Due Date Calculator

Forty weeks. One date. A lifetime of "so when are you due?"

Use this free pregnancy due date calculator to estimate your baby's estimated due date (EDD) from the first day of your last menstrual period (LMP). Enter one date and get the day the whole family will circle on the calendar — and the baby will cheerfully ignore.

The method is Naegele's rule: LMP plus 280 days (40 weeks). It is what most clinics use for the first estimate before an ultrasound refines it. We calculate it precisely, then remind you that only about 4% of babies actually arrive on their due date. Punctuality is learned, apparently.

What This Due Date Calculator Does

Calculates your estimated due date using the standard 280-day (40-week) rule
Gives you the anchor for trimester dates, prenatal appointments, and the "how many weeks are you?" question
Works from the one date most people know without a test: the start of the last period
Sets expectations: it is an estimate with a ±2-week window, not a delivery appointment

How to Use the Due Date Calculator

Inputs:

Last Period — the first day of your last menstrual period (not the last day, not the day it ended)

Steps:

Pick the first day of your last period
Click Calculate
Note the due date; 37–42 weeks from LMP is the normal "term" window
Confirm with your provider — a first-trimester ultrasound may adjust the date by a few days

How the Calculation Works

Due Date = LMP + 280 days (= 40 weeks = 9 months + 7 days)

Assumes a 28-day cycle with ovulation on day 14. Longer cycles → real due date is a bit later; shorter → a bit earlier
Pregnancy is counted from LMP, so you are "4 weeks pregnant" the day your period is late — about 2 weeks after conception
Trimesters: 1st weeks 1–13 · 2nd weeks 14–27 · 3rd weeks 28–40

Worked Example: LMP on March 1

March 1 + 280 days
March has 31 days, so: 30 (rest of March) + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 = 274 days gets you to November 30
280 − 274 = 6 more days → December 6
Realistically: anywhere from November 15 to December 20 is normal. Pack the bag early.

Tips (From People Who Have Made These Mistakes)

Know your conception date instead of LMP? Add 266 days (38 weeks) to it
IVF transfer? Day-5 embryo: transfer date + 261 days. Day-3 embryo: + 263 days. Your clinic will give you the exact date
Irregular cycles make LMP dating unreliable — the early ultrasound (crown-rump length) becomes the better estimate

Frequently Asked Questions

How accurate is a due date calculator?

The LMP method is accurate to about ±2 weeks. Only ~4–5% of babies arrive on the exact date; about 80% arrive between 38 and 42 weeks. An early ultrasound narrows the range.

How many weeks pregnant am I?

Count the weeks from the first day of your last period to today. Example: LMP 10 weeks ago = 10 weeks pregnant, even though conception was ~8 weeks ago. Yes, everyone finds this confusing.

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

Use the conception date + 266 days if you know it, or rely on the first-trimester ultrasound, which dates pregnancy from the embryo's size within a few days.

Can my due date change?

Yes. If an early ultrasound differs from the LMP date by more than about 5–7 days, providers usually adopt the ultrasound date. After the first trimester, the date is rarely changed.

What is considered full term?

39 weeks 0 days to 40 weeks 6 days is "full term." 37–38 weeks is "early term," 41 weeks is "late term," and 42+ is "post-term." All of them are still "please just come out now."

Mini Roast Disclaimer

We roast the number, not you. The baby, however, will roast your sleep schedule for roughly eighteen years.

Reality Check

This is a calendar calculation, not a medical assessment. Cycle length, ovulation timing, and ultrasound measurements all refine the real date, and your healthcare provider's estimate overrides anything a website says. Use this to plan; use them to decide.

Due Date Calculator

Inputs