Due Date Calculators

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A due date is a target date by which a task, project, or biological process is expected to be complete. In obstetrics, the estimated due date (EDD) of a pregnancy is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). In project management and daily life, due dates are calculated by adding a specified number of days or weeks to a start date. Most pregnancies deliver within ±2 weeks of the EDD. Accurate due date calculation is important for prenatal care planning and scheduling.

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Pregnancy Due Date (Naegele's Rule)

EDD = LMP + 280 days (40 weeks)

Or equivalently: EDD = LMP + 9 months + 7 days. Example: LMP = January 1 → EDD = October 8 (9 months later = October 1, plus 7 days = October 8).

In Excel/Sheets: =LMP_date + 280. In date calculators: add 40 weeks to the LMP date.

Ultrasound Dating

First-trimester ultrasound (6–12 weeks) measuring crown-rump length (CRL) is the most accurate method for gestational dating, often more accurate than LMP-based calculation (especially if periods are irregular). If ultrasound EDD differs by more than 5–7 days from LMP-based EDD before 14 weeks, ultrasound dating is used. After 20 weeks, EDD is typically not revised.

Gestational Age

Gestational age = weeks and days elapsed since the LMP. At the EDD, gestational age = 40+0 weeks. Full term = 39+0 to 40+6 weeks. Preterm = delivery before 37+0 weeks. Late preterm = 34+0 to 36+6 weeks. Post-term = 42+0 weeks or beyond.

Project and Task Due Dates

Add days: due date = start date + N days (spreadsheet: =start+N). Add weeks: =start + 7×weeks. Business days only: =WORKDAY(start, N_days) in Excel. For recurring deadlines (monthly, quarterly): use EDATE(start, N_months).

Glossary

Estimated Due Date (EDD)
The expected delivery date in pregnancy: LMP + 280 days (Naegele's rule); approximately 40 weeks of gestational age; only ~5% of births occur on this exact date.
Naegele's Rule
A formula estimating pregnancy due date: EDD = LMP + 9 months + 7 days (= LMP + 280 days); assumes a 28-day cycle with ovulation at day 14.
Gestational Age
Weeks elapsed since the first day of the last menstrual period (LMP); standard measure of pregnancy progression; EDD corresponds to 40 weeks 0 days gestational age.

Frequently Asked Questions

Using Naegele's rule: EDD = first day of last menstrual period (LMP) + 280 days (40 weeks). Alternatively: add 9 months and 7 days to the LMP. Example: LMP March 5 → add 9 months = December 5 → add 7 days = December 12. This assumes a regular 28-day cycle with ovulation on day 14. First-trimester ultrasound is more accurate than LMP calculation, especially with irregular cycles. Only about 5% of babies are born exactly on their EDD; most deliver within ±2 weeks.

Gestational age is measured in weeks and days from the first day of the last menstrual period (LMP), not from conception. At the EDD, gestational age = 40 weeks 0 days (40+0). Key milestones: viability ~23–24 weeks; late preterm 34+0 to 36+6 weeks; early term 37+0 to 38+6; full term 39+0 to 40+6; late term 41+0 to 41+6; post-term ≥ 42+0. Even though fertilization typically occurs at ~2 weeks of gestational age, obstetric convention counts from LMP to allow consistent comparison across patients.

The EDD is an estimate — only about 5% of births occur on the exact EDD. About 80% of births occur within 2 weeks of the EDD. First-trimester ultrasound (CRL measurement at 6–12 weeks) is more accurate than LMP dating (within ±5 days vs. ±7–14 days for LMP). EDD accuracy decreases with later ultrasound dating. Women with irregular cycles, recent contraceptive use, or uncertain LMP dates benefit most from early ultrasound dating. After 20 weeks, gestational age estimates from ultrasound have ±2–3 week uncertainty.

Add calendar days: due date = start date + N days. In Excel: =A1+30 adds 30 days to the date in A1. For business days only: =WORKDAY(A1, 20) adds 20 business days. For weeks: =A1+7*4 adds 4 weeks. For months: =EDATE(A1, 2) adds 2 months. To find days remaining: =deadline_date − TODAY(). For business days remaining: =NETWORKDAYS(TODAY(), deadline_date). Format cells as Date or Number as appropriate. Many project management tools (Jira, Asana, Monday) have built-in due date calculators linked to task dependencies.