Due Date Calculator — Step-by-Step Solutions

The Due Date Calculator estimates your expected delivery date based on your last menstrual period (LMP) or conception date. It adds 280 days (40 weeks) from your LMP to calculate the estimated due date.

The calculator also shows your current week of pregnancy, trimester, and a week-by-week progress tracker. Two modes are available: calculate from LMP date or from known conception date.

First day of your last menstrual period

Your calculations will appear here

How to Use the Due Date Calculator

Choose how you want to calculate: From Last Period (LMP) or From Conception Date.
Pick the date with the date field: the first day of your last menstrual period, or the date of conception if you know it, for example from ovulation tracking or IVF transfer.
The result shows your estimated due date, the current week and day of pregnancy, days remaining, the current trimester and the estimated conception date. The trimester timeline lists the date range for each trimester.
The due date is an estimate and not medical advice. Only a midwife or doctor can confirm dating, usually with an ultrasound.

Formula and Theory

A full-term pregnancy is counted as 40 weeks, or 280 days, from the first day of the last menstrual period. The old shortcut known as Naegele rule (add one year, subtract three months, add seven days) lands within a few days of that, depending on the month. This calculator adds 280 days.

The LMP method assumes a regular 28-day cycle with ovulation around day 14, so conception falls about two weeks after the LMP. If your cycle is longer or shorter than 28 days, adjust the date by the difference: with a 32-day cycle, add 4 days.

When the conception date is known, the due date is conception plus 266 days (38 weeks), and the calculator also works back to an estimated LMP. For the same pregnancy, both methods give the same date.

Gestational age is counted from the LMP, not from conception, so the pregnancy is 2 weeks along at the time of conception. Trimesters here are weeks 1-12, 13-27 and 28 to the due date. Most babies arrive in the weeks either side of the estimated date rather than on it.

Worked Examples

Example 1: Due date from the last period

Problem: The first day of the last period was 1 March 2025. What is the due date?

Solution: Add 280 days. 1 March plus 31 days is 1 April, plus 30 more is 1 May, plus 31 is 1 June, plus 30 is 1 July, plus 31 is 1 August, plus 31 is 1 September, plus 30 is 1 October, plus 31 is 1 November, plus 30 is 1 December. That is 275 days, so add 5 more to reach 6 December. Estimated conception is 14 days after the LMP: 15 March.

Answer: 6 December 2025 (conception about 15 March 2025)

Example 2: Due date from a known conception date

Problem: Conception was on 15 March 2025. What is the due date?

Solution: Add 266 days to 15 March: 15 March plus 261 days is 1 December (the same count as above, 275 minus 14), and 5 more days gives 6 December.

Answer: 6 December 2025, matching the LMP result

Example 3: How far along on a given day

Problem: With the 1 March 2025 LMP, how many weeks pregnant is someone on 1 June 2025?

Solution: Days from 1 March to 1 June = 31 + 30 + 31 = 92. Weeks = 92 / 7 = 13 with 1 left over, so 13 weeks and 1 day. Week 13 is in the second trimester (weeks 13-27). Days remaining to the due date = 280 - 92 = 188.

Answer: 13 weeks 1 day, second trimester, 188 days to go

Example 4: Crossing into the next year

Problem: The first day of the last period was 20 October 2025. What is the due date?

Solution: Step month by month from the 20th. 20 October to 20 November is 31 days, to 20 December 61, to 20 January 92, to 20 February 123, to 20 March 151 (February has 28 days in 2026), to 20 April 182, to 20 May 212, to 20 June 243 and to 20 July 273. The remaining 280 - 273 = 7 days reach 27 July.

Answer: 27 July 2026

Frequently Asked Questions

How is the due date calculated?

By adding 280 days (40 weeks) to the first day of the last menstrual period, or 266 days to the conception date. It assumes an average 28-day cycle.

How accurate is the due date?

It is an estimate. Only about 1 baby in 20 is born on the estimated date, and most are born in the weeks either side of it. A midwife or doctor will confirm dating, often with an early ultrasound.

What if my cycle is not 28 days?

Add or subtract the difference to the result. With a 32-day cycle add 4 days; with a 26-day cycle subtract 2. This calculator does not ask for cycle length, so make that adjustment yourself.

How many weeks pregnant am I?

Weeks are counted from the first day of your last period, so you are already 2 weeks along at conception. The calculator shows the current week and day.

What are the trimesters?

The first trimester is weeks 1-12, the second is weeks 13-27 and the third runs from week 28 to birth. Different sources draw the boundaries one week apart.

Is this a medical tool?

No. It is a date calculator and gives an estimate only. It is not medical advice; ask your midwife or doctor about your own pregnancy.