Free Pregnancy Due Date Calculator Online

How the due date is calculated

The calculator uses Naegele's rule: a pregnancy is counted as 280 days (40 weeks) from the first day of the last period. That assumes a 28-day cycle with ovulation around day 14, so the date is moved by the difference when your cycle is longer or shorter:

Due date = LMP + 280 days + (cycle length − 28) days

Weeks of pregnancy are counted from the LMP (adjusted for cycle length the same way), so you are already about 2 weeks along at conception. The estimated conception date is LMP + 14 days + (cycle length − 28) days.

How to work it out by hand

Suppose the first day of your last period was January 5, 2026 and your cycle is 28 days:

Count 280 days forward from January 5, 2026. That lands on October 12, 2026.
A quicker shortcut for a 28-day cycle: add 7 days (January 12), go back 3 months and add 1 year (October 12, 2026). It gives the same date as counting, or one within a few days, depending on how many days the months in between have.
Adjust for your cycle. With a 32-day cycle, add 4 days: October 16, 2026. With a 25-day cycle, subtract 3 days: October 9, 2026.
Estimate conception: LMP + 14 days = January 19, 2026 for a 28-day cycle.
To find how far along you are, count the days since the LMP and divide by 7. On March 30, 2026, 84 days have passed, so you are 12 weeks 0 days.

Trimesters and term

Example for a 28-day cycle. The first trimester runs to 13 weeks 6 days.
Stage	Gestational age	Date (LMP Jan 5, 2026)
Estimated conception	2 weeks 0 days	Jan 19, 2026
Second trimester begins	14 weeks 0 days	Apr 13, 2026
Third trimester begins	28 weeks 0 days	Jul 20, 2026
Early term begins	37 weeks 0 days	Sep 21, 2026
Full term begins	39 weeks 0 days	Oct 5, 2026
Due date	40 weeks 0 days	Oct 12, 2026
Late term begins	41 weeks 0 days	Oct 19, 2026
Post-term begins	42 weeks 0 days	Oct 26, 2026

The term labels follow the American College of Obstetricians and Gynecologists (ACOG): early term is 37w0d to 38w6d, full term 39w0d to 40w6d, late term 41w0d to 41w6d and post-term 42w0d and beyond.

An estimate, not a promise

Few babies arrive exactly on their due date; most are born in the weeks around it. If you have an ultrasound early in pregnancy, your healthcare provider may adjust your due date based on it. This calculator is for information only and is not medical advice.

Frequently asked questions

What if I don't know the date of my last period?

Use your best estimate, but treat the result as rough. A healthcare provider can date the pregnancy more accurately with an ultrasound.

My cycles are irregular. Is the result accurate?

Naegele's rule assumes regular cycles, so it is less reliable when cycles vary a lot. Enter your average cycle length and confirm the date with your healthcare provider.

Why does a longer cycle push the due date later?

Ovulation usually happens about 14 days before the next period. In a longer cycle, ovulation and conception happen later after the LMP, so the due date moves later by the same number of days.

How many weeks is a full-term pregnancy?

Full term is 39 weeks 0 days through 40 weeks 6 days, counted from the first day of the last period. Births from 37 weeks 0 days to 38 weeks 6 days are early term, and births from 41 weeks 0 days to 41 weeks 6 days are late term.

Why is pregnancy counted from the last period instead of conception?

The first day of the last period is a date most people know, while the exact day of conception usually is not. Counting from the LMP adds about two weeks before conception, which is why a pregnancy is described as 40 weeks long.

Is my information stored?

No. The calculation runs entirely in your browser and the dates you enter are not sent anywhere.

Results are estimates for informational purposes and are not financial, medical or professional advice. See our methodology and disclaimer.