Free Pregnancy Due Date Calculator

Pregnancy Due Date Calculator

Estimate your due date from your last period — 100% private, nothing leaves your browser.

First day of last period

Getting an Estimated Timeline Before the First Ultrasound

In the early weeks after finding out about a pregnancy, one of the first practical questions is simply "when is this due" — this tool calculates an estimated due date from your last menstrual period or a known conception date, giving an initial timeline reference before a clinical ultrasound provides a more precise estimate.

The Calculation Methods

The most common method, known as Naegele's Rule, estimates a due date as 280 days (40 weeks) from the first day of your last menstrual period, based on the standard assumption of a 28-day cycle with ovulation around day 14 — this is the method most healthcare providers use for an initial estimate before any ultrasound is available. An alternative calculation, when the actual conception date is known (from IVF or precise ovulation tracking, for instance), estimates the due date as 266 days (38 weeks) from that conception date instead, since conception itself already accounts for the timing that Naegele's Rule has to estimate.

A Worked Example

For a last menstrual period starting March 1st, the LMP-based method estimates a due date around December 5th, following the standard 280-day calculation. If conception is instead known to have occurred on March 15th (roughly two weeks after that LMP date, matching typical ovulation timing), the conception-based method estimates a very similar due date, since both methods are ultimately estimating toward the same underlying biological timeline, just starting the count from different reference points.

Why an Ultrasound Estimate Is Considered More Precise

Both calculation methods here rely on an assumed, standard cycle pattern — LMP-based calculation specifically assumes a 28-day cycle with ovulation exactly on day 14, an assumption that doesn't hold precisely for everyone, since cycle length and ovulation timing vary meaningfully between individuals. A first-trimester ultrasound measures the actual fetus directly, comparing its size against well-established growth curves, which generally provides a more individually accurate due date estimate than a calendar-based calculation, which is why medical providers typically adjust the estimated date based on ultrasound findings once available.

What "Due Date" Actually Means in Practice

A due date is a statistical estimate of the midpoint of a normal delivery range, not a precise prediction of the actual delivery day — only a small percentage of babies are actually born on their exact calculated due date, with full-term delivery normally considered to span a range of roughly two weeks before to two weeks after that estimated date. Treating the date as a general timeline anchor for planning purposes, rather than an exact prediction, reflects how the estimate is actually meant to be used medically. This is a general wellness estimate, not medical advice, and any pregnancy-related decision should be guided by your doctor or midwife based on your actual prenatal care.

Who This Estimate Helps

Someone newly aware of a pregnancy wanting an initial timeline before their first prenatal appointment. Partners and family planning around an approximate arrival window for practical preparations. Anyone tracking pregnancy week milestones, since many pregnancy resources organize information by week number counted from this same estimated timeline. This is a starting reference point — actual prenatal care and more precise dating should come from a healthcare provider.

Calculated Privately, On Your Device

The date calculation runs entirely with client-side JavaScript in your browser — this is deeply personal health information, and it's never transmitted to a server or stored anywhere outside your own device during the calculation.

A Second Example

Someone with an irregular cycle who isn't confident their last period date reflects a standard 28-day assumption uses the calculation as a rough initial estimate, understanding that their first ultrasound appointment will likely provide a more individually accurate due date once the fetus can be directly measured.

Why did my doctor's due date differ slightly from this calculation?

Ultrasound-based dating measures the fetus directly and is generally considered more precise than calendar-based calculation, especially for anyone whose actual cycle doesn't match the standard 28-day assumption this calculation relies on.

What if I don't remember my exact last period date?

A less certain starting date will produce a correspondingly less certain estimate; an ultrasound-based estimate from a healthcare provider becomes more valuable in that situation since it doesn't depend on remembering that date precisely.

Is it normal to deliver before or after the calculated due date?

Yes — full-term delivery normally spans a range of roughly two weeks before to two weeks after the estimated due date, with only a small percentage of births occurring exactly on the calculated date.

Which method should I use if I know my exact conception date?

The conception-based calculation is generally considered more precise than the standard LMP-based method when the actual conception date is confidently known.

Does this replace seeing a doctor for prenatal care?

No — this provides an initial estimate for reference only; prenatal care, ongoing monitoring, and a more precise due date should come from a qualified healthcare provider.