Free forever, online, no sign-up. Calculate chronological age from date of birth in seconds.

Last updated: 4 October 2026

How to Use the Calculator

1. Enter the date of birth.
2. Enter the reference date in the As-Of Date field. It starts on today’s date.
3. Press Calculate, then use the Copy button to paste the Y;M;D value into your protocol or scoring software.

What Each Result Means

Result	Example	Used for
Years, months, days	7 years, 3 months, 14 days	Reports, parent letters, records
Y;M;D	7;3;14	Most standardized test protocols
Total months	87 months	Screeners that select forms by month, toddlers
Total weeks and days	380 weeks	Infant records, research logs
Day of the week born, next birthday	Tuesday, 51 days	Personal use

What Is Chronological Age?

Chronological age is the time that has passed between a person’s date of birth and a reference date, counted in completed years, months and days. The reference date is usually today, but in testing it’s the day the test was given. A child shown as 7;3;14 has finished 7 years and 3 months and is 14 days into the next month.

Chronological Age vs Biological, Developmental, Mental, Corrected and Bone Age

Chronological age is the yardstick. The other ages describe a person against it.

Type of age	What it measures	How it’s found
Chronological	Time since birth	Date subtraction
Corrected (adjusted)	Age counted from the due date for a baby born early	Chronological age minus weeks born early
Developmental	The level of a child’s skills	Developmental assessment
Mental	Level of intellectual performance	Older intelligence tests
Biological	How fast the body is aging	Biomarkers, health measures
Bone	Skeletal maturity	Hand and wrist X-ray

A 6-year-old whose skills match a typical 4-year-old has a chronological age of 6 and a developmental age near 4. That gap is what clinicians look for, and it only means something if the chronological age is right. Mental age once fed the ratio IQ formula: mental age divided by chronological age, times 100. Modern intelligence tests replaced that formula with standard scores.

Chronological Age Formula and Manual Calculation

The chronological age formula is:

Test date − Date of birth = Years; Months; Days

Write both dates as year, month, day, with the test date on top. Subtract days first, then months, then years. When a column comes out negative, borrow from the column to its left.

The Borrowing Rules

If this happens	Do this
Test day is smaller than birth day	Take 1 from the test month and add the number of days in the month before the test month to the test day
Test month is smaller than birth month	Take 1 from the test year and add 12 to the test month

Worked Examples

Example 1, borrowing days. Born 15 March 2015, tested 10 September 2024.

	Year	Month	Day
Test date	2024	9	10
Date of birth	2015	3	15

Days: 10 minus 15 is negative, so borrow a month. September becomes 8, and August’s 31 days go onto the 10, making 41. Then 41 − 15 = 26 days. Months: 8 − 3 = 5. Years: 2024 − 2015 = 9. The chronological age is 9;5;26.

Example 2, borrowing days and months. Born 20 November 2018, tested 5 February 2025.

Days: 5 minus 20 is negative, so February becomes 1 and January’s 31 days are added: 36 − 20 = 16. Months: 1 minus 11 is negative, so borrow a year. 2025 becomes 2024 and the month becomes 13: 13 − 11 = 2. Years: 2024 − 2018 = 6. The result is 6;2;16, or 74 total months.

You can check any answer by counting forward. From 20 November 2018, add 6 years (20 November 2024), then 2 months (20 January 2025), then 16 days. You land on 5 February 2025.

Edge Cases: Leap-Day Birthdays, Month-End and Year-End

Situation	Date of birth	Test date	Result with the borrowing method	What other tools may show
Leap-day birthday	29 Feb 2016	28 Feb 2025	8;11;30	9;0;0, if 28 February counts as the birthday in non-leap years
Leap-day birthday	29 Feb 2016	1 Mar 2025	9;0;0	Same
Born on the 31st	31 Jan 2020	29 Feb 2024	4;0;29	4;1;0, if the last day of February counts as a full month after 31 January
Across New Year	31 Dec 2019	1 Jan 2025	5;0;1	Same

Why Two Calculators Can Give Different Answers

If you’ve run the same dates through two tools and got results a day or two apart, neither one is broken. They follow different rules, and no competitor page explains which rule to trust.

