so ovulation is about 19 August, not 15 August, and the fertile window is 14 to 19 August. Using the 28-day answer would put four of the six fertile days in the wrong place.
From a birthday. A baby born on Sunday 20 September 2026 at 39 weeks 3 days (276 days) was conceived about 276 − 14 = 262 days before the birth: around Thursday 1 January 2026.
Show Work
Formulas
Where the Six-Day Window Comes From
For a long time the number of fertile days in a cycle was a guess. Allen Wilcox, Clarice Weinberg and Donna Baird of the Epidemiology Branch of the US National Institute of Environmental Health Sciences settled it with a study of 221 healthy women who were planning a pregnancy. The women collected a urine sample every day and recorded the days they had sex; hormone measurements in the urine pinned down the day of ovulation. Across 625 cycles with 192 conceptions, published in the New England Journal of Medicine in 1995, conception happened only after sex in the six days ending on ovulation day.
Four years later, in the same journal, the team used the same samples to time implantation by the first appearance of the pregnancy hormone hCG: in pregnancies that continued, it came 6 to 12 days after ovulation, and in 84% of them on day 8, 9 or 10. Those two results are the windows this calculator draws.
About This Calculator
This calculator estimates the most likely conception date from a last period (with your cycle length and luteal phase), from a due date or from a birth date, then shows the fertile window, the usual implantation days, the next period and the due date on a day-by-day strip. It counts whole calendar days, so time zones never shift the answer. The result is the middle of a window, not an exact day.
Every date stays in your browser; nothing is sent anywhere.
For general information only — not medical advice. Talk to a doctor or other qualified professional about your own health.
Related tools: hCG Doubling Time Calculator, Pregnancy Weight Gain Calculator, and Corrected Age Calculator.
Frequently Asked Questions
How is the conception date estimated?
Conception happens on or just after ovulation, which usually comes about 14 days before the next period. With a last period on 1 August 2026 and a 28-day cycle, the next period is due on 29 August, so ovulation and the most likely conception date are about 15 August 2026. With a 32-day cycle it moves to 19 August.
What is the fertile window?
The six days ending on the day of ovulation. In a study of 221 women and 625 cycles (Wilcox, Weinberg and Baird, New England Journal of Medicine, 1995), conception happened only after sex in that six-day span, with the chance rising from 0.10 five days before ovulation to 0.33 on the day itself. For ovulation on 15 August, the window is 10 to 15 August.
When does implantation happen?
Usually 6 to 12 days after ovulation. In a 1999 study by the same team, 84% of pregnancies that continued implanted on day 8, 9 or 10. For ovulation on 15 August 2026, that is most often 23 to 25 August, and anywhere from 21 to 27 August.
Can I work out conception from a due date?
Yes: take 266 days off the due date. A due date of Saturday 8 May 2027 points to conception around Saturday 15 August 2026. Taking off 280 days instead gives 1 August, which is the start of the last period, two weeks before conception.
Can this tell me the exact day I conceived?
No. Ovulation varies from cycle to cycle, sperm can survive several days, and a due date is itself an estimate: even an early ultrasound dates a pregnancy only to within about 5 to 7 days (ACOG Committee Opinion 700). Treat the result as the middle of a window of a week or so.
How do I use the Conception Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Trying to conceive
With a 32-day cycle starting 1 August 2026, the fertile window is 14 to 19 August, not the 10 to 15 August a 28-day rule would give.
From a due date
A due date of 8 May 2027 places conception around 15 August 2026, with the fertile window 10 to 15 August.
From a birthday
A baby born on 20 September 2026 at 39 weeks 3 days was most likely conceived around 1 January 2026.
Testing early
Ovulation on 15 August means implantation most often on 23 to 25 August, and the next period due on 29 August.
Short luteal phase
A 30-day cycle with a 12-day luteal phase puts ovulation on cycle day 19 (18 days after the period starts), the same as a 32-day cycle with 14 days.
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