How is the due date calculated from the last menstrual period?

The calculator applies Naegele’s rule: it adds 280 days (40 weeks) to the first day of your last menstrual period. If your average cycle is longer or shorter than 28 days, the estimate is shifted by the difference, since ovulation typically occurs about 14 days before the next period rather than a fixed number of days after the last one.

How accurate is a due date calculated this way?

Only about 4-5% of babies arrive exactly on their due date; most are born within two weeks either side of it. Naegele’s rule assumes regular cycles and a known period date, so an early ultrasound (before 14 weeks) usually gives a more precise estimate. Treat the result as a planning window, not a fixed appointment.

What if I know my conception date instead of my period date?

Switch the toggle to conception date and the calculator adds 266 days (38 weeks) instead, which is the average length of pregnancy from fertilization. This is useful after IVF or if you tracked ovulation, and it removes the cycle-length uncertainty entirely, so no cycle adjustment is applied in that mode.

How are gestational age and trimesters counted?

Gestational age is counted in completed weeks plus days from the start of your last period, adjusted for cycle length — so at 6 weeks 3 days you are in your seventh week. The first trimester runs through 13 weeks 6 days, the second through 27 weeks 6 days, and the third from 28 weeks until birth.

Is my information private when I use this calculator?

Yes. All calculations run entirely in your browser using JavaScript; the dates you enter are never uploaded, stored, or sent to any server. You can even load the page, disconnect from the internet, and the calculator will keep working normally.

Pregnancy Due Date Calculator

Enter your last menstrual period or conception date to estimate your due date, current week of pregnancy, and key milestones.

First day of your last menstrual period

Options

Average cycle length (days)

Calculated on your device — nothing is sent anywhere.

How it works

Enter the first day of your last menstrual period, or switch to conception date if you know it.
Open Options to adjust your average cycle length if it differs from 28 days.
Read your estimated due date, current gestational age, trimester, and milestone dates — they update instantly.

Frequently asked questions

How is the due date calculated from the last menstrual period?: The calculator applies Naegele’s rule: it adds 280 days (40 weeks) to the first day of your last menstrual period. If your average cycle is longer or shorter than 28 days, the estimate is shifted by the difference, since ovulation typically occurs about 14 days before the next period rather than a fixed number of days after the last one.
How accurate is a due date calculated this way?: Only about 4-5% of babies arrive exactly on their due date; most are born within two weeks either side of it. Naegele’s rule assumes regular cycles and a known period date, so an early ultrasound (before 14 weeks) usually gives a more precise estimate. Treat the result as a planning window, not a fixed appointment.
What if I know my conception date instead of my period date?: Switch the toggle to conception date and the calculator adds 266 days (38 weeks) instead, which is the average length of pregnancy from fertilization. This is useful after IVF or if you tracked ovulation, and it removes the cycle-length uncertainty entirely, so no cycle adjustment is applied in that mode.
How are gestational age and trimesters counted?: Gestational age is counted in completed weeks plus days from the start of your last period, adjusted for cycle length — so at 6 weeks 3 days you are in your seventh week. The first trimester runs through 13 weeks 6 days, the second through 27 weeks 6 days, and the third from 28 weeks until birth.
Is my information private when I use this calculator?: Yes. All calculations run entirely in your browser using JavaScript; the dates you enter are never uploaded, stored, or sent to any server. You can even load the page, disconnect from the internet, and the calculator will keep working normally.

About this tool

This pregnancy due date calculator estimates when your baby is likely to arrive from either the first day of your last menstrual period (LMP) or a known conception date. Alongside the due date it shows your current gestational age in weeks and days, which trimester you are in, and a timeline of key milestone dates — the end of the first trimester, the typical anatomy scan window, the start of the third trimester, and full term. Everything recalculates live as you change the inputs.

The estimate follows Naegele’s rule, the same method most clinicians use as a first pass: due date = LMP + 280 days. Because that rule assumes a 28-day cycle with ovulation on day 14, the calculator lets you set your average cycle length under Options and shifts the whole timeline by the difference — a 32-day cycle, for example, moves the due date four days later. In conception-date mode it adds 266 days directly, which suits IVF transfers and tracked ovulation. All of this runs client-side in your browser, so the dates you enter never leave your device.

Typical uses include a quick check after a positive home test before your first appointment, sanity-checking the date a clinic gave you, planning maternity leave around the 40-week mark, and knowing when routine events fall — such as the 18-22 week ultrasound or the 28-week point when many providers increase visit frequency.

A practical tip: write down the exact first day of your period as soon as you suspect pregnancy, since recall gets harder over time and a one-week error in the LMP shifts every estimate by a week. If your cycles are irregular or you are unsure of the date, mention it at your first visit — dating ultrasounds in the first trimester are accurate to within about five to seven days and will take precedence over any calendar-based estimate.

Pregnancy Due Date Calculator

How it works

Frequently asked questions

About this tool

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