Baby Age Calculator
Track a baby’s age in weeks, days and months from their date of birth.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
Results
About this calculator
Babies are measured in weeks for the first months and in months thereafter, so both are useful at once. This calculator also shows the corrected age for babies born early.
Worked example
calculated with the default values below
Date of birth 2026-05-01 · Original due date (for premature babies) 2026-05-01 Days old 183 days · Weeks old 26.0 weeks · Months old 5.00 months · Age 26 weeks 1 days · Corrected age 26.1 weeks
Method and assumptions
D = max(round((t − t₀) ÷ 86,400,000), 0); W = ⌊D ÷ 7⌋ Months are counted by calendar-month difference; corrected age subtracts prematurity days: (D − early days) ÷ 7. Babies are measured in weeks for the first months and in months thereafter, so both are useful at once. This calculator also shows the corrected age for babies born early. The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions.
Data sources
The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions.
Disclaimer
These calculators give general date results. Public holidays, working-day conventions and time-zone rules are set locally and change, so confirm against the official calendar for anything legally or contractually binding.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
D = max(round((t − t₀) ÷ 86,400,000), 0); W = ⌊D ÷ 7⌋
- t₀
- Birth date
- t
- Current date, UTC midnight
- D
- Age in days
- W
- Completed weeks
Months are counted by calendar-month difference; corrected age subtracts prematurity days: (D − early days) ÷ 7.
Frequently asked questions
Why is corrected age lower for a baby born early?
Corrected age subtracts the time between the actual birth date and the original due date from age since birth. For a baby born six weeks early, the corrected age is six weeks less than chronological age, down to a minimum of zero. Enter both dates to use this output; if the dates match, the two ages match. Developmental milestones should be discussed with the baby’s clinician.
Which inputs and limits apply to Baby Age Calculator?
Use the units shown beside each field. These are form limits, not recommended settings. D = max(round((t − t₀) ÷ 86,400,000), 0); W = ⌊D ÷ 7⌋ Months are counted by calendar-month difference; corrected age subtracts prematurity days: (D − early days) ÷ 7. Babies are measured in weeks for the first months and in months thereafter, so both are useful at once. This calculator also shows the corrected age for babies born early. The references provide context and limitations. They do not validate this page’s illustrative multipliers, lookup values or predictions.
What is a complete worked example for Date of birth = 2026-04-12; Original due date (for premature babies) = 2026-04-12?
Inputs: Date of birth = 2026-04-12; Original due date (for premature babies) = 2026-04-12. Calculation: D = max(round((t − t₀) ÷ 86,400,000), 0); W = ⌊D ÷ 7⌋. Results: Days old = 183 days; Weeks old = 26.0 weeks; Months old = 6.00 months; Age = 26 weeks 1 days; Corrected age = 26.1 weeks. Reference date: 2026-10-12 (UTC).