Due Date Calculator

A due date calculator helps expectant parents estimate when their baby will arrive. Enter your last menstrual period (LMP) or conception date and the tool works out your estimated due date, current gestational age, and key pregnancy milestones.

How to Use the Due Date Calculator

1. Enter the first day of your last menstrual period, or toggle to conception date if you know it.
2. Select your average cycle length (default is 28 days).
3. Click Calculate to generate your estimated due date.
4. Review the trimester breakdown and milestone dates shown in the results.
5. Use the result as a guide; confirm with your midwife or obstetrician.

The Formula

The most widely used method is Naegele's Rule. Add 280 days (40 weeks) to the first day of your last menstrual period. Alternatively, take the LMP date, subtract 3 months, and add 7 days to get the same result. If your cycle is longer or shorter than 28 days, add or subtract the difference to adjust the estimate. Gestational age is expressed in weeks and days from the LMP.

Real-World Example

Last menstrual period: 1 March 2025. Using Naegele's Rule, subtract 3 months to get 1 December 2024, then add 7 days to arrive at 8 December 2025. Alternatively, add 280 days to 1 March 2025 to confirm the same date. Gestational age at any point is calculated as the number of days from 1 March 2025 to the current date, divided by 7 to express in weeks.

Pregnancy Milestones

Key milestones fall at predictable points. The first trimester ends at 13 weeks and 6 days. The anatomy scan typically occurs around 18 to 20 weeks. The third trimester begins at 28 weeks. A baby is considered full term from 37 weeks. Most healthcare providers will discuss induction options if pregnancy extends beyond 42 weeks.

Frequently Asked Questions

How accurate is an estimated due date? Estimated due dates are accurate to within about two weeks either side. Only around 5 percent of babies arrive on their exact due date. Early ultrasound dating is generally more precise than LMP-based calculation.

What if I have irregular cycles? Irregular cycles make LMP-based calculation less reliable. Your midwife will use an early ultrasound measurement, called the crown-rump length, to date the pregnancy more accurately.

Can I calculate my due date from conception? Yes. If you know your conception date, add 266 days (38 weeks) to get the estimated due date. This is equivalent to using the LMP method for a standard 28-day cycle.

What does gestational age mean? Gestational age is measured from the first day of the last menstrual period, not from conception. A full-term pregnancy is 40 weeks of gestational age, even though fertilisation occurs around week 2.

Reading the method step by step

Naegele's rule has two forms and they give the same answer when both are applied in full. The first adds 280 days to the first day of the last menstrual period. The second takes the same date, adds a year, subtracts three months and adds seven days. Worked on a last menstrual period of 1 March 2025:

The two routes land two days apart, and the difference is worth understanding rather than memorising. The 280 day method is the one to prefer, because 280 days is exactly 40 weeks and weeks are what gestational age counts in. The three month method approximates four weeks to a month, and the calendar months it passes through average 30.4 days, so any date it produces sits a day or two away from the one that adding 280 days gives. The year step belongs in that route as well: a period started on 1 March 2025 gives 8 December 2025 by the month route and 6 December 2025 by the 280 day route. Where the two methods disagree, take the 280 day result, and let the scan override both if one has been done.

Gestational age and the milestone dates it produces

Gestational age counts forward from the first day of the last menstrual period, in weeks and days. On a last menstrual period of 1 March 2025, each milestone falls on a date you can check against a calendar:

Every row is the same arithmetic: multiply the weeks by seven and add the days, then add that total to the last period date. There is no month arithmetic anywhere in it, which is the reason gestational age is quoted in weeks and days rather than months. A pregnancy described as "six months" spans any of five different week ranges depending on which months you count.

Cycle length changes the answer

The 280 day rule assumes a 28 day cycle with ovulation around day 14. A cycle that runs longer or shorter moves the estimate by the difference, because the fertile window sits a different number of days after the period started.

The adjustment is the cycle length minus 28, added to the estimate. A 35 day cycle shifts the date by a week, which is half of the two week range the FAQ describes as normal variation. That is the point at which an early scan becomes the better source.

Dating by conception, and where the 266 days comes from

A conception date can be used directly. Conception happens about 14 days after the first day of the last period in a 28 day cycle, so the conception route uses 266 days instead of 280: 14 plus 266 equals 280.

On the same dates: conception would fall around 15 March 2025, and 15 March 2025 plus 266 days lands on 6 December 2025, the same date the 280 day rule gives. The two methods agree by construction, and that agreement is the check that both were applied with the right offsets.

The conception route is only as good as the conception date, and few people know that date with precision outside a fertility treatment cycle. Where the date is known exactly, as with an embryo transfer, the estimate from it is more reliable than one from a remembered period.

How an early scan dates a pregnancy

An ultrasound measurement of crown-rump length converts directly to gestational age. The Robinson and Fleming formula, published in 1975 and still the reference used for first trimester dating, is:

gestational age in weeks = crown-rump length in mm divided by 10, plus 6.5

Two worked cases show the method and its result:

The due date comes from the scan by taking 280 days, subtracting the gestational age in days, and adding the remainder to the scan date. For the 25 mm case that is 280 minus 63, which is 217 days added to 1 May 2025. For the 45 mm case it is 280 minus 77, which is 203 days added to 2 June 2025.

The formula is validated for a crown-rump length between 5 mm and 84 mm, which spans roughly 6 to 14 weeks of gestation. Above 84 mm the fetus curls and straightens too variably for a single straight-line measurement to date the pregnancy, and the biometric parameters take over instead. At the small end, below about 10 mm, a one millimetre difference moves the calculated age by several days, so the measurement carries more uncertainty than the formula's accuracy suggests.

For dating, crown-rump length is the most precise of the available measurements. Published accuracy figures put it within about 3 to 5 days in the validated window, against roughly 7 to 10 days for second trimester biometry and wider margins later in the pregnancy. That precision is why national guidance in the United Kingdom recommends an ultrasound scan in the first trimester to establish the due date for every pregnancy. Where a scan date and a period based estimate disagree by more than a set threshold, the scan result replaces the period based one rather than being averaged with it. The threshold widens as the pregnancy progresses, from about 5 days in the first trimester to roughly 7 days around 14 to 16 weeks.

What the estimate does and does not claim

Four assumptions sit behind the date this tool produces:

1. Ovulation occurred 14 days after the first day of the last period. Cycle length adjustment moves the assumption, and it cannot fix an irregular cycle, because the length of the current cycle is not known when the period starts.
2. The last period date is remembered accurately. Recall error of a few days is common and shifts the estimate by the same amount.
3. The pregnancy is a singleton. Multiple pregnancies are dated differently and are usually delivered earlier.
4. Gestational age is a dating convention, not a measurement of how far the pregnancy has developed. A pregnancy dated at 34 weeks by a scan and one dated at 34 weeks from a remembered period are two different biological states, and the convention hides that.

The two week spread quoted in the FAQ follows from these assumptions rather than from any error in the arithmetic. Five percent of births land on the estimated date, which tells you the estimate is the centre of a wide distribution rather than a prediction. Any decision that depends on the date, from arranging leave to planning a scan, works better with a range than with a single day.

This page is an arithmetic aid. It does not diagnose anything, and it does not replace a dating scan or the advice of a midwife or obstetrician.

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