Due Date Calculator Guide: How to Calculate Your Pregnancy Due Date
A clear, step-by-step walkthrough of every method used to estimate a pregnancy due date, from Naegele's rule to ultrasound dating, so you understand exactly where your number comes from and how much to trust it.
The moment a pregnancy test turns positive, one question shows up almost immediately: when is the baby actually due? A due date calculator answers that in seconds, but the math behind it, and how much weight to give the answer, deserves a proper explanation.
A pregnancy due date calculator estimates when a baby is likely to arrive by adding 280 days (40 weeks) to the first day of the last menstrual period, a method known as Naegele's rule. If you know your conception date or have an early ultrasound instead, the calculator can use those numbers just as easily, often with better accuracy.
None of these methods predict an exact birthday. They give you a due date, which is really the center of a normal range rather than a deadline. 100 Calculator's Pregnancy Due Date Calculator does this math instantly from your last period, your conception date, or an ultrasound your doctor gave you.
This guide walks through exactly how each calculation method works, why your due date might shift after an ultrasound, what to do if you're not sure when your last period started, and how accurate these numbers really are.
Understanding Your Pregnancy Due Date
A due date, sometimes called an estimated date of delivery (EDD) or estimated date of confinement (EDC), is the calendar date your healthcare provider expects you to give birth if your pregnancy follows an average timeline. It's a prediction built from population averages, not a measurement of your specific baby's readiness.
What a Due Date Actually Represents
Every due date calculation rests on one number: a full-term pregnancy lasts about 280 days, or 40 weeks, counted from the first day of your last menstrual period (LMP). That's the definition doctors and due date calculators use, including ours.
Why "40 Weeks Pregnant" Is a Little Misleading
Here's the part that surprises a lot of people: you're not actually pregnant for the first two weeks of those 40. Doctors count gestational age from the first day of your last period rather than from conception, mainly because most people know that date far more reliably than they know exactly when they ovulated. (Mayo Clinic) Conception typically happens about two weeks after your last period starts, so early on, your baby is technically a couple of weeks younger than your "weeks pregnant" number suggests. This is also why the LMP-based calculation adds a full 280 days, while conception-based dating, covered later in this guide, adds only 266.
Curious how this concept plays out closer to birth? Our companion guide, Pregnancy Due Date Explained, looks at what a due date means from a slightly different angle, including why it's treated as an estimate rather than a promise.
How Is My Due Date Calculated?
Nearly every due date calculator, including the one built into most prenatal apps and the tool your OB-GYN's office uses, relies on the same two-century-old formula: Naegele's rule.
Naegele's Rule, Explained Simply
Naegele's rule is named for the German obstetrician who popularized it in the early 1800s. It takes the first day of your last period and works out your due date in three simple steps:
- Start with the first day of your last period.
- Subtract three calendar months from that date.
- Add seven days, then add one year to land on your estimated due date.
For example, if your last period started on January 10, subtracting three months gives you October 10 of the previous year. Adding seven days brings you to October 17, and adding one year lands you on October 17 of the following year, your estimated due date.
The Math Behind the 280-Day Estimate
This three-step shortcut produces the same result as simply adding 280 days, or 40 weeks, to your LMP. Both approaches assume two things: a menstrual cycle that lasts 28 days, and ovulation happening on day 14 of that cycle. When both assumptions hold true, Naegele's rule tends to land close to your actual delivery window. When they don't, and cycles vary quite a bit from person to person, the estimate drifts, which is exactly why the calculation methods later in this guide exist.
Calculating Your Due Date From Your Last Period
Using your last menstrual period is the most common starting point for a due date calculator, mainly because most people can recall that date more easily than an exact conception date.
Step-by-Step: LMP to Due Date
- Note the first day of your last period, not the last day of bleeding. That's day one.
- Add 280 days, or count forward 40 weeks, from that date.
- If your cycles run close to 28 days, that total is your estimated due date.
- If your cycles are consistently longer or shorter than 28 days, adjust the total using the method in the next section.
Rather than counting 280 days by hand, entering your last period into 100 Calculator's Pregnancy Due Date Calculator gives you the same result instantly, along with your current week of pregnancy and trimester. If you haven't been tracking your cycles yet, the Period & Fertility Tracker is a simple way to start logging your last period and average cycle length.
What Counts as "Day One" of Your Last Period
Day one is the first day of full bleeding, not light spotting beforehand and not the day your period ends. This is the same starting point used for period tracking in general. If you'd like a refresher on identifying day one consistently, our guide on how to track your period accurately covers it in detail, which matters just as much for due date accuracy as it does for predicting your next period.
Calculating Your Due Date From Your Conception Date
If you know roughly when you conceived, maybe from ovulation tracking, an ovulation predictor kit, or a fertility treatment, you can calculate your due date from that date instead of your last period.
