The Pregnancy Due Date Calculator estimates when your baby is likely to arrive. Enter the first day of your last menstrual period (LMP) and your average cycle length, and it works out your estimated due date, how many weeks and days pregnant you are today, your current trimester, and your estimated conception date.
The calculation uses the standard method doctors use for an initial estimate — a 40-week (280-day) pregnancy counted from your last period. Everything runs in your browser, so none of the dates you enter are uploaded or stored.
How to calculate your due date
- Select the first day of your last menstrual period (LMP).
- Set your average cycle length — 28 days is typical; adjust if yours is longer or shorter.
- Read your estimated due date, along with how far along you are and your trimester.
- Use the conception estimate and “days to go” to help plan ahead.
How the due date is calculated
The most common method is Naegele’s rule: add 280 days (40 weeks) to the first day of your last period. This calculator also adjusts for your cycle length, since a longer or shorter cycle shifts the likely date of ovulation and therefore the due date.
Pregnancy “weeks” are counted from the first day of your last period, not from conception, which is why you are considered about two weeks pregnant at the point of conception. Conception itself is estimated at around ovulation, roughly two weeks before your next period was due.
What to keep in mind
- A due date is an estimate — only about 1 in 20 babies are born on it, and a normal, full-term birth can happen from 37 to 42 weeks.
- Your doctor may revise the date after an ultrasound, which dates the pregnancy more precisely, especially in the first trimester.
- Cycle length and irregular periods can affect the estimate, so treat this as a guide rather than a medical diagnosis.
- The first trimester is weeks 1–13, the second 14–27, and the third 28 onward.
Frequently Asked Questions
How is my due date calculated from my last period?
It adds 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your cycle length. This is the standard method used for an initial due-date estimate.
How many weeks pregnant am I?
Pregnancy is counted in weeks from the first day of your last period. This calculator shows your current gestational age in weeks and days based on that start date and today’s date.
Is the due date accurate?
It is a good estimate, but only about 5% of babies arrive on the exact date. A full-term birth can occur any time between 37 and 42 weeks. An ultrasound gives a more precise date.
What if my cycle isn’t 28 days?
Adjust the cycle-length slider to your average. A longer cycle usually means later ovulation and a slightly later due date; a shorter cycle, the opposite.
Is this a substitute for medical advice?
No. This tool is for general information only. Always consult your doctor or midwife for pregnancy dating and any medical questions.