Calculator
What the Pregnancy Calculator does
Pregnancy 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.
Supply First Day of Last Period (LMP) and you get Estimated Due Date and Gestational Weeks (today) back. You can nudge a single number and watch the effect straight away.
Most people who open the Pregnancy Calculator are people tracking a goal. Context matters more than the number itself. Age, build and activity all shift it.
Because it is all local, the tool is as fast as your device and as private as your own notes.
The rest of this page documents each field, the method, and a worked example built from the values the pregnancy calculator loads by default.
What do the Pregnancy Calculator fields mean?
The Pregnancy Calculator uses 1 input. You do not have to fill everything in before seeing an answer; the starting values already produce one.
| Field | What to enter | Default |
|---|---|---|
| First Day of Last Period (LMP) | pick a calendar date | 2026-01-01 |
How does the Pregnancy 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.
The guard clauses run before the formula does, which is why mistakes surface next to the field.
Worked example
These are the values the Pregnancy Calculator loads by default, and the result it produces from them. This is the worked example the page starts from, reproduced here so you can check the arithmetic.
| 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.
- The tool reacts immediately rather than waiting for you to finish the form.
- The results panel then shows Estimated Due Date, Gestational Weeks (today) and What this means.
- Copy Link puts the current inputs in the URL, so bookmarking it brings the same numbers back.
What this tool does not do
- This is a screening estimate, not a diagnosis.
- Body composition, ethnicity and age all shift what a "normal" result means for an individual.
- Nothing is saved between visits, so keep your own record of anything you need again.
Frequently asked questions
First Day of Last Period (LMP). That is the whole form. Every box already holds a working value, meaning you can explore the calculation before committing to real figures.
With the values loaded when the page opens, estimated due date comes out as 2026-10-08. The figure is derived from the inputs above, so it updates whenever they do.
No. Your measurements stay on your device. Health numbers are nobody else’s business, including ours.
No. There is no account, no usage cap and no trial period. The site is funded by advertising rather than by charging for the tools.
The number is exact; its meaning is approximate. Two people with identical results can be in genuinely different situations, which is why this is a screening tool rather than a diagnostic one.