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Free Pregnancy Due Date Calculator

Estimate your baby's due date from the first day of your last menstrual period (LMP) using Naegele's rule, with an optional adjustment for your average cycle length plus first, second, and third trimester date ranges.

Typical is 28. Used to adjust the estimate (range 20–45).

Estimated due date (EDD)
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Enter the first day of your last menstrual period to estimate the due date.

Estimate only. This tool is for general information only and is not medical advice or a diagnosis. Results are estimates based on the formula shown. Consult a qualified healthcare professional for guidance about your own situation.

Quick answer

Your estimated due date (EDD) is the first day of your last menstrual period plus 280 days (40 weeks). If your average cycle is not 28 days, add the difference: EDD = LMP + 280 days + (cycle length − 28) days. For example, an LMP of January 1, 2025 with a 28-day cycle gives a due date of October 8, 2025. This is an estimate; only about 1 in 20 babies arrive on the exact date.

Formula & method

This calculator applies Naegele's rule, the standard method for estimating a due date. It takes the first day of your last menstrual period (LMP) and adds 280 days, which equals 40 weeks of gestational age counted from the LMP. Because the classic rule assumes a 28-day cycle with ovulation on day 14, the tool lets you enter your own average cycle length and adjusts the estimate by (cycle length − 28) days, since a longer cycle means later ovulation and a later due date. All date arithmetic is done in UTC so results never shift by a day across time zones. The tool also derives approximate conception (LMP + 14 days, adjusted for cycle) and trimester boundaries measured as gestational age from the LMP: the first trimester runs through 13 weeks 6 days, the second begins at 14 weeks 0 days (LMP + 98 days), and the third begins at 28 weeks 0 days (LMP + 196 days). "Weeks pregnant so far" is computed only after the page loads, from today's date, so it never causes a display mismatch.

Examples

Example 1: LMP Jan 1, 2025, 28-day cycle
Input
LMP = 2025-01-01, cycle = 28 days
Result
Due date: Wednesday, October 8, 2025
Why
With a 28-day cycle there is no adjustment, so EDD = LMP + 280 days. Counting 280 days from January 1, 2025 lands on October 8, 2025.
Example 2: LMP Mar 10, 2025, 30-day cycle
Input
LMP = 2025-03-10, cycle = 30 days
Result
Due date: Wednesday, December 17, 2025
Why
A 30-day cycle adds (30 − 28) = 2 days, so EDD = LMP + 282 days. Counting 282 days from March 10, 2025 gives December 17, 2025.
Example 3: LMP Jun 10, 2024, 26-day cycle
Input
LMP = 2024-06-10, cycle = 26 days
Result
Due date: Saturday, March 15, 2025
Why
A 26-day cycle subtracts (28 − 26) = 2 days, so EDD = LMP + 278 days. Counting 278 days from June 10, 2024 gives March 15, 2025.

When to use this tool

  • Getting an early estimate of your due date as soon as you know the first day of your last period.
  • Planning prenatal appointments, maternity leave, or other timing around an approximate delivery window.
  • Checking how the estimate shifts when you account for a cycle length that differs from the standard 28 days.

Common mistakes

  • Entering the date your period ended instead of the first day it started. Naegele's rule counts from the first day of bleeding (the LMP).
  • Leaving cycle length at 28 when your cycles are consistently longer or shorter. A 35-day cycle, for instance, pushes the due date about a week later.
  • Treating the estimated due date as a fixed delivery date. It is the midpoint of a normal range; full term spans roughly 37 to 42 weeks.

Frequently asked questions

How is the due date calculated?

It uses Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. If your average cycle is not 28 days, the tool adds or subtracts the difference, because cycle length changes when ovulation occurs.

Why does cycle length change the due date?

The 280-day rule assumes you ovulate around day 14 of a 28-day cycle. A longer cycle usually means you ovulate later, pushing conception and the due date later; a shorter cycle moves them earlier. The tool shifts the estimate by (cycle length − 28) days.

How accurate is the estimated due date?

It is a reasonable estimate, but only about 4–5% of babies are born on their exact due date. Most births occur within a window of roughly two weeks before to two weeks after. An ultrasound dating scan in the first trimester is often more precise, especially if cycles are irregular.

What if I don't know my last period date?

This calculator needs the first day of your last menstrual period. If you are unsure or your cycles are irregular, an early ultrasound is the most reliable way to date a pregnancy; ask your healthcare provider.

How are the trimesters defined here?

Trimesters are measured as gestational age from the LMP. The first trimester runs through 13 weeks 6 days, the second begins at 14 weeks 0 days, and the third begins at 28 weeks 0 days and continues to the due date at 40 weeks.

Is this medical advice?

No. This tool provides general information and an estimate only. It is not a diagnosis or a substitute for prenatal care. Talk to a qualified healthcare professional about your pregnancy.

Sources & references

External references open in a new tab. We are independent and not affiliated with these organizations.

Disclaimer

This tool is for general information only and is not medical advice or a diagnosis. Results are estimates based on the formula shown. Consult a qualified healthcare professional for guidance about your own situation.

  • ✓ Free to use
  • ✓ No sign-up required
  • ✓ Runs entirely in your browser — nothing is uploaded.
  • ✓ Formula and method shown above
Last updated: How we calculateReport a correctionDisclaimer

Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.

Built and reviewed by HIFreeTools against the formula shown above and any authoritative references cited on this page. See our methodology and editorial standards.

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