Calculate Age from Date of Birth in XLSForm

The quick XLSForm formula for age in years is int((today() - ${dob}) div 365.25) in a calculate row, where dob is a date question. It is close but can be one year short on or near a birthday. For an exact age, calculate completed months from the year, month and day parts with format-date(), then take int(${age_months} div 12) for years. Every example below compiles with pyxform and ODK Validate, the check ODK Central runs when you upload a form.

Download example form (.xlsx)All five examples in one XLSForm, one group each. Tested with pyxform and ODK Validate; free to reuse under CC BY 4.0.Download

What is the quick formula for age in XLSForm?

Subtracting two dates gives the number of days between them. Divide by 365.25 for years and cut off the decimals with int():

type	name	label	required	constraint	constraint_message	calculation
date	dob	What is the respondent's date of birth?	yes	. <= today()	The date of birth cannot be in the future.
calculate	age_approx					int((today() - ${dob}) div 365.25)
note	show_age_approx	Age: ${age_approx} years

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Three rules cover most mistakes:

• Divide with div, not /. XLSForm uses XPath, where / means a path.
• Wrap the whole division in int(). int() drops the decimals, which gives completed years. round() would make a 17.6-year-old an adult.
• Reference the question as ${dob}. A bare dob compiles without an error but reads nothing, so the age is blank.

This is the formula on the ODK and KoboToolbox forums, and it is fine for broad age groups. Use the exact method below when the age decides eligibility.

Why is the quick formula sometimes one year off?

A year has 365 or 366 days, and 365.25 is only the average. Someone born on 21 October 2025 is exactly 1 year old on 21 October 2026, but only 365 days have passed: 365 div 365.25 is 0.999, and int() returns 0. The same happens on many birthdays and the day or two around them, depending on how many leap days fall in between.

The mistake is rare per respondent, but a household survey records thousands of ages. Some children turn 5, some adults turn 18, and some women turn 49 on the day of the interview, and those are exactly the cut-offs that send people into or out of a module.

How do I calculate exact age in completed months?

Read the year, month and day of both dates with format-date(), then count calendar months. %Y is the 4-digit year, %n the month number and %e the day of the month, all without leading zeros:

type	name	label	required	constraint	constraint_message	calculation
today	today
date	dob	What is the child's date of birth?	yes	. <= today()	The date of birth cannot be in the future.
calculate	age_months					(int(format-date(${today}, '%Y')) - int(format-date(${dob}, '%Y'))) * 12 + int(format-date(${today}, '%n')) - int(format-date(${dob}, '%n')) - if(int(format-date(${today}, '%e')) < int(format-date(${dob}, '%e')), 1, 0)
calculate	age_years					int(${age_months} div 12)
note	show_age	Age: ${age_years} years (${age_months} months)

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The formula reads: whole years times 12, plus the difference in months, minus one month if the birthday's day of the month has not come yet. A child born on 25 March 2024 is 30 months old on 21 October 2026 (2 × 12 + 7 − 1).

int(${age_months} div 12) then gives exact completed years. There is no rounding anywhere, so the result is right on the birthday itself and in leap years. The today row is a metadata question: it records the date the form was first opened and is never shown to the enumerator.

How do I calculate age in months for children under 5?

Nutrition and immunisation surveys work in months. MUAC screening, for example, applies to children aged 6 to 59 months. Use the months calculate in the relevant column, and show the enumerator the result so they can check it with the caregiver:

type	name	label	required	constraint	constraint_message	relevant	calculation
today	today
date	child_dob	What is the child's date of birth?	yes	. <= today() and . > today() - 1826	The child must be under 5 years old.
calculate	child_age_months						(int(format-date(${today}, '%Y')) - int(format-date(${child_dob}, '%Y'))) * 12 + int(format-date(${today}, '%n')) - int(format-date(${child_dob}, '%n')) - if(int(format-date(${today}, '%e')) < int(format-date(${child_dob}, '%e')), 1, 0)
note	confirm_age	The child is ${child_age_months} months old. Please confirm this with the caregiver.
decimal	muac_cm	What is the child's MUAC in cm?	yes	. >= 6 and . <= 26	MUAC must be between 6 and 26 cm.	${child_age_months} >= 6 and ${child_age_months} <= 59

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The constraint . > today() - 1826 keeps the date of birth within the last 1826 days (5 years including a leap day), so a typing slip in the year is caught on the spot. More date checks are in the XLSForm constraint examples.

How do I show age as years and months?

