What is Naegele’s rule?

Naegele’s rule is a simple formula for estimating the due date: add 280 days (40 weeks) to the first day of the last menstrual period. The rule was developed in 1806 by the German obstetrician Franz Karl Naegele. An equivalent version: LMP + 1 year − 3 months + 7 days (gives the same result). The rule assumes a 28-day menstrual cycle with ovulation on day 14 and full term at 40 weeks. Adjusted formulas exist for people with different cycles, IVF treatment or a known conception date.

How accurate is the due date?

Only about 4% of births happen exactly on the due date. Around 50% occur within ± 1 week and over 80% within ± 2 weeks. First-trimester ultrasound (weeks 8–13) is more accurate than Naegele’s rule and is what your midwife/doctor uses for the official due date. First-time mothers more often deliver slightly after term (median around 5 days late), later births slightly before. The rule is useful as a first estimate before ultrasound dating, but should not drive decisions about induction or caesarean.

What if I don’t remember LMP or have irregular cycles?

Enter the best date estimate you have, i.e. the first day of the last period you remember, and adjust for cycle length: for a long cycle (over 28 days) shift the due date later by the extra days; for a short cycle shift earlier. If you have no date estimate, first-trimester ultrasound is the best method: crown-rump length in weeks 8–12 gives a due date accurate to ± 3–5 days. In IVF/fertility treatment the conception date is known exactly and the due date is conception + 266 days (equivalent to LMP + 280 days for a 28-day cycle).

Does the tool tell me when parental leave should start?

No, this tool only computes the due date and gestational week. For Norwegian parental leave, see our foreldrepenger calculator: the 100% option gives 49 weeks including 3 weeks before term (mother’s quota 15 + father’s quota 15 + shared 16). NAV also pays pregnancy benefits if you must stop working earlier for health reasons. Sweden, Denmark, Germany and other countries have different rules; check the relevant agency (Försäkringskassan, Udbetaling Danmark, Elterngeldstelle, etc.). The tool helps with date maths, not statutory rules.

Pregnancy Due Date Calculator (Naegele)

Due date
Gestational week now
Estimated conception

How it works

The due date is calculated with Naegele's rule: 280 days (40 weeks) from the first day of the last period. Estimated conception is set about two weeks after that. The tool also shows your current gestational week. Everything is calculated locally in your browser.

This is an estimate. Few births happen exactly on the due date, and ultrasound often gives a more precise date. Talk to your midwife or doctor.

About this tool

The due date calculator estimates the expected birth date with Naegele’s rule: 280 days (40 weeks) from the first day of the last menstrual period. It also shows the estimated date of conception (about two weeks after the start of the period) and your current gestational week in the format "week 24 + 3" (24 completed weeks and 3 extra days). Everything is calculated locally in the browser, no data leaves your device. Useful for pregnant people, partners, midwives, HR departments planning parental leave, and caregivers estimating days to the due date.

How to use it

Enter or pick the date of the first day of your last menstrual period (LMP).
Read the due date shown with full weekday and date, e.g. "Saturday 12 April 2026".
Check the current gestational week in the format "week X + Y" (completed weeks and extra days).
The estimated conception date is shown as a reference, about 14 days after LMP.

Examples

Standard 28-day cycle

InputLMP: 5 July 2026

OutputDue date: 11 April 2027. Estimated conception: 19 July 2026.

The rule assumes a 28-day cycle with ovulation on day 14. With a shorter or longer cycle, the actual due date can shift by a week in either direction.

Check what week you are in

InputLMP: 1 January 2026, today's date: 5 July 2026

Output185 days since LMP = week 26 + 3 (end of the second trimester)

"Week 26 + 3" reads as "in the 27th week, day 3". Trimester boundaries fall roughly at week 13 (end of first trimester) and week 27 (end of second trimester).

Common use cases

Parents wanting to know the expected birth date early in pregnancy before the first ultrasound.
Midwives and GPs cross-checking against ultrasound dating and gestational age.
HR departments planning parental leave around the expected due date.
Partners and family counting down weeks and days to birth.
Preschools and schools checking which nursery year or school year the child will enter.

Frequently asked questions

What is Naegele’s rule?: Naegele’s rule is a simple formula for estimating the due date: add 280 days (40 weeks) to the first day of the last menstrual period. The rule was developed in 1806 by the German obstetrician Franz Karl Naegele. An equivalent version: LMP + 1 year − 3 months + 7 days (gives the same result). The rule assumes a 28-day menstrual cycle with ovulation on day 14 and full term at 40 weeks. Adjusted formulas exist for people with different cycles, IVF treatment or a known conception date.
How accurate is the due date?: Only about 4% of births happen exactly on the due date. Around 50% occur within ± 1 week and over 80% within ± 2 weeks. First-trimester ultrasound (weeks 8–13) is more accurate than Naegele’s rule and is what your midwife/doctor uses for the official due date. First-time mothers more often deliver slightly after term (median around 5 days late), later births slightly before. The rule is useful as a first estimate before ultrasound dating, but should not drive decisions about induction or caesarean.
What if I don’t remember LMP or have irregular cycles?: Enter the best date estimate you have, i.e. the first day of the last period you remember, and adjust for cycle length: for a long cycle (over 28 days) shift the due date later by the extra days; for a short cycle shift earlier. If you have no date estimate, first-trimester ultrasound is the best method: crown-rump length in weeks 8–12 gives a due date accurate to ± 3–5 days. In IVF/fertility treatment the conception date is known exactly and the due date is conception + 266 days (equivalent to LMP + 280 days for a 28-day cycle).
Does the tool tell me when parental leave should start?: No, this tool only computes the due date and gestational week. For Norwegian parental leave, see our foreldrepenger calculator: the 100% option gives 49 weeks including 3 weeks before term (mother’s quota 15 + father’s quota 15 + shared 16). NAV also pays pregnancy benefits if you must stop working earlier for health reasons. Sweden, Denmark, Germany and other countries have different rules; check the relevant agency (Försäkringskassan, Udbetaling Danmark, Elterngeldstelle, etc.). The tool helps with date maths, not statutory rules.

Technical background

The computation uses Naegele’s rule with whole-day precision: due date = LMP + 280 days, estimated conception = LMP + 14 days. LMP is parsed as local midnight (`new Date(v + "T00:00:00")`). Dates are formatted via `Intl.DateTimeFormat(locale, {dateStyle: "full"})` for each supported language (nb-NO, sv-SE, da-DK, de-DE, fr-FR, es-ES, en-GB). Gestational week uses `diff = floor((today − LMP) / 86400000)` milliseconds, then `weeks = floor(diff / 7)`, `days = diff mod 7`, with validity range 0–315 days (up to week 45 as a safety margin). Outside the range "-" is shown. Everything runs client-side and no dates are stored or sent to a server.