Calculator
What the Trimester Calculator does
Trimester Calculator: calculates Trimester, Gestational Weeks and Est. Due Date from First Day of Last Period. Example: first day of last period 2026-01-01 gives trimester 3rd trimester.
Trimester, Gestational Weeks and Est. Due Date, derived from First Day of Last Period. You can nudge a single number and watch the effect straight away.
Most people who open the Trimester Calculator are people tracking a goal. Context matters more than the number itself. Age, build and activity all shift it.
Nothing you enter about your body is sent anywhere or stored.
If you want to check the arithmetic, the field table, the method and a worked example are further down.
What do the Trimester Calculator fields mean?
The Trimester 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 | pick a calendar date | 2026-01-01 |
How does the Trimester 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.
Range checks come first, so the formula only ever sees values it can handle.
Worked example
These are the values the Trimester 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 | 2026-01-01 |
|---|
| Trimester | 3rd trimester |
|---|---|
| Gestational Weeks | 35 |
| Est. Due Date | 2026-10-08 |
| What this means | About 35 weeks pregnant → 3rd trimester. Confirm dates with your prenatal provider. |
How to use it
- Select First Day of Last Period from the date picker.
- Watch the results panel as you type: it recalculates on every keystroke.
- The panel reports Trimester, Gestational Weeks, Est. Due Date 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
- Population-level formulas cannot account for medical conditions, medication or pregnancy.
- This is a screening estimate, not a diagnosis.
- Assumptions built into the method are stated above, but they are still assumptions.
Frequently asked questions
The form asks for First Day of Last Period. Every box already holds a working value, letting you treat it as a what-if tool rather than a data-entry form.
With the values loaded when the page opens, trimester comes out as 3rd trimester. The figure is derived from the inputs above, so it updates whenever they do.
Nothing is stored. The page has no back end to send data to, and holds your input only in memory.
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.