The main cause is how many days get borrowed. The standard method adds the real length of the month before the test date. Some test manuals tell examiners to borrow a flat 30 days instead. Born 15 March 2015 and tested 10 March 2024, the real-length method borrows February 2024’s 29 days and gives 8;11;24. The flat-30 method gives 8;11;25. Leap-day birthdays and month-end dates, shown in the edge-case table, account for most of the remaining mismatches.

Follow the method your test manual describes, because its norm tables were built with it. A one-day difference changes nothing unless the age sits within a few days of a norm-band change. In that case, work the age out by hand with the manual’s method before you score. This calculator uses the real-length method [CONFIRM].

Do You Round Chronological Age?

Usually not. Most current norm tables use completed age, so a child who is 7;5;29 is scored in the band that contains 7 years 5 months, even though they’re one day from 7;6. Rounding up would put them in the wrong band.

A few older protocols do tell examiners to round up when the days reach 15 or 16. If your manual says that, follow it. If the manual is silent, don’t round.

How to Write Chronological Age on a Report

Record the full value next to the test date. These forms are all common:

• 7;5;29 (the semicolon format most US publishers use)
• 7-5-29 or 7:5:29
• 7 years, 5 months, 29 days (best for parent-facing reports)
• CA: 7;5 when the protocol only has boxes for years and months

Chronological Age Calculator for Testing and Assessments

Every norm-referenced assessment compares a child’s raw score with children of the same age. Those children are grouped into age bands, and each band has its own norm table. The age you record decides which table you open.

Take a child who is 5;11;30 on test day. One day later they’re 6;0;0 and fall in a different band. The same raw score can convert to a different standard score and percentile rank in each band, which can change an eligibility decision or an IEP goal.

Assessment Age Format Reference

Assessment	Publisher	Age format to record
WISC-V, WPPSI-IV, WAIS-IV	Pearson	Years;Months (record full Y;M;D)
WIAT-4	Pearson	Years;Months (record full Y;M;D)
CELF-5, PLS-5	Pearson	Years;Months (record full Y;M;D)
PPVT-5, EVT-3	Pearson	Years;Months (record full Y;M;D)
GFTA-3	Pearson	Years;Months;Days
Bayley-4	Pearson	Years;Months;Days (corrected age under 24 months)
BRIGANCE IED III / Screens III	Curriculum Associates	[VERIFY: Y;M;D or total months]
ASQ-3, ASQ:SE-2	Brookes Publishing	Months and days
Super Duper tests	Super Duper Publications	Years;Months;Days
Pro-Ed, WPS, Academic Therapy tests	Various	Years;Months;Days

Pearson Chronological Age Calculator Replacement

Pearson no longer hosts a live age calculator. Its old calculator page now sends visitors to a third-party date tool that Pearson says it isn’t affiliated with, and that tool returns a duration between two dates, not a Y;M;D age.

This page replaces Pearson’s old tool for every Pearson test in the table above, and the value it gives can go straight onto the record form or into Q-global.

Super Duper Chronological Age Calculator

Super Duper Publications runs its own calculator, but it has no body copy, no total months and a test-date menu that starts at recent years only. That makes it awkward for scoring an older protocol or checking a past evaluation. This calculator accepts any test date and shows total months beside the Y;M;D value Super Duper’s normed tests require.

BRIGANCE Chronological Age Calculator

BRIGANCE record books ask for the child’s age on the day of testing, and early childhood screens pick the right form by age range. This calculator shows Y;M;D and total months side by side, so whichever value your record book asks for is already on screen. A child aged 4 years 7 months, for example, is 55 months.

ASQ Age Calculator (ASQ-3 and ASQ:SE-2)

The Ages and Stages Questionnaires choose the questionnaire interval by the child’s age in months and days. ASQ adds one rule of its own: a child born 3 or more weeks early is screened on corrected age until 24 months.

Brookes Publishing runs an official ASQ age calculator, and it’s the safest choice for picking the interval because it applies ASQ’s rules directly. Use this calculator to check the age you’ll write on the form, or when the same assessment also needs Y;M;D for another test.

Corrected Age Calculator for Premature Babies

Corrected age, also called adjusted age, is how old a baby born early would be if they’d arrived on their due date. It gives a fairer comparison with developmental milestones in the first two years.

