Compare calculators

Both calculators run independently — change the inputs on either side to compare results.

Conception Date Calculator

Estimates how many days ago conception occurred from a reference point — your due date, last menstrual period, or an ultrasound dating scan — entered as days since that reference (negative if it is a future due date).

Calculate From

Days Since Reference Date (use a negative number for a future due date) (days)

Cycle Length (days)

Ultrasound Gestational Age (weeks)

Fill in the required fields to see your result.

Pregnancy

Conception Date Calculator

Estimates the most likely conception date by counting back 266 days (38 weeks) from your due date. Useful when you know your due date but want to narrow down when fertilization likely occurred.

Days until due date (days)

Fill in the required fields to see your result.

Key differences

	Conception Date Calculator	Conception Date Calculator
Category	Pregnancy	Pregnancy
Inputs required	4	1
Result	Estimated Conception	Days since conception (days ago)
What it does	Estimates how many days ago conception occurred from a reference point — your due date, last menstrual period, or an ultrasound dating scan — entered as days since that reference (negative if it is a future due date).	Estimates the most likely conception date by counting back 266 days (38 weeks) from your due date. Useful when you know your due date but want to narrow down when fertilization likely occurred.