How to use Pregnancy Due Date
Estimate a due date from the first day of your last period.
- Choose the operation and enter every labeled input, keeping units and the time period consistent.
- Calculate, compare the worked example below and inspect any warnings.
- Change one assumption to compare another scenario; keep the model’s limits in view.
Formula & calculation method
Estimated due date = first day of the last menstrual period + 280 days + (regular cycle length − 28) days. The supported cycle input is 21–35 whole days.
Input reference · 2 fields
These labels describe the calculation’s base units. Supported measurements can switch through the header’s region settings; follow the current workspace labels. Prefilled values are examples.
- First day of last period
- Use a valid calendar date; dates carry no time zone.
- Regular cycle length (days)
- Enter a plain number with a decimal point if needed. The workspace shows an illustrative value, not a recommendation.
A quick example
A 28-day-cycle estimate uses 40 weeks from the first day of the last period.
Assumptions, edge cases & limits
- This calendar method assumes a regular cycle and does not date a pregnancy clinically.
- Irregular cycles, uncertain dates and assisted reproduction can require different dating.
- An estimate only. Irregular cycles and uncertain dates reduce accuracy. Ultrasound and your maternity clinician can establish more accurate dating.
Questions about Pregnancy Due Date
Is the calculated due date the day the baby will arrive?
No. It is a planning estimate based on the supplied period date and cycle assumption. Your maternity team may use clinical information or ultrasound to establish or revise pregnancy dating.
Can I use this for an irregular cycle or IVF pregnancy?
This particular method is a regular-cycle estimate. Do not treat it as the appropriate dating method for irregular cycles or assisted reproduction; use the dating supplied by your care team.
Are my files or inputs uploaded?
Calculation and file processing run in your browser without a processing upload. Downloads go to your device. Choosing a next-step transfer creates a temporary local copy for that workflow. Region preferences are stored separately from tool inputs. See our privacy details.
Can I use this tool for free?
Yes. No signup, subscription or payment is required to use the tool or download its output. File tools have device and size limits; a smaller batch may work better on a phone.
Method & references
Read the underlying method or measurement convention in context. These references do not imply affiliation or endorsement.
ACOG: Methods for estimating the due date ↗NHS: Due date calculator ↗Educational health estimates. No independent clinical review is claimed.A result or explanation needs correcting? Tell us the tool name and an example.