Corrected age = chronological age − weeks born early

Weeks born early = 40 minus the baby’s gestational age at birth. A baby born at 32 weeks is 8 weeks early. At 6 months chronological age, their corrected age is about 4 months. To get it from this calculator, enter the due date in place of the birth date.

Three rules catch people out:

• Most pediatric guidance stops correcting at about 24 months, by which point most premature children have caught up.
• Vaccinations follow chronological age, not corrected age.
• For school entry and legal documents, chronological age is the only one that counts.

How Many Months Old Is My Child?

Multiply completed years by 12 and add the leftover months. A child aged 3 years 4 months is 40 months old.

Age	Months	Age	Months
1 year	12	9 years	108
2 years	24	10 years	120
3 years	36	11 years	132
4 years	48	12 years	144
5 years	60	13 years	156
6 years	72	14 years	168
7 years	84	15 years	180
8 years	96	16 years	192

Going the other way, 35 months is 2 years 11 months, 36 months is exactly 3 years, and 18 months is 1 year 6 months.

Everyday Uses: School Cut-Offs, Forms and Past Dates

The same calculation answers plenty of questions outside the clinic.

School enrollment. Schools judge age on a cut-off date, not the day you apply, so set the As-Of Date to the cut-off. In England and Wales, the school year group is set by age on 31 August. In the US and Australia, cut-offs vary by state, so check your district’s date.

How old was I, or how old is my son or daughter, on a certain day? Put that date in the As-Of Date field. This is useful for a medical history, a legal form, or the date a photo was taken.

When will my child reach a certain age? Add the target years and months to the date of birth. A child born 12 April 2011 turns 15 years 9 months on 12 January 2027.

Calculate Chronological Age in Excel or Google Sheets

If you track ages for a whole caseload, a spreadsheet saves time. With the date of birth in A2 and the test date in B2:

Value	Formula
Years	=DATEDIF(A2,B2,”y”)
Months	=DATEDIF(A2,B2,”ym”)
Days	=B2-EDATE(A2,DATEDIF(A2,B2,”m”))
Total months	=DATEDIF(A2,B2,”m”)
Y;M;D in one cell	=DATEDIF(A2,B2,”y”)&”;”&DATEDIF(A2,B2,”ym”)&”;”&(B2-EDATE(A2,DATEDIF(A2,B2,”m”)))

Both Excel and Google Sheets support these functions. Skip the “md” option of DATEDIF for days. Microsoft’s own documentation warns it can return wrong results, which is why the days formula uses EDATE. Format the Days cell as a number, not a date, and spot-check birthdays on the 29th, 30th or 31st against this calculator.

Before You Record the Age: A Quick Check

Most wrong ages on a protocol come from the input, not the arithmetic.

1. Date of birth from the record, not memory. Copy it from the school file, medical record or birth certificate. If none is available, don’t estimate, because a guessed birth date makes the norm comparison unreliable.
2. Day and month in the right order. The calculator’s fields are separate, so the risk is in reading the source record. 03/04 is 3 April in the UK and 4 March in the US, and a swap throws the age off by weeks.
3. The test date, not the scoring date. Protocols are often scored days later. Set the As-Of Date to the day the child sat the test.
4. More than one session? Note every testing date and use the one your manual names for scoring.

Frequently Asked Questions

Is chronological age the same as calendar age?

Yes. Calendar age, actual age and chronological age all mean the time since birth measured by the calendar.

Can you change your chronological age?

No. Chronological age moves at the same rate for everyone, and diet, exercise or health can’t alter it. Biological age is the one lifestyle can affect.

Does the day of birth count as day one?

No. Chronological age counts elapsed time, so on the day a baby is born their age is 0;0;0. They’re 0;0;1 the next day.

How do I work out age from a birth year only?

Subtract the birth year from the current year, then take off 1 if this year’s birthday hasn’t happened yet. Someone born in 2014 is 12 after their 2026 birthday and 11 before it.

Does this work for UK and Australian assessments?

Yes. The calendar math is the same everywhere, so the result applies to UK standardisations such as CELF-5 UK and BPVS-3, and to Australian testing and NDIS early intervention paperwork.

Is chronological age measured in years or months?

Both. Most school-age tests record years, months and days, while infant and toddler screeners usually work in total months. The calculator shows both from one entry.