Health & Fitness

Pregnancy Due Date Calculator

Applies Naegele's rule: it adds 280 days (40 weeks) to the first day of your last menstrual period and returns the estimated due date as a calendar date in YYYY-MM-DD format.

What this calculator does

The Pregnancy Due Date Calculator estimates your result instantly from the values you enter, using standard health & fitness formulas. Applies Naegele's rule: it adds 280 days (40 weeks) to the first day of your last menstrual period and returns the estimated due date as a calendar date in YYYY-MM-DD format. Calculations run in your browser, so nothing you type is sent to a server, and the output is an estimate rather than a certified or official figure.

Pregnancy Due Date Calculator

Result

—
Estimated Due Date

About Pregnancy Due Date Calculator

The Pregnancy Due Date Calculator applies Naegele's rule. It adds 280 days (40 weeks) to the first day of your last menstrual period and returns the estimated due date as a calendar date in YYYY-MM-DD format. Only the last period date is needed.

Formula & Mathematical Calculation

Estimated Due Date = f(First Day of Last Period)

Calculated according to standard health formulas implemented in the calculator core.

Calculation Example

For a last period starting 2026-01-01: adding 280 days gives Estimated Due Date = 2026-10-08, with the note: "Naegele rule: due date is about 280 days (40 weeks) after the last period began."

Key Benefits

  • Estimates the due date from a single input, the last period start date
  • Uses the standard Naegele's rule of 280 days (40 weeks)
  • Returns a plain calendar date you can plan around

How to Use this Calculator

  1. Enter last period date
    Pick the first day of your last menstrual period in the date field.
  2. Calculate
    Click Calculate to run Naegele's rule.
  3. Read the due date
    Read the Estimated Due Date result box, shown as a YYYY-MM-DD calendar date.

Assumptions & Limitations

What the calculation assumes

  • Every value you enter is treated as an exact, known amount.
  • Any rate or interval you enter is assumed to hold for the whole period unless the form asks for it separately.
  • Results use standard published formulas and account only for the values entered above.
  • Rounding is applied for display; the underlying calculation uses your full precision.
  • Results are returned in the same units you enter, unless the tool states the output unit.

What it does not cover

  • Pregnancy Due Date Calculator produces a calculated result for reference and learning, not a certified or official measurement.
  • Real-world outcomes differ when tolerances, rounding rules or local standards apply.
  • Definitions and standards vary by region and change over time; verify the convention that applies to your case.
  • Figures are only as accurate as the inputs you supply — check them before acting on the result.
  • For work where the result carries real consequences, confirm it against an authoritative source.

Frequently Asked Questions

Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period to estimate the due date.

No. Unlike the ovulation and fertile window tools, this calculator uses only the last period date and a fixed 280-day count; it does not ask for cycle length.

280 days is exactly 40 weeks, the conventional length of a full-term pregnancy counted from the last menstrual period.

Free to use, no account needed. Calculations run in your browser, so nothing you type is sent to a server. Results are estimates — how we build and check our calculators.