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Pregnancy Due Date Calculator

Estimate your due date from your last menstrual period using Naegele's rule, with a rule-of-thumb adjustment for your cycle length.

What this tool does

Estimate your due date from your last menstrual period using Naegele's rule, with a rule-of-thumb adjustment for your cycle length. You fill in these fields: First day of last menstrual period, Average cycle length, and Measure progress on. It shows estimated due date, weeks pregnant, extra days, days until due date, trimester, and estimated ovulation.

Inputs

  • First day of last menstrual period: A date. Use the first day of your last period, not the last day.
  • Average cycle length (days): A whole number. Accepts 20 to 45. Starts at 28 days. From the first day of one period to the first day of the next. Most cycles are about 28 days.
  • Measure progress on: A date. Starts as today's date. Defaults to today. Change it to see how far along you will be on another date.

How to use

  1. Pick a date for "First day of last menstrual period".
  2. Enter the average cycle length in days.
  3. Pick a date for "Measure progress on".
  4. Read the result: estimated due date, weeks pregnant, extra days, days until due date, trimester, and estimated ovulation. It appears once every required field has a valid value.

Worked examples

Example 1. Inputs:

  • First day of last menstrual period: January 1, 2026
  • Average cycle length: 28 days
  • Measure progress on: February 12, 2026

Results:

  • Estimated due date: October 8, 2026 (280 days after the first day of the last period, shifted by your cycle length minus 28 days (a rule of thumb))
  • Weeks pregnant: 6 (completed weeks)
  • Extra days: 0 (beyond the completed weeks)
  • Days until due date: 238 (negative once past the estimate)
  • Trimester: First (first to 13w6d, second to 27w6d, third from 28w0d)
  • Estimated ovulation: January 15, 2026 (14 days before the next expected period)

How this is worked out: 28-day cycle, no shift: 280 days after 1 Jan 2026 (day 1) is day 281 = 8 Oct (day 273 is 30 Sep). Ovulation = LMP + (28 - 14) = 1 Jan + 14 days = 15 Jan. Elapsed 1 Jan to 12 Feb = 42 days = 6w0d. 280 - 42 = 238. Week 6 is in the first trimester.

Example 2. Inputs:

  • First day of last menstrual period: January 1, 2026
  • Average cycle length: 28 days
  • Measure progress on: April 8, 2026

Results:

  • Estimated due date: October 8, 2026 (280 days after the first day of the last period, shifted by your cycle length minus 28 days (a rule of thumb))
  • Weeks pregnant: 13 (completed weeks)
  • Extra days: 6 (beyond the completed weeks)
  • Days until due date: 183 (negative once past the estimate)
  • Trimester: First (first to 13w6d, second to 27w6d, third from 28w0d)
  • Estimated ovulation: January 15, 2026 (14 days before the next expected period)

How this is worked out: 8 Apr 2026 is day 31 + 28 + 31 + 8 = 98, elapsed from day 1 is 97 days = 13 x 7 + 6 = 13w6d, the last day of the first trimester. 280 - 97 = 183.

Example 3. Inputs:

  • First day of last menstrual period: January 1, 2026
  • Average cycle length: 20 days
  • Measure progress on: January 1, 2026

Results:

  • Estimated due date: September 30, 2026 (280 days after the first day of the last period, shifted by your cycle length minus 28 days (a rule of thumb))
  • Weeks pregnant: 1 (completed weeks)
  • Extra days: 1 (beyond the completed weeks)
  • Days until due date: 272 (negative once past the estimate)
  • Trimester: First (first to 13w6d, second to 27w6d, third from 28w0d)
  • Estimated ovulation: January 7, 2026 (14 days before the next expected period)

How this is worked out: Shortest cycle allowed. Shift = 20 - 28 = -8 days. Due = 8 Oct - 8 = 30 Sep (day 281 - 8 = day 273). Ovulation = 1 Jan + (20 - 14) = 7 Jan. Effective start 24 Dec 2025, 8 days before 1 Jan, so 8 days = 1w1d. 280 - 8 = 272.

Example 4. Inputs:

  • First day of last menstrual period: January 1, 2026
  • Average cycle length: 45 days
  • Measure progress on: January 18, 2026

Results:

  • Estimated due date: October 25, 2026 (280 days after the first day of the last period, shifted by your cycle length minus 28 days (a rule of thumb))
  • Weeks pregnant: 0 (completed weeks)
  • Extra days: 0 (beyond the completed weeks)
  • Days until due date: 280 (negative once past the estimate)
  • Trimester: First (first to 13w6d, second to 27w6d, third from 28w0d)
  • Estimated ovulation: February 1, 2026 (14 days before the next expected period)

How this is worked out: Longest cycle allowed. Shift = 45 - 28 = +17 days. Due = 8 Oct + 17 = 25 Oct. Ovulation = 1 Jan + (45 - 14) = 1 Jan + 31 = 1 Feb. Effective start 18 Jan, the date measured, so 0 days elapsed and 280 to go.

Limitations

  • The date is an estimate. Few babies arrive exactly on it.
  • The cycle length adjustment and the ovulation date are rules of thumb that assume ovulation 14 days before the next period. Real cycles vary.
  • Weeks pregnant are counted from the start date after the cycle adjustment, so a long or short cycle shifts them too.
  • It does not replace dating by ultrasound or advice from a health professional.
  • This tool gives an estimate. It is based on a general model, not on an exact published formula, so a real figure can differ from the result.

FAQs

Q: How is the due date worked out?
A: It is 280 days after the first day of your last menstrual period, the convention stated in ACOG Committee Opinion 700, shifted by your average cycle length minus 28 days. A 32-day cycle adds 4 days and a 24-day cycle subtracts 4. The cycle shift is a common rule of thumb, not part of the ACOG convention.
Q: Why does the cycle length matter?
A: The 280-day figure assumes ovulation about 14 days after the period starts, as in a 28-day cycle. A longer or shorter cycle moves ovulation, so the estimate moves with it.
Q: How is the ovulation date estimated?
A: It is the first day of the last period plus your cycle length minus 14 days, because ovulation usually falls about 14 days before the next period.
Q: Which weeks count as each trimester?
A: Weeks 0 to 13 are the first trimester, 14 to 27 the second and 28 onward the third, counted in completed weeks from the adjusted start date. Some sources start the second trimester at week 13.

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