Split total months into whole years (int(... div 12)) and the remaining months (mod 12):

type	name	label	required	constraint	constraint_message	calculation
today	today
date	dob	What is the respondent's date of birth?	yes	. <= today()	The date of birth cannot be in the future.
calculate	age_months					(int(format-date(${today}, '%Y')) - int(format-date(${dob}, '%Y'))) * 12 + int(format-date(${today}, '%n')) - int(format-date(${dob}, '%n')) - if(int(format-date(${today}, '%e')) < int(format-date(${dob}, '%e')), 1, 0)
calculate	age_y					int(${age_months} div 12)
calculate	age_m					${age_months} mod 12
calculate	age_text					concat(${age_y}, ' years and ', ${age_m}, ' months')
note	show_age_text	Age: ${age_text}

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Keep age_y and age_m as separate calculates. They export as numbers you can analyse, while age_text is only for the screen.

Why does the age change when I open the form again?

today() is recalculated every time the form is opened, edited or saved. A form filled in March and edited in May will show a new age, and in ODK Central or KoboToolbox an edit on the server recalculates it too. Two fixes:

• Use the today metadata row, as in the examples above. It is set once, when the form is first opened, and every age on the form uses that date.
• Use the interview date when interviews can be entered later from paper. Ask it as a date question and calculate from it instead of ${today}.

This version also handles respondents who do not know their date of birth, which is common in rural surveys. It takes the reported age when the date is unknown:

type	name	label	required	constraint	constraint_message	relevant	calculation
date	interview_date	What is the date of the interview?	yes	. <= today()	The interview date cannot be in the future.
select_one yes_no	dob_known	Does the respondent know their date of birth?	yes
date	dob	What is the respondent's date of birth?	yes	. <= ${interview_date}	The date of birth must be before the interview date.	${dob_known} = '1'
integer	age_reported	How old is the respondent in completed years?	yes	. >= 0 and . <= 120	Age must be between 0 and 120.	${dob_known} = '0'
calculate	age_from_dob					${dob_known} = '1'	int(((int(format-date(${interview_date}, '%Y')) - int(format-date(${dob}, '%Y'))) * 12 + int(format-date(${interview_date}, '%n')) - int(format-date(${dob}, '%n')) - if(int(format-date(${interview_date}, '%e')) < int(format-date(${dob}, '%e')), 1, 0)) div 12)
calculate	age_final						if(${dob_known} = '1', ${age_from_dob}, ${age_reported})
note	show_age_final	Age used in this interview: ${age_final} years

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list_name	name	label
yes_no	1	Yes
yes_no	0	No

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${age_final} is now the one age the rest of the form uses, for example ${age_final} >= 18 in the skip logic of an adult module. The if() that picks between the two is explained in the XLSForm if statement guide.

Common age calculation mistakes

These are the messages ODK Validate returns, which is what you see when ODK Central, KoboToolbox or the XLSForm converter rejects a form:

You wrote	Write instead	Error you will see
(today() - ${dob}) / 365.25	(today() - ${dob}) div 365.25	Unexpected beginning of path
DATEDIF(${dob}, today(), 'Y')	the months formula above	cannot handle function 'DATEDIF'
YEAR(today()) - YEAR(${dob})	format-date(${today}, '%Y')	cannot handle function 'YEAR'
=int((today() - ${dob}) div 365.25)	no leading =	Bad node ... in expression
int(today() - ${dob}) div 365.25)	balance the brackets	Unbalanced brackets or parentheses!
int((today() - dob) div 365.25)	${dob}	No error, but the age is blank
round((today() - ${dob}) div 365.25)	int(...)	No error, but ages are rounded up

The last two rows are the dangerous ones: the form compiles and the age is silently wrong. For other messages, see common XLSForm errors, or paste your form into the free XLSForm validator.

Does this work the same in ODK, KoboToolbox and SurveyCTO?

The logic is the same on all three: today(), int(), div, mod, if() and once() are shared. Two details differ:

Platform	What to know
ODK (XLSForm)	Use the formulas as written. format-date() and the today metadata row both work.
KoboToolbox	Same as ODK when you upload an XLSForm. In the Formbuilder, add a Calculate question and paste the expression.
SurveyCTO	Its function list documents format-date-time() for the date parts, with the same %Y, %n and %e codes, so write format-date-time(${dob}, '%Y'). Skip logic goes in a column named relevance.

format-date-time() also works in ODK, so a form written with it runs on all three platforms.

Why calculate age in the form

An age typed by the enumerator is a second answer that can disagree with the date of birth, and nobody notices until cleaning. A calculated age is consistent by design, and a note such as "The child is 30 months old" lets the caregiver correct a wrong birth year while the enumerator is still in the household. That matters most at the cut-offs that route the interview: under 5, 6 to 59 months, 15 to 49 years.

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