complete.tools

Due Date Calculator

Estimate your baby's due date from the last menstrual period, a conception date or an ultrasound, with how far along you are.

What this tool does

Estimate your baby's due date from the last menstrual period, a conception date or an ultrasound, with how far along you are. You fill in these fields: Calculate from, Date, and Count from. Some fields appear only for certain choices. It shows estimated due date, gestational age, trimester, progress, and estimated conception.

Inputs

  • Calculate from: Switch between: Last period, Conception, Ultrasound.
  • Date: A date. The first day of your last menstrual period, the conception date, or the date of the ultrasound, to match the choice above.
  • Weeks at the ultrasound (weeks): A whole number. Accepts 0 to 42. Starts at 8 weeks. Shown only when "Calculate from" is Ultrasound. The gestational age on your ultrasound report.
  • Extra days at the ultrasound (days): A whole number. Accepts 0 to 6. Starts at 0 days. Shown only when "Calculate from" is Ultrasound.
  • Count from: A date. Starts as today's date. Change it to see how far along you were or will be on another day.

How to use

  1. Choose Last period, Conception, or Ultrasound for "Calculate from".
  2. Pick a date for "Date".
  3. If "Calculate from" is Ultrasound, enter the weeks at the ultrasound in weeks.
  4. If "Calculate from" is Ultrasound, enter the extra days at the ultrasound in days.
  5. Pick a date for "Count from".
  6. Read the result: estimated due date, gestational age, trimester, progress, and estimated conception. It appears once every required field has a valid value.

Worked examples

Example 1. Inputs:

  • Calculate from: Last period
  • Date: January 1, 2026
  • Count from: March 12, 2026

Results:

  • Estimated due date: October 8, 2026 (210 days to go)
  • Gestational age: 10w 0d
  • Trimester: 1 1st: 0w0d to 13w6d, 2nd: 14w0d to 27w6d, 3rd: 28w0d on
  • Progress: 25 % of 40 weeks
  • Estimated conception: January 15, 2026
  • Note: This is an estimate. Few babies arrive on the exact date, and your care provider may change the date after an early ultrasound.

How this is worked out: Naegele's rule (ACOG Committee Opinion 700): due = LMP + 280 days. 2026-01-01 is day 1 of a non-leap year, so day 281 is the due date; January to September have 273 days, so day 281 is October 8. Age on March 12: 31 + 28 + 11 = 70 days = 10w0d. 280 - 70 = 210 to go. 70 / 280 = 25%. Conception = LMP + 14 = January 15.

Example 2. Inputs:

  • Calculate from: Conception
  • Date: January 15, 2026
  • Count from: July 9, 2026

Results:

  • Estimated due date: October 8, 2026 (91 days to go)
  • Gestational age: 27w 0d
  • Trimester: 2 1st: 0w0d to 13w6d, 2nd: 14w0d to 27w6d, 3rd: 28w0d on
  • Progress: 68 % of 40 weeks
  • Estimated conception: January 15, 2026
  • Note: This is an estimate. Few babies arrive on the exact date, and your care provider may change the date after an early ultrasound.

How this is worked out: Conception + 266 days = due date, which is the same as taking the last period as conception - 14 = January 1 and adding 280. January 15 is day 15, so 15 + 266 = day 281 = October 8. January 1 to July 9: 181 days to July 1 (31+28+31+30+31+30) plus 8 = 189 days = 27w0d. ACOG puts 27w6d in the second trimester.

Example 3. Inputs:

  • Calculate from: Ultrasound
  • Date: March 5, 2026
  • Weeks at the ultrasound: 8 weeks
  • Extra days at the ultrasound: 3 days
  • Count from: March 5, 2026

Results:

  • Estimated due date: October 12, 2026 (221 days to go)
  • Gestational age: 8w 3d
  • Trimester: 1 1st: 0w0d to 13w6d, 2nd: 14w0d to 27w6d, 3rd: 28w0d on
  • Progress: 21 % of 40 weeks
  • Estimated conception: January 19, 2026
  • Note: This is an estimate. Few babies arrive on the exact date, and your care provider may change the date after an early ultrasound.

How this is worked out: Scan on March 5 (day 31 + 28 + 5 = 64) at 8w3d = 59 days. Equivalent last period = day 64 - 59 = day 5 = January 5. Due = day 5 + 280 = day 285 = October 12 (October 8 is day 281).

Example 4. Inputs:

  • Calculate from: Last period
  • Date: January 1, 2026
  • Count from: April 9, 2026

Results:

  • Estimated due date: October 8, 2026 (182 days to go)
  • Gestational age: 14w 0d
  • Trimester: 2 1st: 0w0d to 13w6d, 2nd: 14w0d to 27w6d, 3rd: 28w0d on
  • Progress: 35 % of 40 weeks
  • Estimated conception: January 15, 2026
  • Note: This is an estimate. Few babies arrive on the exact date, and your care provider may change the date after an early ultrasound.

How this is worked out: ACOG: the second trimester starts at 14w0d = 98 days. January 1 to April 9: 31 + 28 + 31 = 90 to April 1, plus 8 = 98.

Limitations

  • Assumes a regular 28-day cycle when working from the last period or conception.
  • Does not adjust for IVF transfer dates or for ultrasound measurements beyond the gestational age you type in.
  • It is a date estimate, not medical advice. Your care provider sets the date that counts.
  • 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: The tool finds the first day of the last period that matches what you entered, then adds 280 days. This is Naegele's rule as the American College of Obstetricians and Gynecologists states it.
Q: How is conception handled?
A: The tool takes the last period to be 14 days before the conception date, so the due date is 266 days after conception.
Q: How does the ultrasound option work?
A: It subtracts the gestational age on your report from the scan date to find the matching start of pregnancy, then adds 280 days.
Q: Which weeks are in each trimester?
A: The first trimester runs from 0 weeks to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks on.
Q: Does the tool know my cycle length?
A: No. It assumes a 28-day cycle with ovulation on day 14, so a different cycle can move the real date.

Explore Similar Tools

Explore more tools like this one: