The Gestational Age Calculator is a free tool that helps you calculate gestational age using the formula Naegele's rule: due date = LMP + 280 days. Enter your values above and press Calculate to get estimated due date, current gestational age (weeks) instantly, with a full step-by-step breakdown.
What is the Gestational Age Calculator?
The gestational age calculator is a free online tool that helps you calculate gestational age quickly and accurately. It applies the standard formula Naegele's rule: due date = LMP + 280 days to the values you enter and, when you press Calculate, displays the result together with a clear, step-by-step explanation of how the answer was reached.
How to use the Gestational Age Calculator
- Enter the first day of your last menstrual period.
- See the estimated due date and current week.
Formula used
The gestational age calculator is based on this formula:
Naegele's rule: due date = LMP + 280 daysEach term in the formula corresponds to one of the inputs explained below. The calculator performs the arithmetic for you and shows every step so you can check the working by hand if you wish.
Worked example
Suppose you enter LMP 1 Jan. The calculator works as follows:
| Inputs | LMP 1 Jan |
|---|---|
| Working | +280 days |
| Result | Due about 8 Oct |
Step-by-step calculation
- Apply the formula:
Naegele's rule: due date = LMP + 280 days - Substitute your values: +280 days
- Read the result: Due about 8 Oct
Inputs explained
- First day of last period — the first day of last period used in the calculation.
Understanding your result
- Estimated due date — the calculated estimated due date.
- Current gestational age (weeks) — the calculated current gestational age (weeks).
Benefits and practical uses
The gestational age calculator saves time and removes the risk of manual errors when you need to calculate gestational age. It is useful for students, professionals and anyone who wants a fast, dependable answer. Results update the moment you press Calculate, work on any device, and can be copied or shared in a single click — and because the maths runs in your browser, your figures stay completely private.
Tips for an accurate result
- Keep all related inputs in the same units.
- Use the worked example above to confirm you are entering values in the right places.
- Review the assumptions below so the result fits your situation.
- Press Reset to clear everything and try a different scenario.
Common mistakes to avoid
- Entering values in the wrong unit (mismatched units give wrong answers).
- Leaving a required field empty — the calculator will ask you to fill it in.
- Treating the result as a diagnosis instead of a general guide.
Assumptions and limitations
- Naegele's rule; a regular 28-day cycle.
Frequently asked questions
How accurate is the due date?
Only about 4% of babies arrive on the exact due date; full-term is 37-42 weeks.
How do I use the Gestational Age Calculator?
Enter your values in the fields, then press the Calculate button to see the result. Use Reset to clear the form and start again. Nothing is calculated until you click Calculate.
What formula does the Gestational Age Calculator use?
It uses the standard formula: Naegele's rule: due date = LMP + 280 days. The full step-by-step working is shown with every result so you can verify it.
What information do I need to use the gestational age calculator?
You need: first day of last period. Sensible example values are pre-filled so you can see how it works.
Is the Gestational Age Calculator free to use?
Yes. Every DigiToolkit calculator is completely free, with no sign-up, no login and no usage limits.
Does the Gestational Age Calculator work on mobile phones?
Yes. The calculator is mobile-first and works on phones, tablets and computers in any modern browser.
Do I need to install an app?
No. The gestational age calculator runs directly in your web browser — there is nothing to download or install.
Is my data private when I use this calculator?
Yes. All calculations run inside your browser. The numbers you enter are never uploaded to a server or shared with anyone.
How accurate is the Gestational Age Calculator?
It uses the generally accepted formula for gestational age and computes to several decimal places. Results are accurate for the assumptions listed on this page.
Can I copy or share my result?
Yes. After calculating, use the Copy button to copy the result, or Share to copy the page link.
Why is nothing showing until I press Calculate?
By design, the calculator only computes when you press Calculate, so you can finish entering all your values first. Press Reset any time to clear everything.
Is the Gestational Age Calculator a substitute for a doctor?
No. It gives general estimates for educational purposes only and is not medical advice. Consult a qualified healthcare professional for personal guidance.