Due Date Calculator (Naegele's Rule)
Estimate your due date from your last menstrual period using Naegele's rule, adjusted for your actual cycle length and luteal phase so a non-28-day cycle isn't misdated.
Last updated: May 2026
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About this calculator
Naegele's rule is the clinical standard for dating a pregnancy: the estimated due date (EDD) falls 280 days after the first day of the last menstrual period (LMP). That 280 figure is built from two physiological facts — ovulation and conception occur about 14 days after LMP in a textbook 28-day cycle, and gestation from conception lasts about 266 days, so 14 + 266 = 280. The weakness of the textbook rule is that it assumes everyone ovulates on cycle day 14, which is only true for a 28-day cycle with a 14-day luteal phase. Because the luteal phase (ovulation to next period) is relatively fixed at about 12–16 days while the follicular phase varies, the day of ovulation actually equals cycle length minus luteal phase. This calculator therefore dates conception at (cycle length − luteal phase) days after LMP and adds the fixed 266-day post-conception gestation, then subtracts the days already elapsed since LMP to return how many days remain until the due date. Every input matters: a longer cycle pushes ovulation and the due date later, a shorter luteal phase pulls them earlier, and the elapsed-days input converts the static estimate into a live countdown. A negative result simply means the estimated due date has passed (post-term).
How to use
Worked example. It has been 56 days (8 weeks) since the first day of your last period, your cycles average 28 days, and your luteal phase is 14 days. Step 1 — find the day of conception relative to LMP: cycle length − luteal phase = 28 − 14 = 14 days after LMP. Step 2 — add the 266-day post-conception gestation: 14 + 266 = 280 days total from LMP to the due date (the classic Naegele figure). Step 3 — subtract the days already elapsed: 280 − 56 = 224 days until your estimated due date. Now change one input to see the correction in action: if your cycles were 35 days with the same 14-day luteal phase, conception shifts to day 21, total gestation becomes 287 days from LMP, and the days remaining rise to 231 — a full week later than the naive rule would predict. That cycle-length correction is exactly what separates a clinically sound estimate from a calendar-wheel guess.
Frequently asked questions
How accurate is Naegele's rule for calculating a due date?
Naegele's rule is the long-standing clinical starting point, but only about 4–5% of babies arrive on the exact estimated date; most are born within roughly two weeks on either side, and a normal term spans 37–42 weeks. The rule is most accurate for people with regular 28-day cycles and a known LMP. Its biggest source of error is assuming day-14 ovulation, which this calculator corrects using your cycle length and luteal phase. For the most precise dating, a first-trimester ultrasound (crown-rump length) is considered the gold standard and may revise the date.
How do I calculate a due date if my cycle isn't 28 days?
Adjust for when you actually ovulate. Ovulation falls about cycle length minus luteal phase days after your period starts, not a fixed day 14. For a 35-day cycle with a 14-day luteal phase, that's day 21 — a week later than textbook. Because conception is later, the due date is later too. This calculator builds that correction in: it dates conception at (cycle length − luteal phase) after your LMP, adds the 266-day post-conception gestation, and subtracts days elapsed. Enter your real cycle length and luteal phase rather than assuming 28 and 14.
What is the difference between gestational age and conception age?
Gestational age is counted from the first day of your last menstrual period (LMP), so it includes the roughly two weeks before you actually conceived. Conception (or fetal) age is counted from ovulation/fertilization and is about two weeks less. A pregnancy said to be '8 weeks gestational' is about 6 weeks past conception. Clinicians use gestational age because the LMP is a date most people can identify, whereas the exact conception day usually isn't known. A full-term pregnancy is 40 weeks gestational age, or about 38 weeks from conception.