Chronological age calculator, exact to the day.
Enter a date of birth and a test date and read the chronological age in years;months.days — the form norm tables are banded by — along with the years-and-months figure, the rounded figure and the age in whole months. Nothing is sent anywhere, so it is safe to use with a real case.
Chronological age
Enter a date of birth
Results appear as you type. Nothing is sent anywhere.
CA notation—
- Age in whole months
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- Next birthday
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Every form of the figure
The same age, in each unit a manual might ask for
Copy the one your instrument specifies. The exact years;months.days figure is the safest default — rounding is a rule some manuals impose, not a property of the age itself.
Chronological age (Y;M.D)
The form norm tables are banded by.
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Years and months only
Days dropped, never rounded.
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Rounded to the nearest month
Rounds up at 16 days. Only use it if the manual asks for it.
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Age in whole months
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Decimal age
Growth charts and sports age-grading use this.
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Weeks and days
Used for infants and for gestation-corrected age.
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Total days
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Next birthday
Worth knowing if testing runs across an age band.
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The manual’s 30-day method, for comparison
Manual method result
Columns subtracted borrowing a flat 30 days for a month.
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Against the calendar figure
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If your protocol form was filled in by hand and disagrees with the headline figure by a day, this is almost always why.
What chronological age is, and what it is not
Chronological age is elapsed calendar time between birth and a reference date. Nothing more. It is the denominator of almost every developmental comparison: a mental age, a reading age, a language age or a bone age only means something set against the chronological age of the person it describes. A seven-year-old with a reading age of nine is reading two years ahead; the chronological age is what “ahead” is measured from.
Because it selects the norm table, it is also the one number in a test protocol where an arithmetic slip propagates silently into every derived score. A standard score, a percentile and an age equivalent can all be internally consistent and still be wrong, because they were read off the wrong page.
Reading the Y;M.D notation
Test manuals write chronological age as years;months.days. The punctuation is deliberate: the semicolon marks a boundary between units that are not decimal, so nobody can read the months as a fraction of a year.
| Written | Means | Not to be confused with |
|---|---|---|
| 7;4.12 | 7 years, 4 months, 12 days | 7.412 years |
| 7;4 | 7 years and 4 months, days not stated | 7.4 years, which is 7 years 5 months |
| 0;11.29 | 11 months and 29 days — still under one year | 1;0.0 |
| 16;0.0 | Exactly sixteen, tested on the birthday | 15;11.30 |
Some publishers use a colon or a hyphen instead of the semicolon, and a few write months with a leading zero. The reading is the same.
Working it out by hand
Write the test date above the date of birth in year, month, day columns and subtract each column, borrowing from the left when a column goes negative. Take a child born on 14 November 2018, tested on 2 April 2026.
| Row | Year | Month | Day |
|---|---|---|---|
| Test date | 2026 | 4 | 2 |
| Date of birth | 2018 | 11 | 14 |
| Chronological age | 7 | 4 | 19 |
The day column is 2 − 14 = −12, so borrow the length of March, the month before the test date: 31 − 12 = 19 days, and the months drop by one. The month column is then 3 − 11 = −8, so borrow twelve months from the years: 4 months, and the years drop to 7. The CA is 7;4.19.
A manual using the flat 30-day borrow would get 30 − 12 = 18 days and report 7;4.18 — one day less, because March is 31 days long and the convention pretends it is 30. Neither figure is a mistake; they answer slightly different questions. Follow whichever your instrument’s norms were built on, and be consistent across a caseload.
Rounding, and when not to
Most current instruments band their norms on the exact chronological age, so the days matter and rounding is wrong. A norm table headed 7:4–7:7 covers every age from 7;4.0 up to 7;7.30 — a child at 7;7.29 belongs in that table even though rounding would push them into the next one.
Where a manual does ask for rounding, the usual rule is to round the months up when 16 or more days have passed and down otherwise. That figure is on this page as “rounded to the nearest month”. Use it only on instruction: applying it to a test that expects exact age moves a share of children into the wrong band, and the effect is largest exactly where norms change fastest, which is early childhood.
Corrected age for preterm infants
For a baby born before 37 weeks gestation, developmental norms are usually read against corrected age rather than chronological age. Subtract the weeks of prematurity — 40 weeks minus the gestational age at birth — from the chronological age.
