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Conception Calculator

Conception date from a due date (minus 266 days) or the first day of your last period (plus 14 days), with the five days before it as the likely window. How it works

Use the Conception Calculator

Counts back 266 days (38 weeks) from the due date.

Estimated conception date
Enter the expected due date to see the estimate.
Likely intercourse window —

Sperm can survive for up to 5 days (a maximum, not a typical figure).

About the Conception Calculator

Conception is estimated at 266 days before the due date, or at ovulation counted from the first day of the last period: cycle length minus 14 days later, which is 14 days for a 28-day cycle. A due date of October 1, 2026 gives Thursday, January 8, 2026, with a likely intercourse window of January 3 to January 8. A period that began January 1, 2026 gives Thursday, January 15, 2026 with a 28-day cycle and Thursday, January 22, 2026 with a 35-day cycle.

Estimated conception date by due date

Due dateEstimated conception (due date − 266 days)Likely intercourse window
October 1, 2026Thursday, January 8, 2026January 3 to January 8
December 25, 2026Friday, April 3, 2026March 29 to April 3
January 1, 2027Friday, April 10, 2026April 5 to April 10
March 15, 2027Monday, June 22, 2026June 17 to June 22
June 30, 2027Wednesday, October 7, 2026October 2 to October 7
September 10, 2027Friday, December 18, 2026December 13 to December 18

The window is the conception date and the five days before it.

What happens after the estimated conception date

StageWhen, counted from ovulationFor a conception date of January 15, 2026Source
The egg can be fertilizedAbout 12 to 24 hours after ovulationJanuary 15 to January 16Cleveland Clinic
Implantation, when hCG first appears in urine6 to 12 days after ovulationJanuary 21 to January 27Wilcox and colleagues, 1999
Most implantations (84%)8 to 10 days after ovulationJanuary 23 to January 25Wilcox and colleagues, 1999
A home urine test is most reliableFrom the day a period is missedFrom January 29 with a 28-day cycleCleveland Clinic

The calculator’s conception date is its ovulation estimate, so every row moves with it. Wilcox and colleagues dated implantation by the first rise of hCG in daily urine samples. A blood test at a clinic can detect hCG before a home test.

Methodology

Conception = due date − 266 days (38 weeks), or first day of the last period + (cycle length − 14) days, with a cycle length from 14 to 40 days and 28 by default. The likely intercourse window is the conception date and the five days before it. The last-period method assumes ovulation 14 days before the next period.

How to use this tool

  1. Choose By Due Date or By Last Period.
  2. Pick the date. The field starts empty and no result shows until you choose one.
  3. By Last Period only: enter your usual cycle length, 14 to 40 days. Blank or the default uses 28. A cycle shorter than 21 days, below the normal 21 to 35, shows a warning that the dates are unreliable.
  4. Read the estimated conception date and the five-day window that ends on it.

Frequently asked questions

How do I find my conception date from my due date?
Subtract 266 days, which is 38 weeks. A due date of October 1, 2026 gives Thursday, January 8, 2026, and a due date of January 1, 2027 gives Friday, April 10, 2026. The tool also shows the five days before that date as the likely intercourse window.
How do I find my conception date from my last period?
Add your cycle length minus 14 days to the first day of your last period: 14 days for a 28-day cycle. A period that began January 1, 2026 gives Thursday, January 15, 2026 at 28 days and Thursday, January 22, 2026 at 35 days. Cleveland Clinic gives ovulation as around day 14 of a 28-day cycle on average and normal cycles as 21 to 35 days. Enter your own cycle length in the By Last Period view.
Why does the tool show a window instead of one day?
Conception can happen days after intercourse. Cleveland Clinic gives sperm survival as up to 3 to 5 days, and the tool uses 5 as a maximum, not a typical figure. The window is the five days before the estimated conception date plus that date. Cleveland Clinic puts the fertile window from about 5 days before ovulation through 1 day after; this tool does not count the day after.
Should I use the due date from my scan or from my last period?
The scan date, if you have one. Clinicians move the due date to the ultrasound when the two disagree by more than 5 days on a scan up to 8 weeks 6 days, or by more than 7 days from 9 weeks to 15 weeks 6 days. Every day the due date moves, the conception date moves by the same day, so a week’s difference between the two due dates is a week’s difference here.
When can I take a pregnancy test after conception?
A home urine test is most reliable from the day your period is due. In a study of daily urine samples, hCG first appeared 6 to 12 days after ovulation, and 8 to 10 days in 84% of pregnancies, so a test taken earlier can be negative in a pregnancy. For a conception date of January 15, 2026 with a 28-day cycle, that is from January 29. A blood test at a clinic can detect hCG sooner.
Can this tell me exactly which day I conceived?
No. It returns a calendar estimate and the five days before it. Ovulation itself can shift from one cycle to the next, and sperm can survive for days, so the window is the most the dates can say.

Working back from a due date or forward from a last period

Start from the due date on your pregnancy paperwork if you have one, especially one set by an early scan; it already allows for your own cycle. Start from the last period only when that is all you know, and enter your usual cycle length if it is not 28 days. The result does not confirm a pregnancy and cannot pick out one act of intercourse.

Why 266 days, cycle length − 14 and a six-day window

conception = due date − 266 days (38 weeks)

conception = first day of last period + (cycle length − 14) days, so 14 days for the default 28-day cycle

likely intercourse window = the five days before the conception date, plus that date

The two routes agree because a due date is normally set 280 days after the last period: 280 − 14 = 266, so both point to the same day for a 28-day cycle.

Example: a due date of October 1, 2026 gives Thursday, January 8, 2026, with a window of January 3 to January 8. A last period that began January 1, 2026 gives Thursday, January 15, 2026, with a window of January 10 to January 15; with a 35-day cycle it gives Thursday, January 22, 2026.

The last-period method places ovulation 14 days before the next expected period, so the cycle length you enter (14 to 40 days, blank for 28) moves the date. Ovulation can still shift from one cycle to the next. The window rests on sperm survival. Cleveland Clinic gives sperm survival as up to 3 to 5 days and the fertile window as about 5 days before ovulation through 1 day after; this tool stops at the estimated date and does not count the day after.

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