Gestational Age Calculator
Enter the last menstrual period date to get gestational age in completed weeks and days, plus an estimated due date at LMP plus 280 days. An optional as-of date lets you check any point; otherwise today is used.
What this calculator does
The Gestational Age Calculator estimates your result instantly from the values you enter, using standard date & time formulas. Enter the last menstrual period date to get gestational age in completed weeks and days, plus an estimated due date at LMP plus 280 days. An optional as-of date lets you check any point; otherwise today is used. 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.
Result
Estimate only, not medical advice.
About Gestational Age Calculator
Give this tool the first day of the last menstrual period (LMP) and it counts the days to the as-of date, then splits that total into completed weeks and leftover days. It also projects an estimated due date by adding 280 days to the LMP.
The as-of date is optional; leave it blank to measure up to today. The page notes that the result is an estimate and not medical advice.
Formula & Mathematical Calculation
lmp = new Date(lraw + 'T00:00:00')
totalDays = Math.round((asof - lmp) ÷ 86400000)
Derived directly from the verified calculation function in the tool engine.
Calculation Example
Set the Last Menstrual Period (LMP) to 1 January 2024 and the As Of Date to 1 March 2024. The gap is 60 days, so Math.floor(60 ÷ 7) gives 8 weeks and the remainder gives 4 days.
The estimated due date is the LMP plus 280 days, which lands on October 7, 2024.
Key Benefits
- Splits the elapsed days since the LMP into completed weeks plus leftover days
- Projects an estimated due date by adding 280 days to the LMP
- Accepts an optional as-of date so you can check gestational age at any point, not just today
How to Use this Calculator
-
Enter the LMP
Select the first day of the last menstrual period in the Last Menstrual Period (LMP) field. -
Set an as-of date (optional)
Pick a value in the As Of Date (optional) field to measure to that day, or leave it blank to use today. -
Calculate
Press Calculate to see the Gestational Age in weeks and days and the Estimated Due 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
- Gestational Age 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
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.