A pregnancy week calculator is the fastest way to find out exactly how far along you are, without doing the date math yourself or waiting for your next prenatal appointment. Type in one date β either the first day of your last menstrual period or your due date β and the calculator instantly tells you your current gestational age in weeks and days, which trimester you're in, how many days are left until your due date, and how much of the pregnancy you've completed as a percentage.
Arb Digital built this tool as part of a small collection of free pregnancy and health calculators because so many expecting parents end up doing this same arithmetic by hand, on scraps of paper or in a phone notes app, and it's easy to get wrong. The calculator below does it in a fraction of a second and updates the moment you change the date.
What This Pregnancy Week Calculator Does
The tool takes a single reference date and converts it into a full gestational-age snapshot. If you enter the first day of your last menstrual period (LMP), it adds up the days that have passed since then and expresses them as completed weeks plus extra days β the same "weeks + days" format your doctor or midwife uses on every visit. If you instead enter your due date, the calculator works backward, since a standard due date is always exactly 280 days (40 weeks) after the LMP, and it figures out where "today" falls on that same 280-day timeline.
Alongside the headline weeks-and-days figure, it shows which trimester you're currently in, the number of days remaining until your estimated due date, the due date itself (calculated automatically if you entered an LMP), and a percentage that shows how much of the standard 40-week pregnancy has elapsed. None of this requires you to count on your fingers or scroll through a calendar app β one date in, five numbers out.
How to Use It
- Pick your starting point. Choose "Last menstrual period (LMP) date" if you know the first day of your last period, or "Due date" if your provider has already given you one (from an early ultrasound, for example).
- Enter the date. Use the date field to select the exact day. If you're not 100% sure of your LMP date, use your best estimate β the calculator can only be as accurate as the date you give it.
- Read your results. The big number at the top shows exactly how many weeks and days pregnant you are today. The grid below breaks that down further into trimester, days remaining, due date, and percentage complete.
- Recalculate any time. Bookmark this page and come back weekly β or whenever your provider gives you an updated due date after an ultrasound β to see your progress shift in real time.
The Formula / How It's Calculated
Pregnancy dating uses a deceptively simple rule called Naegele's rule: a full-term pregnancy is estimated at 280 days, or exactly 40 weeks, counted from the first day of the last menstrual period β not from the day of conception. So if you give the calculator your LMP date, it simply counts the number of days between that date and today, divides by seven to get completed weeks, and keeps the remainder as extra days. If you give it a due date instead, it subtracts 280 days to back-calculate an equivalent LMP, then runs the same math. This is the identical method used by obstetric offices and hospitals worldwide, and it's described in detail by the American College of Obstetricians and Gynecologists (ACOG).
Why "6 Weeks Pregnant" Confuses Almost Everyone
Here's the part that trips up nearly every first-time parent: pregnancy weeks start counting from your last period, which is roughly two weeks before you actually ovulated and conceived. That means when the calculator says you're "6 weeks pregnant," your baby has really only existed, biologically speaking, for about four weeks since fertilization. It sounds like a technicality, but it explains a lot of confusion people run into β why an early pregnancy test can show positive at what feels like "4 weeks" when conception only happened two weeks earlier, or why a due date feels like it's counting time that hasn't happened yet.
The reason the medical world dates pregnancy this way instead of from conception is practical: almost nobody knows the exact day they conceived, but most people can identify the first day of their last period reasonably well. LMP dating gives doctors a consistent, repeatable starting point for every patient, even though it technically overstates true fetal age by about two weeks in a standard 28-day cycle. If you want to estimate the actual conception date rather than the LMP-based pregnancy week, our conception date calculator handles that calculation directly.
The Three Trimesters, Explained
Pregnancy is traditionally divided into three trimesters, and this calculator tells you which one you're in based on your current week:
- First trimester (weeks 1β13): This is when the foundational structures of the baby's body form β the neural tube, heart, limbs, and major organs all begin developing. It's also when morning sickness, fatigue, and heightened sense of smell are most common, and when the risk of early pregnancy loss is highest, which is why many people wait until the end of this trimester to share the news.
