Compare calculators

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

Conception Date Calculator

Works backward from how many days remain until a known due date to estimate when conception occurred (and how far along you are). Switch the basis to LMP to account for your cycle length and luteal phase.

Days Until Due Date (days)

Average Cycle Length (days)

Luteal Phase Length (days)

Calculate From

Fill in the required fields to see your result.

Pregnancy

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.

Key differences

	Conception Date Calculator	Conception Date Calculator
Category	Pregnancy	Pregnancy
Inputs required	4	4
Result	Estimated Conception	Estimated Conception
What it does	Works backward from how many days remain until a known due date to estimate when conception occurred (and how far along you are). Switch the basis to LMP to account for your cycle length and luteal phase.	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).