How to use the Due Date Calculator

Quick start

1. Open the Due Date Calculator.
2. Enter the first day of the last menstrual period.
3. Use the first example, "LMP Apr 1: 28-day cycle", if you want to see a filled-out calculation before entering your own values.
4. Calculate, read the formula line, then copy the result only after the units and assumptions look right.

Best uses

Use this guide when one of these tasks matches what you are trying to do.

• Estimate a due date from last menstrual period.
• Adjust the estimate for shorter or longer cycles.
• Find estimated conception date alongside due date.
• Use as a planning reference before clinical confirmation.

What this calculator is for

The Due Date Calculator focuses on expected delivery date from the first day of the last menstrual period and usual cycle length.

Use it when you want to: Estimate a due date from last menstrual period. Adjust the estimate for shorter or longer cycles.

What to enter

Good answers start with clean inputs. Before calculating, check the labels, units, and dates so the tool is solving the same problem you actually have.

• Enter the first day of the last menstrual period.
• Set the usual cycle length if it differs from 28 days.
• Use the result as a planning date until clinical dating is confirmed.

Example walkthrough

Try the calculator example: LMP Apr 1: 28-day cycle. The example result is Estimated due date Jan 6, 2027.

• With Apr 1 as LMP and a 28-day cycle, the calculator adds 280 days.
• A longer cycle moves estimated ovulation later, so the date can shift later.

Formula and steps

The calculator uses Naegele-style dating: LMP plus 280 days, adjusted by the difference from a 28-day cycle.

The formula line on the calculator page is there so the answer is not a mystery. Read it when you need to understand where the number came from.

How to read the answer

Due dates are estimates. Many healthy pregnancies deliver before or after the estimated date.

• A due date is an estimate; many healthy pregnancies deliver before or after it.
• The conception estimate is backward math from the due date, not a confirmed event date.

Common mistakes to avoid

Most bad calculator results come from a small input mistake or from using a good estimate for the wrong decision.

• Do not enter ovulation date into an LMP field.
• Do not treat the due date as an appointment guarantee.
• Do not ignore clinician updates after ultrasound.

What to try next

A related calculator can help check the same topic from another angle instead of relying on one number.

• Use Pregnancy Calculator for gestational age and trimester too.
• Use Pregnancy Conception Calculator to work backward from a due date.

Sources and safety notes

This guide uses public-health, clinical, or peer-reviewed references where the calculator needs a specific formula or interpretation boundary.

Source links are provided for transparency, but they do not turn the calculator into medical advice or a replacement for professional care.

• Johns Hopkins Medicine: Calculating a due date
• Johns Hopkins Medicine: Fertile window basics

Examples from the calculator

Common questions

Related tools

• Pregnancy Calculator Estimate due date, gestational age, conception date, and trimester.
• Pregnancy Conception Calculator Estimate conception date from an expected due date.
• Ovulation Calculator Estimate ovulation date and fertile window from cycle details.

History, privacy, and copying

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