Why Conception-Based Dating Uses 266 Days, Not 280
A full-term pregnancy lasts about 266 days, or 38 weeks, from the moment of conception. That's 14 days shorter than the 280-day LMP figure, because conception typically happens around day 14 of an average cycle. Add 266 days to your conception date, and you land on essentially the same due date Naegele's rule would give you, just calculated from a different starting point.
When This Method Makes Sense
Conception-based dating is most useful if you were closely tracking ovulation while trying to conceive, since it skips the assumption that you have a textbook 28-day cycle. Our guides on how to calculate ovulation and the best time to conceive both explain how to pin down that date more precisely if you're not sure.
If your pregnancy involved fertility treatment, your clinic will usually give you a due date based on your specific procedure date rather than an estimate, which is typically more precise than either LMP or conception-based math on its own.
Due Date Calculation With Irregular Cycles
Naegele's rule assumes a 28-day cycle, but plenty of people have cycles that run shorter, longer, or all over the place. The good news is that the calculation can be adjusted, at least when your cycles are irregular but still somewhat predictable.
Adjusting for Cycles Longer or Shorter Than 28 Days
If your average cycle is longer than 28 days, add the difference to your 280-day estimate. A 35-day cycle, for example, means ovulation likely happens later, so you'd add 7 extra days for a 287-day estimate. If your average cycle is shorter, like 21 days, subtract the difference instead, landing on a 273-day estimate. This works because a longer or shorter cycle shifts when ovulation happens, and ovulation timing is what actually determines conception, not the start of your period.
| Average Cycle Length | Adjustment to 280 Days | Estimated Total |
|---|---|---|
| 21 days | Subtract 7 days | 273 days |
| 28 days (average) | No adjustment | 280 days |
| 32 days | Add 4 days | 284 days |
| 35 days | Add 7 days | 287 days |
When Irregular Cycles Make LMP Dating Unreliable
If your cycles vary widely from month to month rather than just running consistently longer or shorter, this adjustment stops being reliable, since there's no single "average" cycle length to adjust by. The same is true if you're not sure when your last period started. In both cases, an early ultrasound becomes the more trustworthy way to date your pregnancy, which we cover next.
Our guide to signs of an irregular menstrual cycle can help you figure out whether your cycles fall into this category, and our piece on the fertility window explains how ovulation timing shifts with cycle length in more detail.
How Ultrasound Dating Affects Your Due Date
Once you're in prenatal care, your due date usually gets a second opinion, this time from an ultrasound rather than a calendar.
Why First-Trimester Ultrasound Is the Most Accurate Method
An ultrasound performed in the first trimester, ideally before about 14 weeks, measures your baby's crown-rump length to estimate gestational age directly from actual growth, rather than assuming a textbook cycle. According to the American College of Obstetricians and Gynecologists (ACOG), early ultrasound dating is accurate to within roughly five to seven days, compared to LMP-based dating alone, which can be off by a week or two. (ACOG) That precision matters because gestational age affects everything from when screening tests are scheduled to when labor might be induced if a pregnancy runs long.
When Your Due Date Might Change
If your ultrasound-based due date and your LMP-based due date land close together, your provider will usually keep the original LMP-based date. If the two estimates disagree by roughly a week or more in the first trimester, ACOG guidance supports adjusting the due date to match the ultrasound instead, since it's considered the more reliable number at that stage. Once a due date is confirmed by an early ultrasound, later changes are uncommon and typically only happen for a specific medical reason.
Our guide on what if your due date changes walks through what to expect if your provider updates your estimate partway through your pregnancy.
How Accurate Is a Due Date Calculator, Really?
This is probably the most important question in this entire guide, and the honest answer is: not as accurate as a single date implies.
What the Research Shows
According to Cleveland Clinic, only about 4% of babies are actually born on their exact due date. (Cleveland Clinic) Most arrive within about two weeks before or after it, and that's true even when the due date was set using a first-trimester ultrasound. A due date calculator, no matter which method it uses, is estimating the center of a range, not predicting a specific day.
| Dating Method | Typical Accuracy | Best Used When |
|---|---|---|
| Last menstrual period (Naegele's rule) | Within about 1–2 weeks | Cycles are regular and close to 28 days |
| Conception date | Within about 1–2 weeks | Ovulation or conception date is known closely |
| First-trimester ultrasound | Within about 5–7 days | Performed before roughly 14 weeks |
| Second- or third-trimester ultrasound | Within about 2–3 weeks or more | Used later in pregnancy; less precise for dating |
Think of Your Due Date as a Window, Not a Deadline
Rather than circling one date on the calendar, it helps to think of your due date as the middle of a roughly five-week window. Full-term birth spans 37 to 42 weeks, and babies born anywhere in that range are generally considered on schedule. Doctors typically break that range down further:
| Term Classification | Gestational Age |
|---|---|
| Early term | 37 weeks 0 days – 38 weeks 6 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days |
| Post-term | 42 weeks 0 days and beyond |
Your due date is the useful anchor point your care team uses to schedule checkups, order screening tests, and monitor growth, not a prediction you should plan your hospital bag around too literally.
