Due Date Calculator

What This Calculator Estimates

Due date calculators typically estimate an expected delivery date by adding a set number of weeks to the first day of the last menstrual period (Naegele's rule) or by calculating from a known conception date, though only a small percentage of babies are actually born exactly on their calculated due date. Because individual pregnancies vary and clinical ultrasound dating can refine estimates, this calculator provides a general estimate, not a precise medical prediction.

How to Use This Calculator

  1. Choose the first day of your last period.
  2. Enter your typical cycle length.
  3. Click Estimate Due Date.
  4. Review the due date and timing estimate.
  5. Use the result as informational guidance only.

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Frequently Asked Questions

How is a due date typically calculated?

The most common method (Naegele's rule) adds a set number of weeks to the first day of your last menstrual period, assuming a standard cycle length; conception-date-based methods offer an alternative calculation.

How often are babies born exactly on their due date?

Only a small percentage of babies are born on their exact calculated due date — most arrive within a window of a couple weeks before or after, which is considered entirely normal.

Does irregular cycle length affect due date accuracy?

Yes, calendar-based due date estimates assume a fairly standard cycle length, so irregular cycles can make the estimate less precise — clinical ultrasound dating is often used to refine estimates in these cases.

Is an ultrasound more accurate than a calculated due date?

Early ultrasound dating (particularly in the first trimester) is often considered more precise than calendar-based methods alone, especially for those with irregular cycles or uncertain last period dates.

Should I make medical decisions based solely on this estimate?

No, this provides a general estimate only — consult a healthcare provider for personalized prenatal care and more precise due date assessment.

This calculator provides informational pregnancy timing estimates only and is not medical advice. Please verify medical questions with a qualified clinician.

Last updated: May 11, 2026

About the Due Date Calculator

Free Due Date Calculator — This calculator estimates a due date using the first day of your last menstrual period and a cycle-length adjustment. It is for. Due Date Calculator was built to replace the back-of-napkin math with something exact and repeatable.

You'll enter First Day of Last Period and Cycle Length, and the calculator handles the rest.

How to Use It

Getting your number takes just a few steps:

  1. Enter first day of last period.
  2. Add cycle length.
  3. Review the result — adjust any field to see it update live.

Who This Is For

It's built for anyone who'd rather trust a calculation than guess — students, professionals, and everyday users checking a number they need to be right.

This page also answers common questions like "How is a due date typically calculated?" and "Should I make medical decisions based solely on this estimate?" — see the FAQ section above for details.

Good to Know

Does it store my information? No. The calculation happens in your browser and nothing you enter is saved or sent anywhere.

Is there a cost to use this? No — every calculator on Zaculators is free, with no account or sign-up needed.

How accurate is this? This uses the standard approach used for this type of calculation, so the output should match what you'd get working it out by hand — just with less room for error.

A Note on Precision

Small changes in your inputs can shift the result more than you'd expect, so it's worth running the numbers more than once if you're using this for something important. Treat the output as a strong estimate built on the figures you provide, not a substitute for professional advice where real money or decisions are on the line.

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Zaculators has 250+ free calculators spanning finance, business, health, tax, and everyday planning. Browse the full calculator directory if Due Date isn't quite the tool you're after.