Due Date Calculator – Pregnancy Week & Estimated Due Date

An estimated due date anchors every pregnancy milestone — scans, screenings, maternity leave, the nursery deadline that suddenly feels very real. This calculator uses Naegele’s rule, the standard method midwives have used for generations: add 280 days (40 weeks) to the first day of your last menstrual period. Alongside the due date, you get how far along you are today, which trimester you are in, and the key weeks ahead. It runs privately in your browser; no dates are stored anywhere. Due dates organise an entire support system when they are shared early: employers plan cover, families plan travel, and antenatal appointments, screening windows and vaccination timings all hang off the number. It also helps to know what the number is not — it is not a deadline your baby agreed to, and first babies in particular arrive on their own schedule. Keep the date, hold it loosely, and let your care team refine it as measurements come in.

Quick Answer: Enter the first day of your last period. Your estimated due date (LMP + 280 days), current pregnancy week and trimester appear instantly. Only about 1 in 20 babies arrive exactly on the day.

Free Online Tool

Pregnancy Due Date Calculator

First day of last period
Cycle length (days)

—Estimated due date

—You are now

—Trimester

How to Use the Pregnancy Due Date Calculator

Enter the first day of your last menstrual period (LMP).
Optionally adjust the cycle length if yours differs from 28 days.
Read your estimated due date.
See how many weeks and days pregnant you are today, and your trimester.
Note the milestone weeks: first scan window, anatomy scan and full term.

Key Features

Naegele’s rule — the standard clinical method
Adjustable cycle length for more accurate dating
Current week, day and trimester calculated for today
Key milestone weeks listed with dates
Private: dates never leave your browser

Best Practices and Tips

If you know your conception date, subtract 14 days to approximate the LMP this calculator expects (for a 28-day cycle).
A first-trimester dating scan is more precise than LMP maths when cycles are irregular — your provider may adjust the date.
“Full term” spans weeks 39–40; babies arriving between 37 and 42 weeks are considered within the normal window.
Use the due date for planning ranges, not a single deadline — only around 5% of babies arrive on the exact day.
Keep a note of this date for forms; you will be asked for it constantly.
If your cycles are longer or shorter than 28 days, mention it at your first appointment — the dating scan will adjust the estimate, and the adjusted date is the one every later decision uses.

Common Mistakes to Avoid

Entering the last day of your period instead of the first day.
Assuming a 28-day cycle when yours is longer or shorter, which shifts the estimate by days.
Treating the estimated date as a guarantee and over-planning around one day.
Forgetting that IVF pregnancies date from transfer, not LMP — ask your clinic for the equivalent.

Frequently Asked Questions

How is the due date calculated?

By Naegele’s rule: take the first day of your last period, add one year, subtract three months and add seven days — equivalent to LMP + 280 days for a 28-day cycle. This calculator adjusts for other cycle lengths.

How accurate is the estimate?

It is a statistical estimate: about 80% of babies arrive within two weeks either side, and roughly 5% on the exact date. Dating scans refine it further.

What if I do not know my LMP?

Use the first day of your best-guess period, or ask your provider — an early ultrasound can date the pregnancy without it.

Which week am I in right now?

The calculator counts completed weeks and days from your LMP to today and shows your trimester (1: weeks 1–13, 2: 14–27, 3: 28–birth).

Is this calculator free?

Yes — free, private and no sign-up required. It provides estimates, not medical advice.

What if I do not know my last period date?

An early ultrasound measures the pregnancy directly and sets the date — this is routine and often more accurate than recalled dates, so do not worry about the gap.