How is a pregnancy due date calculated?

People type this into search as a due date calculator.

The pregnancy due date calculator follows ACOG Committee Opinion 700. By convention the estimated due date is 280 days after the first day of the last menstrual period. That count assumes a regular 28-day cycle. ACOG says a first-trimester ultrasound dates the pregnancy more accurately than the last period alone. Take the calendar result to a clinician. This page is the arithmetic.

What the 280 days assume

due date = first day of the last period + 280 + (cycle length − 28)

The extra term is zero when the cycle is 28 days. A 32-day cycle adds 4 days. A 26-day cycle subtracts 2. The calculator does that so the 14-day luteal phase stays put and the extra days sit in the follicular part of the cycle. The field allows 20 through 45 days.

280 ÷ 7 = 40 weeks. The calculator adds days. Months are uneven, so a count that steps by calendar months from 15 March lands on a different day. Example 7 runs that comparison.

The same last period, used for a fertile window instead of a due date, is the ovulation calculator and the fertile window post. Leave and childcare money are separate questions. The parental leave calculator and the childcare post start once you already have dates to plan around.

The card prints dates in US medium form, like Dec 20, 2026. It also prints the day count, the cycle adjustment, and the next period, which is the last period plus the cycle length.

Example 1: 15 March 2026, 28-day cycle

This is the calculator default.

• last period: 15 March 2026
• cycle: 28 days

Adjustment: 28 − 28 = +0 days

Days after the last period: 280

Due date: Dec 20, 2026

Next period on this cycle: 15 March + 28 days = Apr 12, 2026

Count it in month lengths if you want to see the 280. From 15 March to 15 December is 275 days (March has 16 days left after the 15th, then April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, and 15 days of December: 16+30+31+30+31+31+30+31+30+15 = 275). Five more days land on 20 December. 275 + 5 = 280.

Example 2: the same last period, 32-day cycle

Cycle length becomes 32. The last period stays 15 March 2026.

Adjustment: 32 − 28 = +4 days

Days after the last period: 280 + 4 = 284

Due date: Dec 24, 2026

Next period: 15 March + 32 = Apr 16, 2026

Dec 24 is four days after Dec 20. The whole shift is the cycle adjustment. Nothing else in the formula moved.

Example 3: a shorter cycle and a longer one

Still 15 March 2026.

21-day cycle. Adjustment 21 − 28 = −7 days. Days after the last period: 273. Due date: Dec 13, 2026. Next period: Apr 5, 2026.

35-day cycle. Adjustment +7 days. Days: 287. Due date: Dec 27, 2026. Next period: Apr 19, 2026.

The two due dates are 14 days apart. That is the full gap between a 21-day cycle and a 35-day cycle, because each day of cycle length moves the due date by one day.

45-day cycle, the top of the field. Adjustment 45 − 28 = +17 days. Days: 297. Due date: Jan 6, 2027. Next period: Apr 29, 2026. The due date has crossed into the next year. The last period was still in March.

Example 4: what if the cycle length changes by four days

The last period stays 15 March 2026. Days after it are 252 + the cycle length. At 28 days, 252 + 28 = 280. Every step of 4 days in the cycle adds 4 days to the count, so the line is straight.

The same addition covers 36, 40, and 44. 15 March + 288 days is 28 December. 15 March + 292 days is 1 January 2027. 15 March + 296 days is 5 January 2027. The next-period column is 15 March plus the cycle length, which stays in April for this whole table.

Example 5: other last periods, 28-day cycle unless noted

The 280-day count does not care which month you start in. The calendar date does, because the months in between have different lengths.

1 January 2026, 28 days. Due Oct 8, 2026. Next period Jan 29, 2026. January has 31 days, so day 28 of the cycle is 28 January and the next period is the 29th.

31 January 2026, 28 days. Due Nov 7, 2026. Next period Feb 28, 2026. 2026 is not a leap year, so February has 28 days and the next period lands on the last day of February.

1 June 2026, 28 days. Due Mar 8, 2027. Next period Jun 29, 2026. The due date is in the following spring.

1 December 2025, 28 days. Due Sep 7, 2026. Next period Dec 29, 2025.

4 July 2026, 30-day cycle. Adjustment +2. Days: 282. Due Apr 12, 2027. Next period Aug 3, 2026.

20 November 2025, 26-day cycle. Adjustment −2. Days: 278. Due Aug 25, 2026. Next period Dec 16, 2025.

Example 6: a leap day

2024 was a leap year. 29 February 2024 is a real date.

• last period: 29 February 2024
• cycle: 28 days

Due date: Dec 5, 2024

Next period: Mar 28, 2024

The day before, 28 February 2024, plus 280 days, is Dec 4, 2024. One day later in the last period, one day later in the due date. The leap day does not add a bonus day on top of that. It is simply a valid start date, and 280 days later is 5 December.

A 28-day cycle from 29 February has its next period on 28 March. February contributes nothing further. The 28 days run through March.

Example 7: the year, month, and week shortcut

A shortcut you will see in other tools is: add one year, subtract three months, add seven days. The calculator on this site adds 280 days. Those two procedures land on the same date in some months and miss by a day or two in others. The miss is the uneven length of the months.

15 March 2026. Plus one year is 15 March 2027. Minus three months is 15 December 2026. Plus seven days is 22 December 2026. The 280-day result is 20 December 2026. The shortcut is 2 days later, 282 days after 15 March.

31 January 2026. Plus one year is 31 January 2027. Minus three months is 31 October 2026. Plus seven days is 7 November 2026. The 280-day result is also 7 November 2026. On this date the two counts agree.

1 December 2025. Plus one year is 1 December 2026. Minus three months is 1 September 2026. Plus seven days is 8 September 2026. The 280-day result is 7 September 2026. The shortcut is 1 day later.

1 June 2026. Plus one year is 1 June 2027. Minus three months is 1 March 2027. Plus seven days is 8 March 2027. The 280-day result is also 8 March 2027.

Use the 280-day date when you are matching this calculator and the ACOG convention. The shortcut is a useful check when the two dates agree, and a two-day question when they do not. An ultrasound in the first trimester is the step ACOG ranks above either calendar trick.

Example 8: dates that are not dates, and cycles the field refuses

31 February 2026. The result is Need a valid LMP date. The calculator checks that the year, month, and day exist before it adds 280. 2026 is not a leap year, and February never has 31 days in any year.

31 April 2026. Same message. April has 30 days.

15 March 2026 with a 19-day cycle. The result is Need a usual cycle length. The accepted range is 20 through 45. A 19-day entry does not get a due date of 280 minus 9.

15 March 2026 with a 46-day cycle. Same message. 45 days is the long end, and Example 3 already ran it: 6 January 2027.

The page’s own inputs stop at 1 to 31 for the day and 20 to 45 for the cycle. The formula still has to refuse a combination that is not a real date, because 31 is a legal day number in January and an illegal one in April.

Irregular cycles are a different limit. The formula wants one usual length. A cycle that swings from 24 to 40 does not have a single honest input. ACOG’s point about first-trimester ultrasound is the practical one in that case. The calendar count will still run on whatever average you type. It will not tell you the average was shaky.

The examples next to each other

Open the due date calculator with the first day of the last period and a usual cycle length. For the 6-day window on that same period, use the ovulation calculator. Confirm the date in care. ACOG ranks a first-trimester ultrasound ahead of the last period alone.