What this calculator does
Adjusted age for premature babies works out corrects a premature baby’s age for weeks born early, for milestone tracking. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaWeeks early = due date − birth date; adjusted age = chronological age − weeks early (floored at zero)
The inputs explained
| Field | What to enter |
|---|---|
| Due date | A calendar date. |
| Birth date | A calendar date. |
| As of date | A calendar date. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Examples across different dates
The same calculation at a range of dates, with every other input held at its starting value. These are generated relative to the day the page was built.
Examples across different dates
| Due date | Adjusted age | Chronological age | Born early by |
|---|---|---|---|
| 2026-08-16 | 0 weeks (0 days) | 13 weeks (90 days) | 97 days (13.9 weeks) |
| 2026-09-08 | 0 weeks (0 days) | 13 weeks (90 days) | 120 days (17.1 weeks) |
| 2026-11-07 | 0 weeks (0 days) | 13 weeks (90 days) | 180 days (25.7 weeks) |
| 2027-02-05 | 0 weeks (0 days) | 13 weeks (90 days) | 270 days (38.6 weeks) |
| 2027-08-09 | 0 weeks (0 days) | 13 weeks (90 days) | 455 days (65.0 weeks) |