🤱 Due Date Calculator

What is Due Date Calculator?

Expecting parents often want a quick estimate of when their baby is likely to arrive, and this calculator gives you exactly that using the same method doctors have relied on for over a century. Rather than needing an ultrasound on day one, you can get a preliminary due date from a single piece of information you already know — the first day of your last menstrual period.

Naegele's Rule Formula

The calculator adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), which assumes a fairly standard 28-day cycle with ovulation around day 14:

Worked Example

If your last menstrual period began on 1 January 2026:

That gives an estimated delivery window centred around early October, roughly nine calendar months and one week after the LMP date.

How to Use This Calculator

1. Select the first day of your last menstrual period using the date field.
2. Click Calculate.
3. View your estimated due date instantly.

Things to Keep in Mind

Naegele's rule is a useful starting estimate, but it assumes a textbook-perfect 28-day cycle — actual delivery dates commonly fall anywhere from about two weeks before to two weeks after this calculated date, and fewer than 5% of babies arrive on the exact predicted day. Irregular cycles, PCOS, or uncertainty about your LMP date can shift the accuracy of this estimate further. Always confirm your due date with a gynaecologist through an early ultrasound, which dates the pregnancy using foetal measurements rather than menstrual history alone.

Trying to plan around ovulation instead? The Ovulation Calculator covers that from the same LMP starting point.

Limitations to Keep in Mind

Naegele's rule, the formula behind this calculator, assumes every pregnancy follows a textbook 28-day cycle with ovulation on day 14 and a 40-week (280-day) gestation from the last menstrual period — in reality, cycle lengths vary widely between individuals and even month to month for the same person, so this estimate can be off by a week or more in either direction. It is not accurate for pregnancies conceived through IVF or other fertility treatments (which are dated from the actual transfer or conception date, not LMP), for people with irregular cycles or uncertain LMP dates, or for anyone who was on hormonal contraception shortly before conceiving, which can delay ovulation. It also doesn't account for multiple pregnancies (twins or more), which frequently arrive earlier than singleton pregnancies. This calculator is meant purely as an early, rough estimate — your doctor's dating ultrasound, performed ideally in the first trimester, remains the most accurate method for confirming your actual due date.

Frequently Asked Questions

Q: My doctor's ultrasound gave a different due date than this calculator — which one should I trust?
A: Trust the ultrasound date, especially if it was done in the first trimester. Early ultrasounds measure the foetus directly and are considered more accurate than LMP-based calculations like Naegele's rule, particularly if your cycles are irregular or you're unsure of your exact LMP date.

Q: What percentage of babies are actually born on their calculated due date?
A: Very few — commonly cited estimates suggest fewer than 5% of babies arrive on their exact due date. Most healthy deliveries happen anywhere within about two weeks before to two weeks after this date, which is why doctors describe it as an estimated date rather than a guaranteed one.

Q: How is the due date calculated differently for an IVF pregnancy?
A: IVF due dates are typically calculated from the known date of egg retrieval or embryo transfer rather than the last menstrual period, since the exact conception timing is already known. This tends to make IVF due date estimates somewhat more precise than LMP-based calculations like the one used here.

Q: Does carrying twins or multiples change the expected due date?
A: Yes — pregnancies with twins or higher-order multiples commonly deliver earlier than singleton pregnancies, often several weeks before the standard 40-week mark. Naegele's rule doesn't adjust for multiples, so if you're expecting more than one baby, rely on your obstetrician's guidance rather than this general calculation.

Q: Should I make travel, work, or medical plans based solely on this calculated due date?
A: It's best treated as a rough planning reference, not a fixed appointment. Because actual delivery dates vary considerably, confirm timing-sensitive decisions (like maternity leave dates or travel plans) with your obstetrician as your pregnancy progresses, and always seek medical care promptly for any concerning symptoms regardless of how far you are from this estimated date.

Q: How is maternity leave typically calculated around this due date in India?
A: Under India's Maternity Benefit Act, eligible women can generally take up to 26 weeks of paid leave, usually structured as a period before and after the expected delivery date. Employers commonly ask for the estimated due date from a doctor to plan this leave window, so confirming it early with your obstetrician helps with workplace planning alongside the personal and medical side of pregnancy.

More Worked Examples

Applying Naegele's rule to a few different LMP dates shows how the 280-day addition plays out across the calendar:

• LMP = 10 March 2026: Due Date = 10 March 2026 + 280 days = 14 December 2026
• LMP = 5 June 2026: Due Date = 5 June 2026 + 280 days = 10 March 2027
• LMP = 22 September 2026: Due Date = 22 September 2026 + 280 days = 28 June 2027

Trimester Breakdown from Your Due Date

Once you have an estimated due date, pregnancy is commonly divided into three roughly 13-week trimesters, each associated with different prenatal checkups and screening tests:

Counting backward from your estimated due date using these week ranges gives a rough sense of which trimester you're in at any point, though your doctor will confirm exact timing using ultrasound-based dating rather than LMP alone.

📅 Last reviewed: July 2026 · Formulas verified against RBI/SEBI/IT Dept guidelines.