Free Pregnancy Due Date Calculator Online

Pregnancy Due Date Calculator: Estimate a Due Date from Your Last Period with Naegele's Rule

A pregnancy due date calculator takes the first day of your last menstrual period and adds a fixed number of days to project an estimated delivery date — the same basic approach midwives and doctors have used for nearly two centuries, formalized as Naegele's rule. Type in one date, and the tool immediately shows the projected due date alongside how many weeks and days along the pregnancy currently is, which trimester that falls in, how many days remain, and a rough estimate of the conception date.

It's worth being direct about what this number is and isn't from the very first paragraph: a due date produced this way is a statistical estimate built from an average pregnancy length, not a prediction of the actual day labor will start. Only a small minority of babies — commonly cited as somewhere around 4 to 5 percent — are actually born on their calculated due date, with most arriving anywhere within a couple of weeks on either side. The calculation runs entirely as JavaScript in your browser, and the identical Naegele's-rule math also powers a metered API endpoint that other pregnancy-tracking tools can call directly instead of reimplementing the date logic themselves.

How to Use the Due Date Calculator

The interface is a single date field with results that update automatically:

1. Enter the first day of your last menstrual period (LMP) using the date picker.
2. As soon as a date is entered, the estimated due date appears in large text, calculated as the LMP plus 280 days.
3. A progress bar beneath the due date fills in based on how far along the pregnancy is relative to that 280-day span, with the percentage complete shown just below it.
4. Three smaller cards show the current week and day of pregnancy, which trimester that falls in (first, second, or third), and the number of days remaining until the estimated due date.
5. A line at the bottom shows an estimated conception date, calculated as roughly 14 days after the LMP (assuming ovulation occurred partway through the cycle), along with a reminder that all of the above is an estimate and not a substitute for a doctor's or midwife's assessment.

There's no field for cycle length in the current LMP-only view of the tool, though the underlying calculation can also account for cycles that run longer or shorter than 28 days when a cycle length is supplied through the API — a due date shifts a day earlier or later for each day a cycle deviates from the 28-day assumption Naegele's rule is built around.

What Naegele's Rule Actually Calculates

Naegele's rule, credited to 19th-century German obstetrician Franz Naegele, is built on two assumptions: that a typical pregnancy lasts about 280 days (40 weeks) counted from the first day of the last period rather than from conception, and that ovulation happens around day 14 of a 28-day cycle. The rule is often stated as subtracting three months from the LMP and adding seven days and a year, which produces the same result as simply adding 280 days directly — this calculator uses the direct addition, doing the arithmetic in fixed calendar-day steps so a date-only input like "2026-01-01" can't drift by a day regardless of where in the world the calculation runs.

Milestone	How it's calculated
Estimated due date	Last menstrual period + 280 days
Estimated conception	Last menstrual period + 14 days
Current gestational age	Days elapsed since LMP, shown as weeks and remaining days
Trimester	First: weeks 0-13; Second: weeks 13-27; Third: week 27 onward

One quirk worth understanding: because gestational age is conventionally counted from the last period rather than from conception, a pregnancy is already considered "2 weeks along" at the estimated moment of conception itself — this is a dating convention, not a claim that anything measurable was happening in the first two weeks.

Why Only a Small Fraction of Births Land on the Exact Due Date

The 280-day figure is a population average, and averages by definition have a spread around them rather than describing every individual case. A full-term pregnancy is medically considered anywhere from 37 to 42 weeks, which is a five-week window, not a single day — so the "due date" this calculator returns is better read as the center of a several-week range than as a firm appointment. Commonly cited figures put the share of babies born on their exact calculated due date at only around 4 to 5 percent, with the majority arriving within roughly a week or two of that date in either direction. Factors like whether this is a first pregnancy, the length of a person's typical cycle, and individual variation in pregnancy length all shift where within that range an actual birth is likely to fall — none of which a simple date-plus-280-days formula can see.

Why an Ultrasound Often Gives a More Precise Estimate

Naegele's rule depends entirely on knowing the LMP date accurately and on ovulation having occurred close to day 14 of the cycle — both of which are shaky assumptions for anyone with irregular periods, anyone unsure of their last period's exact start date, or anyone who conceived while breastfeeding or shortly after stopping hormonal birth control, when cycle timing can be unpredictable. An early ultrasound measures the size of the developing fetus directly and is generally considered a more accurate way to establish gestational age and a due date in those situations, which is exactly why clinical due dates are often adjusted after a first-trimester ultrasound rather than left as the original LMP-based calculation. This calculator only ever performs the LMP-based math — it has no way to incorporate ultrasound measurements or any other clinical data.

