Free Due Date Calculator
Estimate your pregnancy due date based on LMP, conception, or IVF transfer date.
Key Milestones
A due date calculator is a free online tool that estimates when your baby will be born based on the first day of your last menstrual period (LMP) or your conception date. Enter your dates and cycle length to get your estimated due date, current pregnancy week and trimester, and key milestone dates — instantly, with no signup required.
What makes this tool different: Pregnancy due date calculators typically only support the LMP method. CalcInstant adds conception date mode, IVF 5-day blastocyst transfer calculation, fertility window estimation, trimester progress tracking with a visual bar, and the baby's Chinese zodiac — giving expecting parents more insight than any other free tool.
How the due date is calculated
Due date calculation follows Naegele's rule, the standard method used by obstetricians for over a century. A full-term pregnancy lasts approximately 280 days — 40 weeks — from the first day of your last menstrual period. This assumes a standard 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator adjusts by adding or subtracting the difference from 28 days.
When calculating from conception date instead, the formula uses 266 days (38 weeks) from fertilisation. Conception typically occurs within 24 hours of ovulation, which happens about 14 days before your next expected period. The due date from conception is independent of your cycle length. Both methods provide an estimate within a week or two of accuracy, which is why your healthcare provider will typically confirm or adjust the date based on a first-trimester ultrasound.
The calculator also displays your current pregnancy week and day based on today's date relative to your LMP or conception date. This is updated automatically whenever you visit the page, so you can track your progress throughout the entire pregnancy without re-entering your information. The visual progress bar shows at a glance how far along you are in the 40-week journey.
When to use a due date calculator
Use a due date calculator as soon as you suspect pregnancy to get an initial estimate before your first ultrasound. It is also useful for pregnancy planning, tracking your current week and trimester, preparing for milestones such as the 12-week scan or the 20-week anatomy scan, and planning maternity leave or birth preparations. Remember that the estimated due date is a guide — only about 5% of births occur exactly on the due date, and full-term deliveries between 37 and 42 weeks are considered normal.
The due date calculator on CalcInstant automatically saves your dates in your browser, so you can come back throughout your pregnancy to check your current week, trimester, and days remaining without re-entering any information. This makes it a convenient tool for expectant parents who want to track their pregnancy progress across multiple visits, share milestone dates with family members, or plan ahead for each stage of prenatal care. The milestones section shows your 12-week and 20-week dates — key points when important prenatal screenings and anatomy scans typically occur.
Stages of pregnancy
Pregnancy is divided into three trimesters. The first trimester (weeks 1–13) covers early development of the embryo, including the formation of the heart, brain, and major organs. During this period, many women experience morning sickness, fatigue, and hormonal changes as the body adapts to pregnancy. The second trimester (weeks 14–27) is when the baby grows rapidly and movements become noticeable. This is often when the "baby bump" becomes visible and energy levels return. The third trimester (weeks 28–40) involves further growth and preparation for birth, including Braxton Hicks contractions, increased fetal movement, and the baby settling into a head-down position.
The calculator shows your current trimester alongside your due date to help you track where you are in your pregnancy journey. It also calculates the number of days until your due date and displays your progress as a percentage of the full 40-week term. This information is helpful for planning prenatal appointments, childbirth education classes, hospital pre-registration, and arranging maternity or paternity leave from work. Knowing your exact week and day also helps you understand which developmental milestones your baby is reaching at each stage, since fetal development is typically described in terms of pregnancy weeks.
Cycle length and due date accuracy
The standard 28-day cycle is used as the default in most due date calculations, but many women have cycles that are shorter or longer. A 35-day cycle, for example, means ovulation typically occurs around day 21 instead of day 14, which would push the due date approximately one week later. The due date calculator on CalcInstant allows you to adjust the cycle length anywhere from 20 to 45 days, giving a more personalized estimate for women with irregular or non-standard cycles. If you are unsure of your average cycle length, a range of 28 to 30 days is a reasonable starting point that can be refined later by your healthcare provider based on ultrasound measurements.
Frequently asked questions
How do I calculate my pregnancy due date from my last period?
Add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is Naegele's Rule — the standard method used by OB-GYNs worldwide. For example, if your LMP was January 1, your estimated due date is October 8. This assumes a 28-day cycle with ovulation on day 14. Enter your LMP date above for an instant calculation.
How accurate are due date calculators?
Only 4–5% of babies are born on their exact due date. About 80% of births occur within 2 weeks before or after the EDD. Due dates calculated from LMP assume a 28-day cycle — women with longer or shorter cycles may have earlier or later actual conception dates. An early ultrasound (before 13 weeks) is the most accurate dating method, within 5–7 days.
How many weeks pregnant am I?
Count the weeks from the first day of your last menstrual period (LMP) to today's date. Pregnancy is measured in weeks from LMP, not from conception. If your LMP was 10 weeks ago, you are 10 weeks pregnant. Enter your LMP date in the calculator to see your exact gestational age in weeks and days, plus your trimester and key milestone dates.
What is the Naegele's Rule for calculating due dates?
Naegele's Rule: add 7 days to the first day of your LMP, subtract 3 months, and add 1 year. LMP = March 10: add 7 days = March 17, subtract 3 months = December 17, add 1 year = December 17 of the following year. This equals adding 280 days to the LMP. It was established in 1812 and remains the universal standard despite its simplicity.
What are the key pregnancy milestones by week?
Week 12: end of first trimester, miscarriage risk drops significantly. Week 13: second trimester begins. Week 20: anatomy ultrasound, typically when sex can be determined. Week 24: viability threshold. Week 28: third trimester begins. Week 37: considered full term. Week 40: estimated due date. Week 42: typically when induction is considered if labor hasn't started.
Can I calculate my due date if I know my conception date?
Yes — add 266 days (38 weeks) to your conception date. Conception typically occurs 14 days after LMP in a 28-day cycle, which is why LMP-based calculations add 280 days (14 + 266). If you know your exact conception date from IVF or other tracking, the calculator above accepts conception date as the starting point for a more precise estimate.