An LMP due date calculator estimates your baby's due date from the first day of your last menstrual period using Naegele's rule: add 280 days (40 weeks) to that date. It is the dating method providers start with worldwide, because the date of your last period is usually known long before any scan.
The twist: the 280-day count assumes a 28-day cycle with ovulation on day 14. Real cycles range from about 21 to 45 days, so this calculator also asks for your average cycle length and shifts the result accordingly. That single adjustment fixes the most common inaccuracy in DIY due date math.
How to Use the LMP Due Date Calculator
Enter the first day of your last period — the day full bleeding started, not the day it ended.
Enter your average cycle length — count from the first day of one period to the first day of the next. Leave it at 28 if you are unsure.
Get your estimated due date — the tool applies Naegele’s rule plus your cycle-length adjustment instantly.
Note your gestational age — the calculator also shows how far along you are today, which is what your provider will reference at appointments.
Compare with your scan later — bring your LMP-based date to your first visit so your provider can confirm it against the dating ultrasound.
How the Calculation Works
EDD = LMP + 280 days + (Cycle Length − 28 days)
The foundation is Naegele’s rule, the classic obstetric shortcut: add one year, subtract three months, and add seven days to the first day of your last period. Mathematically it is identical to adding 280 days (40 weeks) to your LMP. The 40 weeks includes roughly two weeks before conception, because gestational age is counted from your period, not from fertilization.
The cycle-length correction handles the ovulation assumption. Ovulation typically happens about 14 days before your next period, so in a 35-day cycle it falls around day 21 — a week later than the standard formula assumes. Adding (35 − 28) = 7 days to the 280-day result corrects for that later ovulation. For a 21-day cycle, the correction subtracts 7 days.
Reading Your Result: What the Numbers Mean
Your calculator result is more than a single date — it anchors every number your provider will use for the rest of the pregnancy. Understanding the three ways dates get expressed (due date, gestational age, and trimester) helps you read your own chart without confusion.
Your estimated due date (EDD)
The EDD is the 40-week mark: LMP + 280 days, adjusted for your cycle. Providers treat it as the center of a delivery window, not a deadline — full-term birth is considered normal from about 38 to 42 weeks, which is roughly five weeks of perfectly ordinary timing. Studies estimate only a small fraction of babies arrive on the exact date.
Your gestational age today
Gestational age is expressed as weeks and days — for example, “12 weeks 3 days.” It is simply how many days have passed since your LMP. This is the number your provider uses at every appointment, the number screening schedules are built on, and the number our week-by-week pregnancy guide follows. If your cycle is longer than 28 days, your cycle-adjusted EDD keeps this count honest.
Which trimester you are in
As a rough map: the first trimester runs through about week 13, the second from week 14 to about week 27, and the third from week 28 onward. Exact boundaries vary slightly between references, but the trimester labels mainly describe phases of development and care — the precise week counts are what providers actually schedule from.
Why your app and your doctor may show different weeks
Discrepancies of a day or two usually come from rounding or from whether a cycle-length correction was applied. A gap of a week or more often means your provider has switched you to an ultrasound-confirmed date, which is routine — ask which date they are using so your records stay consistent everywhere.
Worked Example
Standard 28-day cycle: Your last period started January 1, 2026. Add 280 days with no cycle adjustment:
January 1, 2026 + 280 days = October 8, 2026 — your estimated due date.
Longer 35-day cycle: Same LMP of January 1, 2026, but your average cycle is 35 days. The formula adds the 7-day difference:
January 1, 2026 + 280 days + 7 days = October 15, 2026 — a full week later than the unadjusted result, matching your later ovulation.
Shorter 21-day cycle: Same LMP of January 1, 2026, with a 21-day average cycle. The correction subtracts 7 days: January 1, 2026 + 280 days − 7 days = October 1, 2026 — earlier, because ovulation arrived sooner.
How Accurate Is It?
LMP dating is the global starting standard, and with a regular cycle and a well-remembered LMP date, it is often within a few days of the true gestational age. Its weak points are human memory and the day-14 ovulation assumption: irregular cycles, recent hormonal birth control, or breastfeeding can all make the LMP date misleading. Studies estimate only about 5% of babies are born on their exact due date — treat the result as the middle of a roughly five-week window from 38 to 42 weeks.
There is also a subtler source of error: people often remember the wrong day. Entering the day your period ended instead of the day it began shifts the estimate by nearly a week, and mistaking mid-cycle spotting for a period can shift it by much more. When in doubt, enter the earliest date you are confident was genuine period bleeding — and for the most reliable confirmation, an ultrasound due date calculation before 14 weeks is the gold standard. You can track what is happening at each stage with our week-by-week pregnancy guide.
When to Talk to Your Doctor
Your LMP-based estimate is a planning tool, not a medical date — your provider will confirm or revise it, usually at a dating scan between 8 and 14 weeks. Reach out sooner if you are unsure of your LMP, have very irregular cycles, or experience vaginal bleeding or severe cramping in early pregnancy. If a scan later moves your due date by more than a week, your provider will explain which date they are using for screening windows and monitoring — use that one for all your planning.
Finally, do not wait for your first appointment to start the basics: begin folic acid if you have not already, avoid alcohol and smoking, and note any medications you take so your provider can review them. The due date is a calendar anchor — the everyday habits of early pregnancy matter just as much.
This calculator provides estimates for educational purposes and is not medical advice. Always confirm with your healthcare provider.
Frequently Asked Questions
Common questions answered by our medical team
Take the first day of your last period and add 280 days (40 weeks). If your cycle is not 28 days, adjust by the difference: add extra days for longer cycles, subtract for shorter ones. For example, a 35-day cycle adds 7 days to the standard result. The calculator above handles both steps automatically.