Pregnancy due date calculator: find your estimated due date

Just found out you are pregnant and want to know when your baby might arrive? Use the free pregnancy due date calculator on ToolHQ to get your estimated due date (EDD) from your last menstrual period or conception date.

A pregnancy due date calculator estimates the date your baby is likely to be born based on the standard 40-week pregnancy timeline. It also shows your current gestational age and which trimester you are in.

This is an estimate, not a medical prediction. Fewer than 5% of babies are born on their exact estimated due date, and most arrive within one to two weeks of it. Your healthcare provider will confirm and refine your due date through clinical assessment and ultrasound. That said, having an early estimate helps you plan prenatal appointments, tell family, and begin thinking about maternity leave and newborn preparation.

Key Takeaways

• Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period for your EDD
• If you know your conception date, your EDD is 266 days (38 weeks) from that date
• Only about 5% of babies are born on their exact due date; 90% arrive within two weeks of the estimate
• The calculator shows your gestational age in weeks and your current trimester
• No data is stored or transmitted, all calculations happen in your browser. This tool is not a substitute for medical care.

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How due dates are calculated

The most widely used method for estimating a due date is Naegele's rule, named after German obstetrician Franz Karl Naegele. According to Wikipedia's Naegele's rule article, the method estimates the due date by:

1. Starting from the first day of the last menstrual period (LMP)
2. Adding one year
3. Subtracting three months
4. Adding seven days

In practice this equals LMP + 280 days (40 weeks). So if your last period started on January 1, your estimated due date is October 8 of the same year.

Naegele's rule assumes a regular 28-day menstrual cycle with ovulation on day 14. Women with longer or shorter cycles may have a due date that is a few days earlier or later than the standard calculation. A first-trimester ultrasound is the most accurate way to confirm gestational age.

The reason dates are counted from LMP rather than conception is that the exact date of ovulation and fertilization is usually not known, while the start of the last period almost always is. Pregnancy is therefore described in gestational weeks, which start two weeks before conception actually occurs.

According to the American College of Obstetricians and Gynecologists (ACOG), a full-term pregnancy lasts between 39 and 40 weeks and 6 days. Babies born between 37 and 38 weeks and 6 days are considered early term; those born at 41 weeks or later are late term.

Disclaimer: This calculator provides an estimate only. Always consult your healthcare provider for personalized medical guidance, confirmation of your due date, and prenatal care.

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When you need a due date calculator

Right after a positive pregnancy test. Most people want an approximate due date immediately, before their first doctor's appointment. The calculator gives you a useful starting number within seconds.

Planning prenatal appointments. Knowing your gestational age helps you schedule the right tests at the right time. The first trimester ultrasound (typically between 8-12 weeks) is the critical appointment for confirming dates.

Telling family and planning leave. If you want to share your news around the 12-week mark, knowing your approximate due date helps you figure out when that milestone falls.

Tracking your trimester. The calculator tells you which trimester you are in and shows how many weeks you have been pregnant, which helps you follow week-by-week pregnancy guides.

Take Amara, who got a positive home test on a Thursday morning. She knew her last period had started three weeks earlier. She entered that date into the pregnancy due date calculator and saw her estimated due date was 37 weeks away, putting her in early week 3 of pregnancy. The calculator also showed she was in her first trimester and that the standard first prenatal appointment would typically fall between weeks 8 and 10. She called her OB that same day to schedule, which meant she got an appointment two weeks earlier than she might have if she had waited to figure out the timing.

Find your due date now

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How to use the ToolHQ pregnancy due date calculator

You can enter your information in two ways.

Using your last menstrual period (LMP):

1. Select "Last menstrual period" as your input method.
2. Enter the first day of your most recent period before becoming pregnant.
3. The calculator adds 280 days to that date.
4. Your estimated due date, current gestational age, and current trimester appear instantly.

Using your conception date:

1. Select "Conception date" as your input method.
2. Enter the date you believe conception occurred, whether from tracked ovulation data or a known date.
3. The calculator adds 266 days (38 weeks) to that date.
4. Results appear instantly.

