Health tool
Due date and week calculator
Enter the first day of your last period and your usual cycle length to see how many weeks and days along you are on a given date, plus the estimated due date.
Your dates
How the estimate works
Pregnancy dating conventionally starts from the last period, not conception. This tool adjusts both the due date and gestational age for cycle length: due date = last period + 280 + (cycle − 28) days; adjusted age = check date − last period − (cycle − 28) days. This keeps the due date at 40 weeks. It is an estimate, not confirmation of pregnancy; ultrasound dating takes precedence.
This page uses under 14 weeks for the first trimester, 14 to under 28 for the second, and 28 onward for the third. A due date is not an appointment: if the pregnancy reaches or passes the due date, follow the monitoring and delivery plan from your maternity team rather than waiting based on this calculator.
Method and sources
- Method / source
- Cycle-adjusted estimate: due date = last period + 280 + (cycle − 28) days; age in days = check date − last period − (cycle − 28). Regular cycles and ovulation timing are approximate assumptions; ultrasound dating takes precedence.
- Who it suits
- Early-pregnancy adults with reasonably regular cycles who remember their last period and want a first estimate of gestational age and due date.
- Who it does not suit
- Anyone who cannot recall their last period, has persistently irregular cycles, or conceived via assisted reproduction (e.g. IVF) — dating should come from an early ultrasound or the fertility clinic instead.
- Limitations
- It cannot confirm pregnancy or replace antenatal care. Near or past the due date, follow your maternity team's monitoring and delivery plan. This tool stops showing pregnancy results beyond 42 weeks. Shared-browser users can see locally saved dates.
- Explanation updated
- Explanation updated: 2026-11-03 (not clinically reviewed)
Primary sources: ACOG: methods for estimating the due date (Committee Opinion 700) · NHS: due date calculator
Frequently asked questions
How is the due date calculated?
The usual Naegele rule adds 280 days (40 weeks) to the first day of the last period. When cycles differ from 28 days, ovulation shifts accordingly, so this tool adjusts the estimate by (cycle length − 28) days.
Why count from the last period, not conception?
Conception day is hard to pin down, while most people can recall when their last period started — so clinical dating conventionally starts there, even though ovulation and conception happen about two weeks later.
How accurate is a due date?
It is an estimate. Your clinic's assessment takes precedence. Near or past the due date, follow your maternity team's monitoring and delivery plan rather than using this calculator to decide whether to wait.
What if I don't remember my last period?
Do not guess. An early ultrasound can date the pregnancy reliably — just tell your obstetrician the rough window you remember.