Due Date Calculator

How Due Date Calculator works

Estimate the due date from your last period and cycle length, with current week and trimester. Everything happens inside this page: the tool reads your input, runs the calculation or conversion locally, and hands the result back to you. Nothing is transmitted, because there is no backend to transmit it to.

That design choice is deliberate. Most free tool sites upload your files to a server, process them there, and quietly keep a copy. Toolcin cannot do that — the site is a set of static files. If your connection dropped the moment the page finished loading, every tool would still work.

The 280-day rule

The estimated due date is the first day of the last menstrual period plus 280 days — 40 weeks. That is Naegele's rule, and it is still the default because it needs only one date and no ultrasound.

The tool adjusts for cycle length: a 32-day cycle adds 4 days, a 24-day cycle subtracts 4. The reasoning is that ovulation shifts with cycle length, so a longer cycle means a later conception for the same last period.

Weeks, trimesters and the range

Alongside the date, the tool shows the current week and trimester. Trimesters are counted as 0–13, 14–27 and 28–40 weeks, which is the common clinical division.

Only about 4% of births land exactly on the estimated date. Most arrive within two weeks either side, which is why a due date is described as an estimate and never as an appointment.

What this cannot replace

A dating ultrasound in the first trimester is more accurate than any calculation from a reported date, especially when the cycle is irregular or the last period date is uncertain. Clinicians adjust the estimate after that scan, and their date should replace this one.

This is a planning aid, not a diagnosis, and it says nothing about the health of the pregnancy.

Good to know

• Works offline once loaded, including on flights and in locked-down networks.
• No file size limit beyond your device's own memory.
• Free to use, with no account and no usage counter.
• Results are produced locally, so they are not logged or shared with anyone.

Questions

How this number is produced

The calculation is Naegele's rule: last menstrual period + 280 days, adjusted by (cycle length − 28), following Naegele's rule. The formula is repeated here so the result can be checked by hand rather than taken on trust.

This is a calculator, not advice. No clinician, accountant or adviser has reviewed it, and it cannot see your circumstances. For anything involving health, a loan, a tax filing or a religious obligation, confirm the figure with a qualified professional or the authority that publishes the rule.