Understanding Your Pregnancy by Trimester
Once you have an estimated due date, it splits your pregnancy into three roughly equal stages, called trimesters. Knowing which trimester you're in helps you understand what appointments, tests, and changes to expect next.
Early Development
Your baby's major organs and systems begin forming. This is also when a dating ultrasound is most valuable, and when early symptoms like fatigue and nausea are most common.
Growth and Movement
Many early symptoms ease up, and you'll likely feel your baby move for the first time. An anatomy scan around week 20 checks development in detail.
Final Preparation
Your baby gains most of their birth weight, and appointments become more frequent. Labor can begin any time from about week 37 onward and still be full term.
If you're not sure which week you're currently in, entering your last period or due date into 100 Calculator's Pregnancy Due Date Calculator shows your current week and trimester automatically, no manual counting required. For a closer look at what changes week by week, our pregnancy week-by-week guide breaks down each stage of development in more detail.
What to Do After Calculating Your Due Date
Your due date is a starting point, not a finish line. Here's what typically comes next once you have an estimate.
Book Your First Prenatal Appointment
Many providers schedule the first prenatal visit around 8 weeks of pregnancy, though this varies. That first appointment often includes a dating ultrasound, which is exactly why getting an estimated due date early, even a rough one from a calculator, helps you know when to make that call. ACOG considers a pregnancy without an ultrasound confirming or adjusting the due date before about 22 weeks to be less precisely dated, so earlier is generally better.
Track Milestones Beyond the Due Date Itself
Once your due date is set, it becomes the reference point for a lot of what follows: when screening tests are offered, when your second-trimester anatomy scan happens, and when your provider starts monitoring more closely as you approach term. A few tools make that easier to stay on top of between visits:
- 100 Calculator's Pregnancy Weight Gain Calculator helps you see whether your weight gain is tracking within a healthy range for your stage of pregnancy.
- The Nutrition Calculator for Women can help you check whether your diet is meeting increased needs like iron and protein during pregnancy.
- Once your baby arrives, the Baby Growth Memory Calendar is a simple way to log early milestones without needing yet another app.
None of these replace your prenatal appointments, but they're useful ways to stay engaged with your pregnancy between visits.
Common Due Date Calculator Mistakes to Avoid
A few small mix-ups are responsible for most of the "my due date feels wrong" confusion. Here's what to watch for:
- Counting from the wrong day. Day one is the first day of full bleeding, not spotting, and not the day your last period ended.
- Assuming every calculator uses the same method. LMP-based, conception-based, and ultrasound-based estimates can land on slightly different dates, and that's normal, not an error.
- Ignoring cycle length. If your cycles regularly run longer or shorter than 28 days, an unadjusted LMP calculation will drift, sometimes by a week or more.
- Panicking over a due date change after an ultrasound. A shift of a few days to a week after your first dating scan is common and expected, not a sign anything is wrong.
- Treating the due date as a hard deadline. Planning travel, work leave, or a hospital bag around the exact date instead of a two-week window on either side sets you up for unnecessary stress.
Related Calculators
Put what you just read into practice, try these free tools instantly, no sign-up required.
Period & Fertility Tracker
Track your menstrual cycle and predict fertile days with ease.
Pregnancy Due Date Calculator
Estimate your baby's due date based on your last period.
Baby Growth Memory Calendar
Track and celebrate your baby's growth milestones month by month.
Nutrition Calculator for Women
Calculate daily nutrient and calorie needs tailored for women's health.
Pregnancy Weight Gain Calculator
Monitor healthy weight gain throughout each trimester of pregnancy.
Medical disclaimer: This article is for general educational purposes only and isn't a substitute for professional medical advice, diagnosis, or treatment. Due date calculations are estimates, not guarantees, and only your doctor or midwife can confirm your due date and monitor your pregnancy properly. Always talk to a licensed healthcare provider about your specific situation.
More From Women's Health Guides
Still building your knowledge of pregnancy timing, fertility, and cycle tracking? These related guides dig deeper into the topics covered above.
Frequently Asked Questions
How is my pregnancy due date calculated?
Most due date calculators use Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. You can also calculate your due date from your conception date by adding 266 days (38 weeks), or your doctor can estimate it directly from an early ultrasound measurement, which is generally the most accurate method available.
