A due date calculator turns a single date — the first day of your last menstrual period — into a precise estimated delivery date (EDD), your current pregnancy week, and the number of days remaining. This page is built around the arithmetic of Naegele's rule rather than general pregnancy education: you give it dates and cycle length, and it returns exact calendar dates.
The math is simple once you see it: a textbook pregnancy is 280 days (40 weeks) from the last period, and every day your own cycles run longer or shorter than 28 days shifts that estimate. Type your figures and the result card updates instantly.
How to Use
- Enter the First Day of Last Period (LMP) — the first day of the last period before you conceived, not the day you think you ovulated.
- Set your Average Cycle Length. Leave it at 28 if your periods are regular; raise it to 30, 32, or 35 if your cycles typically run longer, since longer cycles push the due date later.
- Set Calculate Pregnancy As Of to the date you want measured. It defaults to today, which gives your current week right now.
- Read the result card: the highlighted Estimated Due Date, your Estimated Conception Date, the Current Pregnancy Week, total Days Pregnant, and Days Until Due Date.
Because every step is pure date arithmetic, you can stress-test it: change the cycle length by a few days and watch the due date slide by the same number of days.
The Naegele Formula
The standard rule (Naegele's rule) is:
EDD = LMP + 280 days + (your cycle length − 28)
The base 280 days is the conventional 40-week pregnancy. The cycle adjustment exists because ovulation happens around the middle of your own cycle, not automatically on day 14. If your cycles average 32 days, you likely ovulate around day 18, so the due date moves four days later: +32 − 28 = +4.
Estimated conception uses the midpoint of your cycle:
Conception = LMP + cycle length ÷ 2
For a 28-day cycle this lands on LMP + 14 days, the familiar ovulation estimate. Your current gestational age in weeks is simply the whole weeks elapsed since the LMP — obstetric weeks start counting on the first day of bleeding, which is why you are counted as roughly two weeks pregnant at conception.
Examples
Example 1 — Regular 28-day cycle. LMP of March 1, 2026 and a 28-day cycle gives an estimated due date of December 6, 2026. Measured against October 24, 2026, you are 237 days pregnant, week 33, with 43 days to go. Conception is estimated around March 15, 2026.
Example 2 — Longer 30-day cycle. LMP of February 15, 2026 with a 30-day cycle shifts the due date to November 24, 2026 — two days later than the raw 280-day estimate. On October 24 you are 251 days along, week 35, with 31 days remaining.
Example 3 — The cycle-length shift. Two people share a March 1 LMP, but one has 28-day cycles and the other has 35-day cycles. The second due date lands seven days later. Only about 5% of babies arrive on their exact due date, yet getting the cycle length right keeps the estimate inside the normal two-week window.
Why the Due Date Is an Estimate
Only around 5% of pregnancies deliver on the calculated due date. Most babies arrive within roughly two weeks on either side — 37 to 42 weeks is considered full-term. The 280-day rule assumes a textbook ovulation on day 14 of a 28-day cycle, which does not hold for everyone. If your early ultrasound disagrees with the LMP date, the ultrasound measurement is usually the more accurate anchor, especially in the first trimester.
Think of the due date as a planning target, not a deadline. It schedules prenatal checkups and maternity leave, and it frames how far along you are on any given day — which is exactly what the arithmetic on this page computes.
FAQ
How is the due date calculated?
Using Naegele's rule: due date = last period + 280 days, then add the difference between your usual cycle length and 28 days. A 28-day cycle needs no adjustment; a 32-day cycle adds four days.
What is Naegele's rule?
Naegele's rule is the classic obstetric shortcut: subtract three months from the first day of your last period, add one year, and add seven days. Mathematically this is the same as LMP plus 280 days, the 40-week standard pregnancy length.
Does a longer cycle change my due date?
Yes. Longer cycles usually mean later ovulation, so the due date should move later by roughly (your cycle length − 28) days. Enter your real average cycle length here instead of leaving it at 28.
When did I actually conceive?
The estimate is the midpoint of your cycle: LMP + half your cycle length. For a 28-day cycle that is about LMP + 14 days. Conception is an estimate because sperm can survive for several days before the egg is released.
How many weeks pregnant am I?
Take the whole number of days from your last period to today and divide by seven. Obstetric weeks count from the first day of bleeding, so you are about two weeks further along than the time since conception. Set the "As Of" date on this page to any date to see that week on demand.
Why does my ultrasound due date differ from this calculator?
Early first-trimester ultrasound measures the embryo directly and is more accurate than guessing ovulation from cycle history. When the two differ by more than about a week or two, clinicians usually adopt the ultrasound date.
Disclaimer
This due date calculator is provided for informational purposes only. It is not medical advice, diagnosis, or a substitute for professional prenatal care. Estimates follow Naegele's rule and your entered cycle length; actual delivery and gestation vary widely and should be confirmed with your obstetrician or midwife.