Due Date Calculator

Free Due Date Calculator - Know When Your Baby Is Coming

Finding out you're pregnant brings one question straight to the front of your mind: when is my baby due? This due date calculator gives you a clear, instant answer using either the first day of your last period or your estimated conception date, whichever you know with more confidence.

Along with the due date, you get a likely date range, a live countdown, your current week of pregnancy, your trimester, an estimated conception date, and a full calendar of pregnancy milestones with real dates attached. You can even download a calendar reminder for the big day, export your timeline as a spreadsheet, and check a couple of light-hearted extras like your baby's star sign. No sign-up, no app download, everything updates the moment you change a date.

How This Due Date Calculator Works

This tool uses the same formulas doctors, midwives, and hospitals rely on every day. If you calculate from your last menstrual period (LMP), it adds 280 days, which is exactly 40 weeks, to the first day of that period. If you calculate from a known conception date instead, it adds 266 days, since conception typically happens about two weeks into a standard cycle.

Both routes are built to land on roughly the same due date for a typical 28-day cycle. The small difference is where the countdown starts: from your last period (which most people can pin down easily) or from the day you actually conceived (which is only known for sure in some cases, like ovulation tracking or a single instance of intercourse).

Why Doctors Count From the Last Period, Not Conception

It feels a little strange at first: you're not even pregnant yet on the date your due date calculation starts from. But there's a good reason. Most people don't know their exact conception date. What they usually do know is when their last period started. Using that fixed, memorable date gives every pregnancy a consistent starting point, called gestational age.

This is why, at 4 weeks pregnant, the embryo itself is really only about 2 weeks old. It can be confusing the first time you hear it, but every calculator, app, and doctor's office uses the same counting method, this one included, so your numbers will always match what your provider tells you.

This Formula Behind the Due Date Estimate, Explained Simply

The standard method used here is called Naegele's Rule, named after the doctor who popularized it in the early 1800s. It's simple on purpose: take the first day of your last period and count forward 280 days, or 40 weeks. That's your estimated due date.

Because not everyone has a textbook 28-day cycle, this calculator also lets you enter your average cycle length. A longer cycle usually pushes ovulation, and conception, later than the standard day 14 assumption, so the tool nudges your due date forward slightly to match. If you know your conception date instead, that step isn't needed since you're already starting from a firmer point.

Due Date by Last Period vs Due Date by Conception

Both methods aim for the same target, just from different starting lines.

• By last period (LMP): the most common choice, since it only needs a date most people track anyway. It assumes ovulation around day 14, adjusted here for your real cycle length.
• By conception date: more precise if you know exactly when conception happened, for example from ovulation testing, a fertility app, or a single known encounter. This removes the guesswork around ovulation timing entirely.
• Both should point to roughly the same week 40 due date for an average cycle. If your numbers are far apart, double-check which date you actually entered, since LMP and conception dates are typically about two weeks apart.

What the Countdown, Range, and Percentage Actually Mean

The live countdown ticks down the days, hours, minutes, and seconds until your estimated due date. It's just for fun and motivation. It is not a deadline; think of it as a rough marker rather than an appointment you have to keep.

The likely due date range shown alongside it covers two weeks before and two weeks after the calculated date. That's because full-term, healthy births regularly happen anywhere in that window, and only a small fraction of babies actually arrive on their exact calculated day. The percentage complete simply shows how far through a standard 280-day, 40-week pregnancy you currently are.

The Three Trimesters and What Typically Happens in Each

This calculator automatically shows which trimester you're in based on your current week, and marks the start date of each one on your personal timeline.

• First trimester (weeks 1-12): rapid early development, the highest miscarriage risk window, and often the toughest stretch for fatigue and morning sickness.
• Second trimester (weeks 13-27): energy often returns, this is usually when people learn the baby's sex, and first movements are typically felt.
• Third trimester (weeks 28-40+): fast growth, more frequent checkups, and the final stretch of preparation before labor and delivery.

Key Milestones This Calculator Maps Out for You

Beyond the due date itself, the calendar below turns important pregnancy milestones into actual dates rather than vague week numbers: your estimated conception date, the typical window for your first prenatal visit, the end of the first trimester (when miscarriage risk drops meaningfully), the mid-pregnancy anatomy scan window, the viability milestone around week 24, the start of the third trimester, the point a baby is considered full term at week 37, the due date itself at week 40, and a post-term marker at week 42 where providers often bring up induction.

Seeing these as real calendar dates instead of just week counts makes it much easier to plan appointments, request time off, and get ready for each stage as it approaches.

How Accurate Is a Due Date Calculator, Really?

It's worth being upfront about this: a due date is a well-informed estimate, not a guarantee. Only around 5% of babies are actually born on their exact calculated due date. The large majority of healthy, full-term births happen somewhere within two weeks before or after that date, and that's completely normal, not a sign anything is wrong.

A first-trimester ultrasound is generally considered the most accurate dating method available, often accurate within three to five days, because early fetal measurements are remarkably consistent from one pregnancy to another. Due date calculations based on your last period work well for people with regular, well-remembered cycles, but get less reliable with irregular cycles, which is exactly why providers often adjust the estimate after that first scan.

Common Mistakes to Avoid When Calculating Your Due Date

A handful of mix-ups show up again and again, so it helps to know them in advance.

• Mixing up conception date with last period date. They're roughly two weeks apart and produce noticeably different due dates if swapped by accident.
• Assuming irregular cycles don't matter. They can shift real ovulation well away from the standard day-14 assumption used by the basic formula.
• Treating the due date as a strict deadline instead of a window. Healthy, full-term births happen anywhere from around week 37 to week 42.
• Skipping the update after a first-trimester ultrasound, which is usually more reliable than a last-period-based estimate alone.
• Forgetting that gestational age counts from the last period, not conception, which is why an early pregnancy test at 'week 4' can feel confusingly early.

Tips to Get the Most Accurate Estimate

A due date calculator is only as good as the date you feed it, so a little care here goes a long way. Try to pin down the actual first day of bleeding for your last period, not the day you noticed it or a rough guess from memory. If you track your cycles in an app or a calendar, that record is usually more reliable than trying to recall it off the top of your head.

• Use your average cycle length over the last few months, not just your shortest or longest cycle ever.
• If you're not sure whether to use LMP or conception date, use whichever one you're more confident about, then switch methods here to compare the two results.
• Re-check your estimate after any early ultrasound, since that scan is usually more accurate than a date-based calculation alone.
• Keep in mind that this calculator, like every online due date tool, gives an estimate for planning purposes, not a medical diagnosis.

Who Should Use This Due Date Calculator

This tool is built for anyone who wants a fast, clear estimate: someone who just found out they're expecting and wants an immediate answer, someone planning maternity leave or travel around key dates, or simply anyone curious how far along a pregnancy is and what's coming up next.

This calculator is for general, informational tracking only and is not a substitute for medical care. Actual due dates, viability windows, and milestone timing can shift based on individual health factors, and only a doctor or midwife using clinical exams and ultrasound measurements can give a medically confirmed due date. Anyone who is pregnant or trying to conceive should be under the care of a qualified healthcare provider throughout.