What is Naegele's rule, and how is it used to calculate a due date?
Naegele's rule is a two-century-old formula that estimates your due date from your last menstrual period. To use it, subtract three calendar months from the first day of your LMP, then add seven days and one year. The result is the same as simply adding 280 days to your LMP. It assumes a 28-day cycle with ovulation on day 14, so it works best for people with regular cycles.
Can I calculate my due date from my conception date instead of my LMP?
Yes. If you know roughly when you conceived, add 266 days (38 weeks) to that date instead of using the 280-day LMP calculation. Both methods should land on a similar due date for someone with a typical 28-day cycle, since conception usually happens about two weeks after the last period starts. Conception-based dating can be more precise if you were tracking ovulation closely.
How do I calculate my due date if I have irregular periods?
If your cycles are consistently longer or shorter than 28 days, adjust the standard 280-day estimate by the difference. For example, a 35-day cycle adds about 7 extra days. But if your cycles vary widely and unpredictably, that adjustment isn't reliable. In that case, an early ultrasound, which measures your baby directly rather than relying on cycle assumptions, is a much more accurate way to establish your due date.
What if I don't know the first day of my last menstrual period (LMP)?
If you're not sure when your last period started, or you have irregular cycles, an early ultrasound becomes the most reliable way to date your pregnancy. Your provider can estimate gestational age directly from your baby's measurements instead of relying on a calendar date you can't confirm. It's worth mentioning this uncertainty at your first prenatal visit so your provider can prioritize early dating.
Can an ultrasound change my estimated due date?
Yes. If a first-trimester ultrasound measurement differs from your LMP-based due date by roughly a week or more, your provider will typically update your due date to match the ultrasound, since it's considered more reliable at that stage. This is a normal, expected part of early prenatal care rather than a sign that something is wrong with your pregnancy.
Can my due date change during pregnancy?
Yes, though it's less common than you might think. Your due date is most likely to change early in pregnancy, if a first-trimester ultrasound shows a measurement that differs noticeably from your LMP-based estimate. According to ACOG, once a due date is confirmed with early ultrasound data, later changes are reserved for rare, medically specific situations, not routine adjustments as your pregnancy progresses.
How accurate is a due date calculator?
A due date calculator gives you a reasonable estimate, but not a guarantee. LMP-based calculations are typically accurate within one to two weeks, while a first-trimester ultrasound narrows that down to roughly five to seven days. Even with the most accurate method, only about 4% of babies are born on their exact due date, so it's best treated as the midpoint of a range rather than a fixed appointment.
What percentage of babies are born on their actual due date?
Only about 4% of babies are born on their exact due date, according to Cleveland Clinic. Most arrive within roughly two weeks before or after it, which is why doctors treat the due date as the center of a normal range rather than a firm prediction. A due date that comes and goes without labor starting is common and not usually a cause for concern on its own.
How many weeks pregnant am I based on my due date?
Count backward from your due date: subtract the number of weeks remaining until your due date from 40 to find your current week of pregnancy. For example, if your due date is 12 weeks away, you're currently 28 weeks pregnant. 100 Calculator's Pregnancy Due Date Calculator does this calculation automatically and shows your current week alongside your due date.
Is a due date calculator accurate for twins or multiples?
A standard due date calculator estimates the same way for twins as it does for a single baby, using your LMP, conception date, or ultrasound. However, multiples are statistically more likely to arrive earlier than a singleton pregnancy's due date, so your provider will likely discuss a different expected delivery window based on your specific pregnancy rather than relying on the calculator estimate alone.
What should I do after calculating my due date?
Use your estimated due date to schedule your first prenatal appointment, ideally one that includes a dating ultrasound. From there, your due date becomes the reference point for screening tests, growth monitoring, and deciding when your pregnancy reaches full term. Tools like a pregnancy weight gain calculator or nutrition calculator can also help you track how your pregnancy is progressing between appointments.
Sources & References
- American College of Obstetricians and Gynecologists (ACOG). "Methods for Estimating the Due Date," Committee Opinion No. 700. acog.org
- Cleveland Clinic. "Due Date Calculator." my.clevelandclinic.org
- Mayo Clinic. "Fetal Development: The First Trimester." mayoclinic.org
About 100 Calculator
100 Calculator is a free hub of online calculators and educational guides covering health, finance, trading, crypto, education, and more. Every tool is built to be fast, accurate, and genuinely free, with no account or signup required.
We built this guide, and the Pregnancy Due Date Calculator referenced throughout it, to make one of the first questions every expecting parent asks a little easier to answer. Head back to the 100 Calculator homepage to explore the full library of tools, or learn more about the team behind them on our About Us page.