Reverse Due Date Calculator to Calculate Conception Dates

Reverse Due Date Calculator Sign

This free online conception calculator will determine the conception date -- including the day of the week -- based on your pregnancy due date, ultrasound, or the 1st day of your last menstrual period (LMP).

Plus, unlike other conception calculators, this calculator includes selectable dates dating back 100 years, which means it can also be used to calculate conception dates and days of the week for any dates of birth occurring within the past 99 years.

Want to know what date and day of the week you were likely conceived on? Simply select your birthday (or due date if you were born premature or late) and click the "Calculate Conception Date" button.

Note that if you want to calculate what date your baby is due to be born on (EDD), please visit the Pregnancy Due Date Predictor -- which also includes How to Calculate Due Date based on Naegele's Rule.

Or, if you'd like to track your (or a loved one's) pregnancy term, be sure to visit the Pregnancy Countdown Calculator, which tells you the days, weeks, and months along and remaining. It also includes a countdown meter and printable calendar.

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Reverse Due Date Calculator

Calculate conception date to estimate date and day of the week your baby was conceived on.

Special Instructions

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Selected Data Record:

A Data Record is a set of calculator entries that are stored in your web browser's Local Storage. If a Data Record is currently selected in the "Data" tab, this line will list the name you gave to that data record. If no data record is selected, or you have no entries stored for this calculator, the line will display "None".

DataData recordData recordSelected data record: None
Event:Event:Event to base on:Event to base calculations on:

Event to base calculations on:

Select the event that will correspond to the date you will be selecting. The entry form will update to accommodate your selection.

Due date:


Select the month, day, and year that corresponds to the selected date type.

Ultrasound weeks & days:Ultrasound weeks & days:Ultrasound weeks & days along:Ultrasound weeks and days along:

Ultrasound weeks and days along:

If you selected "Ultrasound date" as the event, select the number of weeks and days along you are according to your ultrasound. If you selected a different date event, the fields on this row will be disabled.

Average cycle:

Average menstrual cycle:

If you selected "1st day of last period" date to base the calculation on, select the number of days your average menstrual cycle lasts. If you selected a different date type, then the field on this row will be disabled.

Date:Conceive date:Conception date:Estimated date of conception:

Conception date:

Based on your entries, this is the estimated conception date.

If the calculation is based on Due or birth date, the result is 266 days subtracted from the selected due or birth date (266 is the average gestation period per Naegele's Rule).

If the calculation is based on the 1st day of last period, the result is equal to the 1st day of your last period plus 14 days, plus your average menstrual cycle minus 28 days (30 days - 28 days = 2 + 14 = 16 days).

If the calculation is based on the ultrasound weeks and days along, the result is equal to the ultrasound date minus the total days along (1st day of last period), plus 14 days (14 days after 1st day of last period).

Period:Last period:Last period date:Estimated last period date:

Last period date:

This is typically 14 days prior to the conception date (assuming ovulation occurred on day 14 of your menstrual cycle), though this can vary if a woman's average menstrual cycle is different from the 28-day norm.

Due date:Due date:Due date:Due date:

Due date:

This is 266 days from your conception date.

If you would like to save the current entries to the secure online database, tap or click on the Data tab, select "New Data Record", give the data record a name, then tap or click the Save button. To save changes to previously saved entries, simply tap the Save button. Please select and "Clear" any data records you no longer need.

Help and Tools


How to calculate a conception date or plan a baby birth date.

How to Calculate Conception Date

According to Naegele's Rule, the average human gestation period is 266 days. Therefore, to figure out the conception date from a birth date we need to subtract 266 days from the due date (or birth date).

But since it's difficult to subtract days from a calendar date without a date calculator, the easiest way to determine conception date by birth date manually is to count backward on a calendar.

Here is a conception calendar calculator that illustrates counting from 1 to 266 days backward from the due date.

Conception Calendar Calculator
Select Due or Birth Date
Due DateConception Date1st Day of Last Period (LMP)Possible Conception Date

How to Plan Baby Birth Date

If you select the desired due date in the Reverse Due Date Calculator on this page, the calculator will tell you when you will need to get pregnant to increase the odds of delivering on the desired date.

If you would rather know the month ranges, here is an at-a-glance chart showing target conception ranges for delivery months.

Birth Month Conception Ranges
PlanConception Range (Mid)
January babyApr 10 - May 10 (Apr 24)
February babyMay 11 - Jun 7 (May 24)
March babyJun 8 - Jul 8 (Jun 21)
April babyJul 9 - Aug 8 (Jul 22)
May babyAug 9 - Sep 7 (Aug 21)
June babySep 8 - Oct 7 (Sep 21)
July babyOct 8 - Nov 6 (Oct 21)
August babyNov 7 - Dec 7 (Nov 20)
September babyDec 8 - Jan 6 (Dec 21)
October babyJan 7 - Feb 6 (Jan 20)
November babyFeb 7 - Mar 9 (Feb 21)
December babyMar 10 - Apr 9 (Mar 23)

Of course, there is no guarantee whether or not your baby will be born on its due date. So the best chance for having the baby in the desired month is to plan conception in the middle of the ranges listed above.

Adjust Calculator Width:

Move the slider to left and right to adjust the calculator width. Note that the Help and Tools panel will be hidden when the calculator is too wide to fit both on the screen. Moving the slider to the left will bring the instructions and tools panel back into view.

Also note that some calculators will reformat to accommodate the screen size as you make the calculator wider or narrower. If the calculator is narrow, columns of entry rows will be converted to a vertical entry form, whereas a wider calculator will display columns of entry rows, and the entry fields will be smaller in size ... since they will not need to be "thumb friendly".

Show/Hide Popup Keypads:

Select Show or Hide to show or hide the popup keypad icons located next to numeric entry fields. These are generally only needed for mobile devices that don't have decimal points in their numeric keypads. So if you are on a desktop, you may find the calculator to be more user-friendly and less cluttered without them.

Stick/Unstick Tools:

Select Stick or Unstick to stick or unstick the help and tools panel. Selecting "Stick" will keep the panel in view while scrolling the calculator vertically. If you find that annoying, select "Unstick" to keep the panel in a stationary position.

If the tools panel becomes "Unstuck" on its own, try clicking "Unstick" and then "Stick" to re-stick the panel.