Age in Weeks, Months and Days: When Each Unit Makes Sense

We describe age in whole years for most of life, but at the beginning and in some special contexts other units are far more useful.

Why babies are counted in weeks and months

In the first year, development is fast, so a span of a few weeks matters. Paediatric check-ups, vaccination schedules and feeding guidance are usually tied to ages in weeks or months. Parents commonly say "eight weeks" or "fourteen months" for the same reason.

Converting years to months is easy; weeks are not

Months convert cleanly from years because there are always twelve in a year. Weeks are trickier because a year is not a whole number of weeks: 365 days is 52 weeks and 1 day, and a leap year is 52 weeks and 2 days. Two people born a year apart can therefore differ by 52 or 53 weeks depending on the dates involved.

Months are not fixed in length

A "month" can be 28, 29, 30 or 31 days. A child who is "three months old" born on 31 May reaches three months on 31 August, but one born on 30 November would reach "three months" on 28 February (or 29 in a leap year) — the anniversary date has to be adjusted at month ends. A good calculator handles this rather than dividing by 30.

Where weeks are the natural unit

• Pregnancy is tracked in weeks of gestation.
• Training plans for races and courses are built in weeks.
• Project and contract periods often run in whole weeks.

Getting from an exact age to weeks

Total days divided by seven gives full weeks plus leftover days. For example, 100 days is 14 weeks and 2 days. Presenting the remainder avoids the false precision of decimals like 14.29 weeks.

Choosing the unit for the audience

For a new baby, use weeks up to about three months and months after that. For a toddler, months until roughly two years, then years and months. For adults, years is normally enough, unless a form specifically asks for months.

Corrected age for premature babies

Babies born early are often described by "corrected age", counted from the original due date rather than the birth date, for developmental comparison. If a health professional gives you a corrected age, keep both figures handy, since different services may ask for one or the other.

An age calculator that shows several units at once removes the guesswork.

View age in years, months, weeks and days with Age Calculator →