How To Calculate Due Date From LMP: Naegele's Rule

Learning how to calculate due date from lmp is one of the first things many expectant parents want to do after a positive test, because a clear due date helps with planning antenatal visits, work leave, and the arrival of the baby. The date you are given is called the estimated due date, and it is worked out from the first day of your last menstrual period rather than from the day

you conceived, which is often harder to pinpoint. To check your own date in seconds, use our free OB Wheel Calculator. This guide explains the rule in plain language, shows each step with a worked example, and flags the mistakes that most often shift the date by a week or more.Remember that the result is an estimate, not a promise that labour will start on that day. Only a small share of babies arrive exactly on their due date, so treat the date as the centre of a window of a few weeks and confirm it at your first antenatal appointment, especially if an early scan gives a different date.

The Formula

Naegele’s rule gives a simple way to turn the first day of your last period into an estimated due date. The formula used by the tool is: EDD = LMP + 280 days + (cycle length – 28 days). In plain words, add 40 weeks to the LMP, then adjust for a cycle that is shorter or longer than the usual 28 days.

Each term has a clear job. LMP is the first day of your last menstrual period, not the last day and not the day you think conception happened. The 280 days stand for a full-term pregnancy of 40 weeks counted from that first day, which is why the count starts earlier than many people expect. Cycle length is your usual number of days from the first day of one period to the first day of the next. If your cycle is 28 days, the adjustment is zero. If it is longer, the extra days are added because ovulation probably happened later; if it is shorter, days are subtracted. Gestational age follows the same count: it is simply how many days and weeks have passed since the LMP.

How to Calculate Your Due Date Step by Step

Find the first day of your last menstrual period. Use a calendar or period app, and check that you have the first day, not a day in the middle of bleeding.
Work out your usual cycle length. Count from the first day of one period to the first day of the next, over a few months if you can, and use the typical length rather than one unusual month.
Add 280 days to the LMP. The easiest way is to add 40 weeks on a calendar, moving forward week by week, or to use the tool and check the date it returns.
Adjust for cycle length. Subtract 28 from your usual cycle length, then add that number of days to the date you found in step 3. A 32-day cycle adds 4 days; a 26-day cycle subtracts 2 days.
Check the result and note the gestational age too. The final date is your estimated due date, and the weeks since the LMP tell you how far along you are on that date.

Worked Example

Take an LMP of 1 January 2026 and a usual cycle length of 28 days, the same values used in the tool page example. First, apply the main part of the formula: 1 January 2026 plus 280 days gives 8 October 2026. Next, work out the cycle adjustment: 28 minus 28 equals 0 days, so nothing is added or subtracted. The estimated due date is therefore 8 October 2026.

If the cycle had been 35 days instead, the adjustment would be 35 minus 28, or 7 days, giving 15 October 2026. That small change shows why the cycle length matters: using the standard 28 days for every person would move some due dates by a full week.

Common Mistakes / Good to Know

Using the conception date as the LMP. Counting from conception instead of from the first day of the last period usually makes the date about two weeks too late.
Ignoring cycle length. The tool page notes that a longer cycle usually means later ovulation, so the extra days must be added; skipping this step is the most common reason for a date that is a week out.
Treating the estimate as exact. The tool page stresses that only about 5 percent of babies arrive on the exact due date, so plan around a window, not a single day.
Always confirm with a clinician. This method does not replace medical advice, and an early ultrasound can give a more reliable date, especially if cycles are irregular or the LMP is uncertain.

Frequently Asked Questions

What is Naegele’s rule in simple terms?

Naegele’s rule estimates the due date by adding 280 days, or 40 weeks, to the first day of the last menstrual period, then adjusting for a cycle length different from 28 days.

How does a longer or shorter cycle change the due date?

Add the difference between your cycle length and 28 days. A 35-day cycle adds 7 days, while a 26-day cycle subtracts 2 days, because ovulation is assumed to happen later or earlier.

What if I do not know my LMP?

If the LMP is unknown or cycles are very irregular, the LMP method is unreliable. An early ultrasound usually gives a better date, so ask your clinician to date the pregnancy that way instead of guessing.

How accurate is a due date worked out from LMP?

It is an estimate, not a promise. Only about 5 percent of babies arrive on the exact due date, so treat it as the centre of a window of a few weeks and confirm the final date with your clinician.

Now that you know how to work out your due date, use the result to plan appointments and preparations, keeping in mind that it is an estimate. For a related check, try our Pregnancy Calculator in Tamil, and for a different health calculation, read our guide How to Calculate TDEE.

How to Calculate Your Due Date from LMP: Naegele’s Rule