Sources & Methodology
Methodology: Age in months is calculated by counting complete calendar months from date of birth to the reference date, then adding remaining days as a decimal fraction. For adjusted (corrected) age, the same formula is applied from the original due date rather than birth date. Week counts use 7-day periods from birth.
Last reviewed: March 2026 — formulas verified against CDC and WHO guidelines.
How to Calculate Age in Months
Calculating age in months is straightforward but requires careful handling of month boundaries. The basic approach is to count complete calendar months elapsed since birth, then account for remaining days.
Formula
Why Age in Months Matters
For children under 2 years old, development happens so rapidly that measuring age in years isn't precise enough. A 6-month-old and a 12-month-old are at vastly different developmental stages despite being less than a year apart. The CDC and WHO both publish developmental milestone charts using months as the unit of measurement up to age 24 months.
Adjusted Age for Premature Babies
For babies born prematurely (before 37 weeks gestation), pediatricians use adjusted age (also called corrected age) instead of chronological age for developmental assessments. Adjusted age is calculated from the original due date rather than the actual birth date. For example, a baby born 8 weeks early who is now 6 months old chronologically has an adjusted age of approximately 4 months — and would be expected to meet 4-month milestones, not 6-month milestones.
Common Age in Months Reference
| Age | In Months | Key Milestones |
|---|
| 3 months | 3 months | Smiles, tracks faces, holds head up |
| 6 months | 6 months | Rolls over, babbles, sits with support |
| 9 months | 9 months | Crawls, stands with help, says "mama/dada" |
| 1 year | 12 months | First steps, first words, waves bye-bye |
| 18 months | 18 months | Walks independently, 10+ words |
| 2 years | 24 months | Runs, 2-word phrases, points at pictures |
💡 Pro Tip: When sharing your child's age with a pediatrician or entering it into vaccine eligibility checks, always use complete months (whole numbers). A child who is 5 months and 28 days old is still considered 5 months old for clinical purposes — they are not yet 6 months old.
Frequently Asked Questions
How do I calculate age in months? +
Multiply the number of complete years by 12, then add the remaining complete months. For example: 2 years and 3 months = (2 × 12) + 3 = 27 months. For partial months, count the remaining days and divide by the total days in that month.
Why is age in months important for babies? +
Babies develop so quickly in the first two years that years alone aren't precise enough. The CDC and WHO track developmental milestones monthly — a 6-month-old and a 12-month-old have vastly different expected abilities. Pediatricians also use monthly age to schedule vaccines, assess growth on weight-for-age charts, and identify developmental delays early.
What is adjusted age vs chronological age? +
Chronological age counts from actual birth date. Adjusted (corrected) age counts from the original due date, accounting for prematurity. A baby born 10 weeks early who is now 6 months old has an adjusted age of about 3.5 months. Pediatricians use adjusted age for developmental assessments for premature babies typically until age 2.
At what age do doctors stop counting in months? +
Most pediatricians and developmental specialists count age in months up to 24 months (2 years). After age 2, age is typically expressed in years (e.g., "2 years 6 months" or "2.5 years"). CDC milestone checklists go up to 5 years but switch to years after 24 months.
Can I calculate future age in months? +
Yes — this calculator lets you select any "Calculate As Of" date, past or future. This is useful for planning milestone-based activities, calculating when a baby will reach 6 months for solid food introduction, or checking eligibility for programs with age cutoffs.