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Pregnancy Due Date Calculator

Estimate your pregnancy due date from the first day of your last menstrual period using Naegele's rule. Adjust for a non-28-day cycle to see your expected due date, current trimester, weeks pregnant, and days remaining.

Examples

Standard 28-day cycle

Last period started 1 April 2026, average 28-day cycle.· Jun 15, 2026

First day of last period
Apr 1, 2026
Average cycle length
28 days
Estimated due date
2027-01-06
Weeks pregnant
10 weeks
Days remaining
205 days
Trimester
First
Approximate conception date
2026-04-15
Days pregnant
75 days

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How It Works

Formula

dueDate=LMP+280 d+(cycleLength−28) d\text{dueDate} = \text{LMP} + 280\,\text{d} + (\text{cycleLength} - 28)\,\text{d}

conceptionDate≈LMP+14 d+(cycleLength−28) d\text{conceptionDate} \approx \text{LMP} + 14\,\text{d} + (\text{cycleLength} - 28)\,\text{d}

daysPregnant=today−LMP\text{daysPregnant} = \text{today} - \text{LMP}

weeksPregnant=⌊daysPregnant7⌋\text{weeksPregnant} = \left\lfloor \frac{\text{daysPregnant}}{7} \right\rfloor

Variables, symbols and units

LMP\text{LMP}

First day of the last menstrual period

cycleLength\text{cycleLength}

Average length of the menstrual cycle(days)

dueDate\text{dueDate}

Estimated date of delivery

conceptionDate\text{conceptionDate}

Approximate date of conception (LMP + ~14 days, cycle-adjusted)

daysPregnant\text{daysPregnant}

Whole days elapsed since the LMP(days)

weeksPregnant\text{weeksPregnant}

Completed weeks of pregnancy(weeks)
Calculation method explained

Enter the first day of your last menstrual period (LMP). The calculator adds 280 days (40 weeks) to estimate the due date — Naegele's rule. If your cycle is shorter or longer than 28 days, the due date shifts by the same number of days, because ovulation usually happens about 14 days before the next expected period. Today's date is compared against the LMP and the due date to show your current trimester, weeks pregnant, and days remaining.

References and source material

Examples

Standard 28-day cycleApr 1, 2026 · 28 days → 2027-01-06

Last period started 1 April 2026, average 28-day cycle.

First day of last period
Apr 1, 2026
Average cycle length
28 days
Estimated due date
2027-01-06

Jun 15, 2026

Longer 30-day cycleJan 15, 2026 · 30 days → 2026-10-24

Last period started 15 January 2026, average 30-day cycle.

First day of last period
Jan 15, 2026
Average cycle length
30 days
Estimated due date
2026-10-24

Jun 15, 2026

Shorter 24-day cycleFeb 1, 2026 · 24 days → 2026-11-04

Last period started 1 February 2026, average 24-day cycle.

First day of last period
Feb 1, 2026
Average cycle length
24 days
Estimated due date
2026-11-04

Jun 15, 2026

Frequently Asked Questions

How accurate is the estimated due date?
Naegele's rule gives a rough target from the last menstrual period. The estimate can differ from the actual delivery date, especially if cycle timing is irregular or the LMP date is uncertain.
How does the cycle-length adjustment work?
The 280-day rule assumes a 28-day cycle with ovulation around day 14. If your cycle is longer, ovulation happens later and the due date shifts later by the same number of days; a shorter cycle shifts it earlier. The calculator adds (cycle length − 28) days to the standard 280-day estimate.
What is Naegele's rule?
Naegele's rule estimates the expected delivery date as the first day of the last menstrual period plus 280 days (40 weeks). It assumes a regular 28-day cycle and ovulation on day 14, which is why a cycle adjustment is offered for non-standard cycles.
How are trimesters defined here?
Using the common clinical convention measured from LMP: first trimester covers weeks 1–13, second trimester covers weeks 14–27, and third trimester runs from week 28 to week 40. After week 40 the calculator reports "post-term".
Why does it ask for the first day of the last period and not the conception date?
The exact day of conception is rarely known, but the first day of the last menstrual period is usually easy to recall and is the standard clinical anchor for pregnancy dating.

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