A baby born at 30 weeks is ten weeks early. At a chronological age of 24 weeks, the corrected age is 14 weeks. The weeks-and-days figure above gives you the chronological side of that subtraction directly. Correction is conventionally applied until a chronological age of two years, after which the difference is small enough that most instruments stop asking for it.
Where chronological age goes wrong
- Scoring date instead of test date. The age is fixed at administration. Scoring a week later does not make the child a week older.
- An ambiguous date on the referral. 04/05/2019 is two different birth dates depending on who wrote it. The boxes here are labelled Day, Month and Year individually, so the order on screen cannot change the value.
- Rounding by habit. Rounding a test that bands on exact age is the most common way a child ends up in the wrong norm table.
- Mixed dates across a battery. If sessions ran a fortnight apart, one age was used for the whole battery or one per session — record which.
- Transcription. The CA travels from protocol to scoring software to report. Copy it once, from one place. The “Copy results” button gives you the birth date, test date and CA as a single block to paste.
Who needs it
Where an exact chronological age is the whole ballgame
Anywhere a score is read from an age-banded table, the age has to be right before anything downstream can be.
Cognitive and educational assessment
WISC, WPPSI, WAIS, WIAT, Woodcock-Johnson, KABC and the rest select a norm table from the child’s chronological age at the moment of testing. A one-day error can move a child into the neighbouring age band and shift a standard score.
Speech and language therapy
CELF, PLS, GFTA and language sample norms are all age-banded, often to the month. Reports usually quote the CA in Y;M.D beside the test date so the band is auditable later.
Paediatrics and growth charts
Height, weight and BMI centiles are plotted against decimal age. For infants born preterm, the corrected age — CA minus the weeks of prematurity — is what gets plotted for the first two years.
School placement and eligibility
Entry cut-offs, retention decisions and special-education eligibility all turn on age at a fixed date rather than age today. Enter that date as the test date.
Sports age grouping
Age-graded competition uses age on a cut-off date, or a decimal age at the date of the event. Both are on this page.
Research and data cleaning
Coding age at assessment across a sample needs one rule applied consistently. Whole months, decimal years and exact Y;M.D are each defensible; mixing them within a dataset is not.
FAQ
Questions about chronological age
What is chronological age?
Chronological age is the time elapsed between a person’s date of birth and a reference date, expressed in calendar units. It is what standardised tests mean by “age” — as opposed to mental age, developmental age, bone age or reading age, which are all scores derived by comparing performance against what is typical at a given chronological age.
What does 7;4.12 mean?
Seven years, four months and twelve days. The semicolon separates years from months and the full stop separates months from days. The notation exists so the figure cannot be misread as a decimal: 7;4 is seven years and four months, whereas 7.4 years is closer to seven years and five months.
Which date do I use as the test date?
The date the test was actually administered, not the date you are scoring it. If a battery ran over two or three sessions, most manuals tell you to use the date of the first session; a few ask for the date of the subtest being scored. Check the manual before assuming.
Should I round chronological age to the nearest month?
Only if your manual says so. Most current tests band their norms on the exact years;months.days figure, so rounding would put some children in the wrong table. A minority of older instruments round up when 16 or more days have passed. Both figures are shown above so you can follow whichever rule applies.
Why does my manual’s date table give a slightly different answer?
Many test manuals show the subtraction with a flat 30-day borrow: if the day column is negative, add 30 days and take one from the months column. Real calendar months are 28 to 31 days, so the two methods agree only when the month before the test date happens to have 30 days. This page shows both — the calendar figure as the headline and the manual method beside it — and they differ by at most a day or two.
How do I calculate corrected age for a premature baby?
Take the chronological age and subtract the number of weeks the birth came before 40 weeks gestation. A baby born at 32 weeks is eight weeks early, so at a chronological age of six months the corrected age is four months. Correction is conventionally applied until 24 months. Use the weeks-and-days figure above and subtract the prematurity in weeks.
What happens with a 29 February date of birth?
A leap-day birth has no anniversary in a common year. This calculator follows the widespread civil convention and treats 1 March as the anniversary in non-leap years. The days figure changes by one either way, which almost never crosses an age band — but if the child sits exactly on a band boundary, check how the manual handles it.
Is the date of birth I enter sent anywhere?
No. The arithmetic runs entirely in your browser. Nothing is uploaded, logged or stored on a server. The dates do appear in the page URL so a result can be bookmarked or shared, so clear the field before handing the screen to someone else if the case is identifiable.