What is the Gestational Age Calculator?
The gestational age calculator is a free online tool that helps you work out gestational age quickly and accurately. It uses the standard formula Naegele's rule: due date = LMP + 280 days and shows the result instantly as you type, along with a full breakdown so you can see exactly how the answer was reached.
How to use this calculator
- Enter the first day of your last menstrual period.
- See the estimated due date and current week.
Formula used
Naegele's rule: due date = LMP + 280 daysExplanation of each input
- First day of last period โ the first day of last period used in the calculation.
Understanding your result
- Estimated due date โ the calculated estimated due date.
- Current gestational age (weeks) โ the calculated current gestational age (weeks).
Step-by-step calculation
For the example values LMP 1 Jan:
- Apply the formula:
Naegele's rule: due date = LMP + 280 days - Substitute the values: +280 days
- Result: Due about 8 Oct
Worked example
| Inputs | LMP 1 Jan |
|---|---|
| Working | +280 days |
| Result | Due about 8 Oct |
Benefits and practical uses
This calculator saves you time and reduces errors when you need gestational age. It is useful for students, professionals and anyone who wants a fast, reliable answer without manual calculation. Results update instantly, work in your browser and can be copied or shared in one click.
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.
Disclaimer: This calculator provides general estimates for educational purposes only and is not medical advice. Consult a qualified healthcare professional for personal guidance.