- Second trimester (weeks 14β27): Often called the "golden trimester" because early symptoms typically ease up while energy returns. This is when most people learn the baby's sex via ultrasound, start to feel fetal movement (usually between weeks 16β25), and undergo the mid-pregnancy anatomy scan that checks organ development in detail.
- Third trimester (weeks 28β40): The baby gains most of its birth weight during this stretch, lungs and brain continue maturing rapidly, and prenatal visits become more frequent β often weekly by the final month. This trimester ends with labor, whenever it naturally begins.
What "Full Term" Actually Means
A pregnancy isn't considered a single fixed length β there's a defined window. Babies born between 37 and 42 weeks are considered full term, with more precise sub-categories used by providers: "early term" (37β38 weeks 6 days), "full term" (39β40 weeks 6 days), "late term" (41β41 weeks 6 days), and "postterm" (42 weeks and beyond). Only around 5% of babies actually arrive on their exact calculated due date β most arrive somewhere in that broader full-term window, which is completely normal. The CDC tracks preterm birth (before 37 weeks) as a distinct category because babies born earlier than that often need extra medical support, particularly for lung development.
Arb Digital builds fast, high-converting websites and useful free content β this pregnancy calculator is one of dozens of free tools we maintain. Check out our conception date and due date calculators next.
Try the Due Date Calculator All Free ToolsCommon Mistakes to Avoid
- Confusing LMP-based weeks with weeks since conception. They're offset by roughly two weeks β don't be alarmed if a fertility app and a pregnancy app show different numbers for the "same" pregnancy.
- Treating irregular cycles as if they were exactly 28 days. If your cycles run longer or shorter than average, your actual ovulation date shifts, and an early ultrasound is more reliable than LMP math for dating the pregnancy.
- Assuming the due date is a deadline. It's a statistical midpoint of a normal range, not a guarantee β full term spans a full five weeks (37 to 42).
- Forgetting to update the calculation after an ultrasound. If your provider revises your due date based on an early scan, re-enter that new due date here rather than your original LMP guess.
- Ignoring the difference between "weeks pregnant" and "week of pregnancy." Being "in your 6th week" and being "6 weeks pregnant" mean the same thing, but people sometimes read baby-development articles a week ahead of where they actually are.
Related Free Tools From Arb Digital
If you found this useful, try the due date calculator to double-check your estimated delivery date, the conception date calculator to estimate when fertilization actually happened, or the ovulation calculator if you're still trying to conceive. Managing gestational diabetes or blood sugar during pregnancy? Our blood sugar converter and A1C calculator can help you make sense of lab results. Browse our full free online tools hub for more.
Frequently Asked Questions
It's as accurate as the date you enter. LMP-based dating is standard practice and generally reliable within a few days for people with regular 28-day cycles, but an early ultrasound (before 14 weeks) is considered the most accurate dating method overall, especially for irregular cycles.
Small differences usually come down to which reference date was used β LMP versus an ultrasound-adjusted due date. If your provider adjusted your due date after a scan, use that adjusted due date in this calculator instead of your original LMP guess for a matching result.
The second trimester generally begins at week 14 and runs through week 27, though some sources round it to the start of month four. This calculator uses the 14β27 week range, which is the most commonly cited standard.
No. Forty weeks is the average and the basis for calculating the due date, but full term officially spans 37 to 42 weeks. Only a small percentage of babies are born exactly on their calculated due date.
Yes, use your best estimate, but understand the result will only be approximate. If you have an ultrasound-based due date from your provider, switching the calculator to "Due date" mode will generally give a more accurate current week.
Because pregnancy weeks are counted from the first day of your last period, not from conception, which typically happens about two weeks later. This is a standard medical convention, not an error.
This tool provides general estimates for educational purposes only and is not medical advice. Every pregnancy and body is different β confirm anything important with your doctor.
