calculate – Nextra

The calculate question type runs a formula silently in the background. It stores the result in a named field that other questions can reference — but it shows no input or label to the enumerator. Use it to derive values, concatenate strings, run conditional logic, or look up data that you need later in the form.

Basic syntax

type	name	label	calculation
calculate	total		`${price} * ${quantity}`

type: Always calculate.
name: A unique field name. Other questions reference this with ${name}.
label: Leave blank — calculate fields are never shown to the enumerator.
calculation: The formula to evaluate. Supports all XLSForm functions and field references.

Common patterns

Arithmetic

type	name	label	calculation
integer	price	Unit price
integer	quantity	Quantity
calculate	total		`${price} * ${quantity}`
note	total_note	Total: `${total}`

String concatenation

type	name	label	calculation
text	first_name	First name
text	last_name	Last name
calculate	full_name		`concat(${first_name}, ' ', ${last_name})`

Conditional value

type	name	label	calculation
integer	score	Score (0–100)
calculate	grade		`if(${score} >= 90, 'A', if(${score} >= 75, 'B', if(${score} >= 60, 'C', 'F')))`

Age from date of birth

type	name	label	calculation
date	dob	Date of birth
calculate	age_years		`int((today() - ${dob}) div 365.25)`

Using `once()` to lock the value

By default, calculate re-evaluates whenever any referenced field changes. Wrap the formula in once() to evaluate only on first load and freeze the result:

type	name	label	calculation
calculate	interview_start		`once(now())`

Use once() for timestamps, initial values, or any case where recalculation on change is undesirable.

Pulling data from external sources

From a CSV file

pulldata('staff.csv', 'department', 'name', ${staff_id})

See pulldata() for the full reference.

From a bundled SQLite database

pulldata('rawquery',
  concat(${family_path}, '/locations.db::provinces'),
  'SELECT name FROM provinces WHERE id = ?',
  ${province_id})

See Local Database Search for path construction details.

From app context

pulldata('app-api', 'user.username')
pulldata('app-api', 'family_path')

See App API for all available keys.

Using calculate in relevant and constraint

Other fields can reference a calculate result just like any field:

type	name	label	relevant	constraint
calculate	bmi			`${weight} div (${height} * ${height}) * 10000`
note	bmi_warning	BMI is very high — please verify weight and height.	`${bmi} > 40`

Best Practices

Give calculate fields meaningful names — total_cost is clearer than calc1 and easier to trace in exports.
Break complex formulas into multiple chained calculate fields instead of one deeply nested expression.
Use once() for timestamps and initial snapshots to prevent values from changing after first capture.
Place calculate fields before the first question that references them — evaluation order follows form order.
Keep the label column empty. Some tools display the label; leaving it blank avoids confusing output.

Limitations

Calculate fields are never displayed directly to the enumerator — use a note field to show the result.
The calculation column is required; a calculate row with no formula always stores an empty string.
Results re-evaluate on every form change unless wrapped in once() — very complex formulas in large forms can affect rendering performance on low-end devices.
Calculated values are stored as strings in the submission. Use number(), int(), or boolean() to cast when passing the result to arithmetic or logic expressions.

note meta