Calculator
What the Due Date Calculator does
Due Date Calculator: calculates Estimated Due Date and Gestational Weeks (today) from First Day of Last Period (LMP). Example: first day of last period (LMP) 2026-01-01 gives estimated due date 2026-10-08.
Point it at First Day of Last Period (LMP) and it returns Estimated Due Date and Gestational Weeks (today). Each change is reflected immediately, which suits quick what-if comparisons.
A single number is most useful as a trend over time rather than a one-off judgement. The Due Date Calculator exists to take that particular chore off your hands.
Nothing leaves the tab. You can watch your browser network panel while you use it and see for yourself.
The sections that follow cover the fields, the method, and one full example computed from the defaults.
What do the Due Date Calculator fields mean?
The Due Date Calculator uses 1 input. Starting values are already in place, which saves the usual blank-form hesitation about what a field wants.
| Field | What to enter | Default |
|---|---|---|
| First Day of Last Period (LMP) | pick a calendar date | 2026-01-01 |
How does the Due Date Calculator work?
Each output is derived from the inputs above in a single pass; there is no hidden state carried between runs, so the same inputs always give the same calculation.
Values are checked first: an out-of-range or non-numeric entry names the offending field instead of returning nonsense.
Worked example
These are the values the Due Date Calculator loads by default, and the result it produces from them. The figures are the tool’s live defaults and its live output, so you can reproduce them exactly.
| First Day of Last Period (LMP) | 2026-01-01 |
|---|
| Estimated Due Date | 2026-10-08 |
|---|---|
| Gestational Weeks (today) | 35 |
| What this means | Estimated due date is 2026-10-08 (Naegele’s rule: LMP + 280 days). Confirm with your clinician / ultrasound. |
How to use it
- Select First Day of Last Period (LMP) from the date picker.
- Each keystroke triggers a fresh calculation, which is cheap because it happens locally.
- You get Estimated Due Date, Gestational Weeks (today) and What this means back.
- The link encodes your inputs, so saving it is enough to reproduce the result later.
What this tool does not do
- Body composition, ethnicity and age all shift what a "normal" result means.
- These formulas come from population averages and cannot account for your medical history.
- The output is a number, not a judgement. Deciding what it means is still your job.
Frequently asked questions
You need First Day of Last Period (LMP). Defaults are loaded for all of them, which means a single edit is enough to get a meaningful answer.
With the values loaded when the page opens, estimated due date comes out as 2026-10-08. That number tracks the inputs, so a small edit shows its sensitivity immediately.
No. The calculation is local, so no health information ever reaches a server.
Free, and there is no upsell. The whole tool is the free version.
Reliable as a rule of thumb, unreliable as a verdict. Equations of this kind are fitted to large groups, and the spread around the fitted line is wide enough that any individual can sit well off it.