| name | tmdl-standards |
| description | TMDL syntax ground rules for Power BI semantic models. Use this whenever writing or reviewing TMDL for tables, columns, measures, DAX UDFs, or calculation groups. Always consult before generating any TMDL — these rules govern indentation, object declaration, property syntax, and measure formatting. For UDF type system and examples see dax-udf/SKILL.md. For calculation group patterns see calc-groups/SKILL.md.
|
| version | 3 |
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.
Core Syntax Rules
- TAB indentation only — Never spaces. Mixed tabs/spaces cause parse errors.
- Object declaration:
objectType objectName or objectType 'Name With Spaces'
- Properties:
propertyName: value (colon-delimited)
- Expressions:
objectType Name = expression or property =\n\t<expression> (equals-delimited)
- Single quotes around names containing spaces, dots, colons, equals, or apostrophes
→
'Order Date', 'Sales YTD', 'Customer''s Key' (double the apostrophe to escape)
- Boolean shortcut:
isHidden alone means isHidden: true — no colon needed
- Descriptions: Triple-slash
/// on the line immediately above the object declaration
- Multi-line expressions: Indent one extra level beyond the property line
- Case: Keywords use camelCase (
dataType, formatString, isHidden). Parsing is case-insensitive but camelCase is convention.
- Comments: Use
// for inline comments inside expression blocks
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
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.
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.
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.
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].
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 |
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 |
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
Version: 3.0
Last Updated: 2026-05-02