If your menstrual cycle is significantly longer or shorter than 28 days, your healthcare provider may adjust the estimate accordingly, especially after a first-trimester ultrasound.

No data is stored or transmitted. All calculations run entirely in your browser.

For related planning, the countdown timer can count down to your estimated due date, and the date calculator helps you calculate exactly how many days, weeks, or months away your due date falls.

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Trimester breakdown and key pregnancy milestones

Pregnancy trimesters at a glance

Trimester	Gestational weeks	Key milestones
First trimester	Weeks 1-13	Positive test, embryo implantation, heartbeat (week 6-7), first prenatal visit, nuchal translucency scan (weeks 11-13)
Second trimester	Weeks 14-27	Anatomy ultrasound (weeks 18-22), quickening (first movements, weeks 16-20), glucose screening (week 24-28)
Third trimester	Weeks 28-40+	Baby's position confirmed, Group B strep test (weeks 35-37), baby "drops" into pelvis, full term at 39 weeks

Gestational age vs. fetal age

These two terms are sometimes confused.

Gestational age counts from the first day of your last menstrual period. This is the number used by healthcare providers and in the due date calculation. It includes approximately two weeks before conception actually occurred.

Fetal age (or conceptual age) counts from the actual date of conception and is therefore roughly two weeks less than gestational age. When your doctor says "you are 10 weeks pregnant," they mean 10 gestational weeks, which corresponds to about 8 weeks of fetal development.

Due date accuracy

Studies show that:

• Fewer than 5% of babies are born on their exact estimated due date
• About 60% of births occur within a week of the estimate
• About 90% occur within two weeks of the estimate
• The most common birth week is actually weeks 39-40

This is why due dates are better understood as a two-week window around a central estimate rather than a precise calendar day.

Rachel was 6 weeks along by gestational age when she found out she was pregnant. Her LMP had started on a specific date she had tracked in her period app. She entered it into the due date calculator and got an EDD. When she had her first ultrasound at 8 weeks, the sonographer measured the baby and said she was actually 8 weeks and 4 days along, shifting her due date forward by four days. This is normal: first-trimester ultrasounds are the gold standard for confirming dates, and small adjustments are common.

The age calculator can be used after your baby arrives to track their exact age in days, weeks, and months, and the countdown timer is a fun way to visualize how many days remain until your due date.

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Frequently asked questions

How is a due date calculated?

Naegele's rule adds 280 days (40 weeks) to the first day of your last menstrual period. If you know your conception date, add 266 days (38 weeks) instead. Most healthcare providers confirm and refine this estimate with a first-trimester ultrasound.

How accurate is a due date calculator?

The estimate is a starting point. Only about 5% of babies arrive on their exact due date, and 90% arrive within two weeks of the estimate. First-trimester ultrasound measurements are the most accurate method for confirming gestational age.

What if I do not know my last period date?

If you do not know your LMP, use your best estimate or enter a known conception date if you have one. Your healthcare provider can estimate gestational age through an ultrasound based on fetal measurements even without LMP data.

What is gestational age?

Gestational age is the number of weeks since the first day of your last menstrual period. It is the standard measurement used by healthcare providers and is typically two weeks more than fetal age (time since conception).

When should I see a doctor after a positive pregnancy test?

Most healthcare providers recommend scheduling a first prenatal appointment between 6 and 10 weeks of gestational age. Early appointments allow for blood work, confirmation of pregnancy, and dating by ultrasound. Contact your provider promptly after a positive test.

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The short version

A pregnancy due date calculator gives you an estimated due date within seconds using Naegele's rule: your last menstrual period plus 280 days. ToolHQ's calculator also shows your gestational age and current trimester, whether you enter your LMP or your known conception date. It is a useful first estimate; your healthcare provider will confirm it.

Find your estimated due date now

For related planning tools, use the countdown timer to track the days until your due date and the date calculator to calculate exactly how far away your due date is in days, weeks, and months.

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