Manual Date Math vs. Using a Calculator

Working out a due date without a dedicated tool introduces a few common failure points:

• Counting months on a calendar by hand. Adding "nine months" instead of the more precise 280 days is a common shortcut that can land a week or more off, since calendar months don't divide evenly into that day count.
• Local-timezone date bugs in spreadsheets. A formula built in a spreadsheet that parses a date string and reformats it can silently shift by a day depending on the timezone setting of the device doing the calculating — this tool sidesteps that by doing every step of the date arithmetic in fixed, timezone-independent day increments.
• A paper wheel or printed gestation calculator. Cardboard pregnancy wheels handed out at clinics use the same Naegele's-rule logic but require lining up a printed dial by eye, which is harder to read precisely than a typed date producing an exact result.
• A phone's built-in calendar app. Setting a reminder 280 days out by scrolling through months on a phone calendar works in a pinch, but manually counting nine-and-change months forward is slower and more error-prone than a tool that adds the exact day count in one step.
• This calculator's API. A pregnancy-tracking app or a prenatal appointment scheduler that needs the same LMP-based due-date math server-side, rather than a person reading it off this page, can request it as a metered endpoint; see the API reference for the parameters accepted and how credits are charged.

Why Gestational Age Is Counted From Your Last Period, Not Conception

It can feel counterintuitive that a pregnancy is described as "2 weeks along" at roughly the moment conception is estimated to have happened, but this dating convention exists for a practical reason: the first day of a last period is a concrete, rememberable date, while the exact moment of conception almost never is. Clinicians standardized on LMP-based dating precisely because it gives every pregnancy a consistent, comparable starting point even when the exact conception date is unknown, which is nearly always the case. The tradeoff is that "gestational age" and "how long the embryo has actually been developing" differ by roughly two weeks throughout the pregnancy — a distinction worth knowing so the week count this calculator returns isn't mistaken for a count of developmental time since fertilization.

How Cycle Length Adjusts the Naegele Estimate

Naegele's rule in its classic form assumes a 28-day cycle with ovulation around day 14, but the underlying calculation this tool is built on can also take a different average cycle length as an input through the API, shifting the estimated due date accordingly — a day earlier in the due date for each day shorter than 28 the cycle runs, and a day later for each day longer. That adjustment matters because someone with, say, a consistently 35-day cycle likely ovulated later in that cycle than someone with a 28-day cycle sharing the exact same LMP date, which pushes their realistic conception date, and therefore due date, later as well. The web page shown here always assumes the standard 28-day cycle for simplicity; anyone with a well-established longer or shorter average cycle may get a more representative estimate by supplying that cycle length directly to the API rather than relying on the 28-day default.

How accurate is a due date calculated this way?

It's a reasonable starting estimate but not a precise prediction — full-term pregnancies commonly span 37 to 42 weeks, and only a small percentage of babies are born on the exact calculated date. An early ultrasound typically refines the estimate further; always confirm timing with a doctor or midwife.

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

The calculation depends entirely on that date being accurate, so an uncertain LMP produces an uncertain due date. A clinical ultrasound is generally a more reliable way to establish gestational age when the LMP date isn't well known.

Does this account for an irregular cycle?

The core Naegele's-rule formula assumes ovulation around day 14 of a 28-day cycle, which is less reliable for anyone with an irregular or notably longer or shorter cycle. Discuss dating options with a doctor or midwife if your cycle doesn't fit that typical pattern.

Is this tool a substitute for prenatal care?

No. It performs one specific date calculation and nothing else — it doesn't monitor pregnancy health, assess risk, or replace checkups, bloodwork, or ultrasounds. Treat the output as informational and confirm all pregnancy-related decisions with a qualified healthcare provider.

Does a twin or other multiple pregnancy change the calculation?

The Naegele's-rule math itself doesn't distinguish a single pregnancy from a multiple one — it's the same LMP-plus-280-days arithmetic either way. In practice, pregnancies with more than one baby are commonly delivered earlier than a single-baby pregnancy on average, which is one more reason a doctor's individualized assessment matters more than this calculator's generic estimate as a pregnancy progresses.

Related Health Tools

Cycle-based estimates come up elsewhere in family planning too — the ovulation calculator projects a fertile window from the same kind of last-period and cycle-length inputs used here. For pregnancy-related nutrition planning, the calorie calculator and water intake calculator offer general daily targets that a doctor can help adjust for pregnancy-specific needs.