The Pregnancy Calculator is a practical tool to estimate gestational age and due date from the last menstrual period. Instead of making you translate the problem into a spreadsheet, it keeps the calculation focused on the values that actually matter.
The Pregnancy Calculator is designed for a quick estimate rather than a personal diagnosis or prediction. Small changes in the inputs can move the result, so use the number together with the limitations explained below.
How to use the Pregnancy Calculator
Enter the requested values for First day of last menstrual period. Check the units and any selected mode before calculating. Change one input at a time when you want to compare scenarios, then use Reset to return to the starting values.
How the calculation works
The Pregnancy Calculator uses this approach: it will count elapsed days from the first day of the last menstrual period and express them as completed weeks plus remaining days; the due date is also estimated at 280 days from LMP.
Formula or method: Gestational age ≈ today − first day of LMP
Worked example
After you enter the first day of the last menstrual period, the tool counts completed weeks and days from that date and also shows the estimated due date. This shows how the Pregnancy Calculator turns the entered values into a result you can compare with the method above.
Understanding the result
The result expresses estimated gestational age in completed weeks and days, with an estimated due date. Clinical dating can differ, especially when cycle timing is uncertain.
Important: Gestational age from last menstrual period is an estimate and may differ from clinical dating.
When Pregnancy Calculator is useful
- Getting a quick planning estimate.
- Comparing how changed inputs affect the estimate.
- Preparing questions or targets for a more informed discussion.
If you compare scenarios, change one input at a time and keep measurement units consistent. Do not use a formula-based estimate as the sole basis for a medical or nutritional decision.
Inputs used by the Pregnancy Calculator
Before calculating, check each field rather than focusing only on the final number. Clear inputs make the Pregnancy Calculator easier to verify and make comparisons between scenarios more meaningful.
- First day of last menstrual period: Enter the calendar or clock value exactly as you want it used in the calculation.
What can change the Pregnancy Calculator result?
Pregnancy Calculator is an estimate built from the personal inputs and formula shown on the page. Body measurements, age, activity or calendar assumptions can materially change the output. Use consistent units and remember that formula-based estimates cannot capture every individual biological or clinical factor.
Common Pregnancy Calculator mistakes to avoid
Most calculator errors come from inputs, units or assumptions rather than the arithmetic itself. Before using a result, check these common issues:
- Mixing metric and US units.
- Treating a population formula as a diagnosis or individualized prescription.
- Changing several assumptions at once and then not knowing what caused the result to move.
If you are comparing two scenarios in this calculator, keep everything else the same and change only the variable you want to test. That produces a cleaner comparison and makes the reason for the difference easier to understand.
How to interpret this calculator responsibly
This tool turns the entered measurements into a formula-based estimate. Check that the units match the labels, and treat the result as context rather than a diagnosis or individualized prescription. Population formulas can be useful for screening and planning while still missing important differences between individuals.
If a result will influence a medical, pregnancy, nutrition, or exercise decision, use it as a starting point for discussion with an appropriately qualified professional rather than as the only source of guidance.
Measurement and rounding considerations
Measurements taken at different times, on different devices, or with different techniques can change the output. Use consistent measurement methods when comparing results over time, and avoid interpreting tiny changes as meaningful when the underlying inputs are estimates.
Related calculators for the next step
This calculator works best as part of a clear workflow. If your next question changes the formula, unit or planning goal, use a focused calculator rather than forcing a different problem into the same tool.
- Due Date Calculator — Estimate pregnancy due date from the first day of the last menstrual period.
- Date Difference Calculator — Calculate the number of days and weeks between two calendar dates.
Sources and reference notes
Where a calculator relies on a recognized standard, research equation or clinical convention, the source below provides context for the method. The calculator page simplifies that material for calculation purposes and does not replace the source itself.
Frequently asked questions
What does this calculator calculate?
It is designed to estimate gestational age and due date from the last menstrual period. The calculation runs in your browser from the inputs you enter.
Which inputs should I check first?
Check First day of last menstrual period. Make sure the units and selected options match the problem before comparing the result with another source.
Is this result medical advice?
No. It is a general formula-based estimate and cannot account for every individual health factor. Use it for information or planning, not diagnosis or treatment.
Is an estimated due date guaranteed?
No. A due date is an estimate based on the information entered and the selected method. Actual delivery timing varies, and clinical dating may use additional information.
Why might another calculator show a different answer?
Differences can come from rounding, unit conventions, formula variants, timing assumptions, optional inputs or different external rules. Compare the method and inputs, not only the final number.
Can I change the values and calculate again?
Yes. Change any input or option and recalculate. When comparing scenarios, change one assumption at a time so the reason for the difference is clear.