The Due Date Calculator is a practical tool to estimate pregnancy due date from the first day of 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 Due Date 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 Due Date 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 Due Date Calculator uses this approach: it will add 280 days, or 40 weeks, to the first day of the last menstrual period entered.
Formula or method: Estimated due date = LMP + 280 days
Worked example
If the first day of the last menstrual period is January 1, 2026, adding 280 days gives an estimated due date of October 8, 2026. This shows how the Due Date Calculator turns the entered values into a result you can compare with the method above.
Understanding the result
Treat the number as an estimate produced by the stated formula. Individual physiology, measurement error and factors not included in the formula can make real needs or outcomes different.
Important: Pregnancy dating is an estimate; clinical dating and ultrasound findings can change the estimated due date.
When Due Date 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 Due Date Calculator
Before calculating, check each field rather than focusing only on the final number. Clear inputs make Due Date 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 Due Date Calculator result?
Due Date 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 Due Date 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.
- Pregnancy Calculator — Estimate gestational age and due date from the last menstrual period.
- Date Calculator — Add or subtract years, months, weeks and days from a date.
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 pregnancy due date from the first day of 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.
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.