Tmdl standards

TMDL Syntax Standards

TMDL (Tabular Model Definition Language) is Power BI's indentation-based model definition format. Every object is declared once, properties use colon syntax, expressions use equals syntax, and indentation is always TABs — never spaces.

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Core Syntax Rules

1. TAB indentation only — Never spaces. Mixed tabs/spaces cause parse errors.
2. Object declaration: objectType objectName or objectType 'Name With Spaces'
3. Properties: propertyName: value (colon-delimited)
4. Expressions: objectType Name = expression or property =\n\t<expression> (equals-delimited)
5. Single quotes around names containing spaces, dots, colons, equals, or apostrophes → 'Order Date', 'Sales YTD', 'Customer''s Key' (double the apostrophe to escape)
6. Boolean shortcut: isHidden alone means isHidden: true — no colon needed
7. Descriptions: Triple-slash /// on the line immediately above the object declaration
8. Multi-line expressions: Indent one extra level beyond the property line
9. Case: Keywords use camelCase (dataType, formatString, isHidden). Parsing is case-insensitive but camelCase is convention.
10. Comments: Use // for inline comments inside expression blocks

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Table Structure

A table file contains the table declaration, columns, and measures. Measures may also live in a separate dedicated _Measures table.

/// Sales transaction facts
table Sales

	column SalesKey
		dataType: int64
		isHidden
		sourceColumn: SalesKey
		summarizeBy: none

	column 'Order Date'
		dataType: dateTime
		formatString: "yyyy-mm-dd"
		sourceColumn: OrderDate
		summarizeBy: none

	column SalesAmount
		dataType: decimal
		formatString: "$#,0.00"
		sourceColumn: SalesAmount
		summarizeBy: sum

	measure 'Total Sales' = SUM(Sales[SalesAmount])
		formatString: "$#,0.00"

Column Data Types

TMDL type	Description
string	Text
int64	64-bit integer
decimal	High-precision decimal (financial data)
double	Floating-point
boolean	True/False
dateTime	Date and time
date	Date only

Key Column Attributes

column MonthName
	dataType: string
	sourceColumn: MonthName
	summarizeBy: none
	sortByColumn: MonthNumber     ← sort a text column by a numeric column

column CustomerKey
	dataType: int64
	isHidden                      ← hide from report field list
	sourceColumn: CustomerKey
	summarizeBy: none             ← prevent implicit aggregation

summarizeBy values: none, sum, average, count, min, max

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Measure Syntax

All measures are deployed with a createOrReplace block targeting the _measures table. The nesting is:

createOrReplace               ← root, no indent
	ref table _measures       ← 1 TAB
		measure 'Name' = …    ← 2 TABs  (inline DAX after =)
		— or for multi-line —
		measure 'Name' =      ← 2 TABs
				DAX body      ← 4 TABs  (jumps TWO levels — no intermediate row)
			property: …       ← 3 TABs  (properties come AFTER the body)

There is no expression: keyword for measures in a createOrReplace block. The expression follows = directly on the measure name line (inline) or on the next line at 4 TABs (multi-line). Properties sit at 3 TABs after the body.

Canonical Full Example

createOrReplace

	ref table _measures

		/// Total revenue from all sales transactions
		measure '$ Revenue' = SUM(Sales[SalesAmount])
			formatString: "$#,0.00"
			displayFolder: "Revenue"

		/// Year-over-year growth as a percentage
		measure '% Revenue vs PY' =
				VAR _current = [$ Revenue]
				VAR _py =
					CALCULATE(
						[$ Revenue],
						SAMEPERIODLASTYEAR('Date'[Date])
					)
				RETURN DIVIDE(_current - _py, _py)
			formatString: "0.0%"
			displayFolder: "Time Intelligence"

When showing individual measure bodies in other examples the createOrReplace / ref table _measures wrapper is omitted for brevity — always include it in generated output.

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DAX UDFs in TMDL

UDFs use a createOrReplace block with the function declaration directly at 1 TAB — no ref table wrapper. The parameter list and body sit 2 levels deeper (3 TABs):

createOrReplace

	/// Evaluate any measure in the prior year
	function 'PriorYearValue' =
			( expression : Scalar Variant Expr, dateColumn : AnyRef ) =>
			CALCULATE(expression, SAMEPERIODLASTYEAR(dateColumn))

	/// Simple scalar — no context shift needed, Val mode is fine
	function 'CircleArea' =
			( radius : Scalar Numeric ) =>
			PI() * radius * radius

For the full UDF type system (Scalar subtypes, Val/Expr modes, AnyRef), see dax-udf/SKILL.md.

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Calculation Group Tables

Calculation groups use createOrReplace with a table declaration at 1 TAB. calculationItem entries are nested inside calculationGroup at 3 TABs. column entries are siblings of calculationGroup at 2 TABs.

Multi-line calculationItem uses = on the declaration line — no expression: property. DAX body sits 2 levels deeper (5 TABs). Properties (ordinal:) sit at 4 TABs. sourceColumn: Name — no brackets. Always include an Ordinal column.

createOrReplace

	table 'Time Intelligence'

		calculationGroup
			precedence: 10

			calculationItem 'None' = SELECTEDMEASURE()

			calculationItem 'YTD' =
					CALCULATE(
						SELECTEDMEASURE(),
						DATESYTD('Date'[Date])
					)
				ordinal: 2

		column Period
			dataType: string
			isDefaultLabel
			sourceColumn: Name
			sortByColumn: Ordinal
			summarizeBy: none

		column Ordinal
			dataType: int64
			formatString: 0
			sourceColumn: Ordinal
			summarizeBy: sum

For the complete calculation group patterns (Time Intelligence, Scenario, Currency, precedence rules, None item), see calc-groups/SKILL.md.

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Naming Conventions

Object	Convention	Examples
Fact tables	PascalCase, Fact prefix	FactSales, FactInventory
Dimension tables	PascalCase, Dim prefix	DimCustomer, DimProduct
Calculated tables	PascalCase	_Measures, CalcOffsets
Columns	PascalCase	SalesKey, OrderDate, IsArchived
Measures	'Title Case In Quotes'	'Total Sales', 'Sales vs PY %'
Display folders	Title Case	"Time Intelligence", "KPIs"

Measures with spaces in their names must be wrapped in single quotes in both the TMDL declaration and in DAX references: [Total Sales].

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formatString Quick Reference

Pattern	Result
"$#,0.00"	$1,234.56
"€#,0.00"	€1,234.56
"#,0"	1,235
"0.0%"	12.3%
"0.00%"	12.35%
"yyyy-mm-dd"	2026-01-31
"mmm yyyy"	Jan 2026
"Short Date"	1/31/2026

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Common Pitfalls

Problem	Cause	Fix
TMDL parse error	Mixed tabs and spaces	Use only TABs; check all indentation
Measure shows error	Invalid table/column name	Wrap names with spaces in single quotes: Sales[Order Date]
UDF won't evaluate in context	Wrong parameter mode	Use Expr when the argument must be re-evaluated inside CALCULATE
Calc group format wrong	Hardcoded format string	Use SELECTEDMEASUREFORMATSTRING() unless format genuinely changes

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Related References

• measures/SKILL.md — DAX measure patterns, naming conventions, format strings, display folders
• calc-groups/SKILL.md — Calculation group TMDL structure, calculationItem syntax, precedence
• dax-udf/SKILL.md — DAX User-Defined Function syntax, parameter types (Val/Expr/AnyRef)
• svg-measures/SKILL.md — SVG DAX measures, TMDL declaration patterns for image-type measures

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Version: 3.0 Last Updated: 2026-05-02