Due Date Calculator from Last Period (LMP)
Enter the first day of your last period and cycle length to get your due date, how far along you are, likely ovulation date and a week-by-week calendar.
If you know the first day of your last menstrual period (LMP), you can estimate your due date in seconds. This calculator applies the standard 280-day rule, corrects it for your cycle length, tells you how many weeks and days pregnant you are, and builds a week-by-week calendar with trimester dates and key milestones.
How to use this tool
- Enter the first day of your last period, the day bleeding started.
- Enter your usual cycle length; leave 28 if you are not sure.
- Keep “Show progress on” as today, or choose a future date such as a scan appointment.
- Read your due date and current week, then scroll the calendar to see what is coming.
Formula
Due date = LMP + 280 days + (cycle length − 28); ovulation ≈ LMP + cycle length − 14
This is Naegele’s rule with a cycle-length correction. For a 28-day cycle, weeks start at the LMP; otherwise this calendar uses a cycle-adjusted LMP-equivalent start, about two weeks before estimated conception, so at conception you are already "2 weeks pregnant". The first trimester runs to 13 weeks 6 days and the third starts at 28 weeks. Cycle adjustment assumes ovulation roughly 14 days before the next period; real cycles can differ. Use the due date confirmed by your clinician.
Example
Your last period started on 1 January 2026 and your cycle is 28 days. The due date is 1 January + 280 days = 8 October 2026, and ovulation was probably around 15 January. On 1 March 2026 you are 8 weeks 3 days pregnant. With a 32-day cycle instead, the due date moves four days later to 12 October 2026.
Frequently asked questions
What if I don’t remember my LMP exactly?
Use your best guess and treat the result as approximate. Your first ultrasound, usually between 8 and 14 weeks, measures the baby and gives a more reliable date.
Does an irregular cycle make the LMP date wrong?
Irregular cycles make this approximation less reliable because ovulation timing varies. An early ultrasound and your history help your clinician establish the due date.
Why am I counted as pregnant before conception?
Doctors count from the LMP because it is a date most people know, while conception usually happens about two weeks later and is rarely known exactly.
Sources
This tool gives an estimate for general information only. It is not financial, tax, legal or medical advice — check with a qualified professional before you rely on the result.
Last updated: 2026-10-30