Quick answer
Match the unit to the decision. Weeks and days are exact; months are calendar-shaped; hours and smaller are derived from days and are only as precise as the birth time you have.
Exact units and derived units
Days and weeks are exact. A week is always seven days, so a week count is a division of an exact quantity with an exact remainder. Nothing is assumed.
Hours, minutes and seconds are derived by multiplying the day count. They are exact as multiples of whole days — 11,594 days is exactly 278,256 hours — but they are not exact as a measure of how long you have been alive, because that depends on the time of day you were born. Without a birth time, a second-level figure is precise to the day and unknown within it.
Use this guide with the right tool: Open the Age in Specific Units Calculator for the same elapsed age represented across multiple time units. If the question shifts, compare it with the Age in Weeks Calculator or the Age in Hours Calculator.
Months are a different kind of thing
A month count is not a division; it is a calendar count. Age in months advances on the matching day of each month, so a month is 28 to 31 days depending on which month it is. Converting a day count to months by dividing gives a different answer from counting calendar months, and both are legitimately called "age in months".
For forms and records, calendar months are almost always what is meant. For statistical work, a derived figure from days may be wanted. They differ by up to a couple of days at any given moment, which matters more for infants than for adults.
Which unit suits which question
Weeks suit the first few months of life, short developmental timelines, sprint and rota planning, and anything with a weekly rhythm. Calendar months suit forms, milestone charts and anything anchored to a monthly cycle. Years suit conversation and most legal thresholds.
Hours and smaller are essentially recreational at the personal scale — interesting to see, not useful for decisions. They come into their own only in contexts where a birth time is recorded precisely, which is a small minority of cases.
Rounding at the right moment
Convert once, at the end. Rounding an age to whole years and then converting to days compounds the rounding into an error of up to six months' worth of days. Calculate the exact interval first, then express it in whatever unit you need.
Where a converted figure is approximate — a decimal month count, an hour figure without a birth time — label it as such. A number presented to six figures reads as measured, and readers reasonably assume it is.
Worked example
The example is there to show the shape of the working. Your own dates will behave the same way; the rule that governs them may not.
Common mistakes to avoid
- Treating four weeks as a month. Four weeks is 28 days; no calendar month is. The two counts diverge immediately.
- Presenting an hour or second figure as exact. Without a birth time it is precise to the day only. Say so.
- Rounding before converting. Convert once from the exact interval. Rounding first compounds into a large error.
Use the right calculator
These are the practical next step. Each opens with the calculator, states its method, and links onward when the question turns out to be a different one.
Frequently asked questions
Which unit do baby forms want?
Usually calendar months, occasionally weeks in the first few months. Give the calendar figure rather than a converted one.
Is age in seconds meaningful?
As a multiple of whole days, yes. As a measure of elapsed life, only if you know your birth time and time zone.
Why does age in months vary between tools?
Some count calendar months, some divide the day count by an average. Check the stated method.