A static Revit family is just a dumb 3D block. Revit formulas are what transform it into intelligent, responsive BIM. You alter the overall frame height, and the shelf counts, panel divides, glass clearances, and visibility toggles recalculate themselves in milliseconds. Below is your complete field guide to formula syntax, conditional if statements, roundup/rounddown math, trigonometry for angles and slopes, and the dimensional algebra behind “Inconsistent Units” errors. All formulas are tested and verified for Autodesk Revit 2027.
Quick Answer (TL;DR)
In the Family Editor, open Create tab > Family Types and type your equation directly into the parameter’s Formula cell.
A decision is written if(condition, result if true, result if false), for example if(Length < 3000 mm, 200 mm, 300 mm). To round up, use roundup(x). Always append a unit to any number representing a physical dimension.
Revit Parameter Formulas: What They Are and Where to Type Them
Autodesk describes formulas as mathematical and logical relationships where one parameter derives its value from others. Typical production applications include calculating areas and volumes, driving clearances by overall element size, calculating whole counts of arrayed items, and adding mullions or structural diagonal braces as geometry scales.
- Inside Families (The Primary Engine): In the Family Editor, open Create tab → Properties panel → Family Types. Every parameter features a dedicated Formula column. This is where Revit family formulas live, and where this guide focuses.
- Inside Schedules (Calculated Values): Used to calculate costs, occupant densities, or areas on the fly using fields already present in your schedule.
- Not Available in Standard Project Parameters: Native project parameters in the project environment do not possess a Formula cell. Calculations must take place in the loadable family or inside a schedule.
A parameter driven by a formula referencing two or more other parameters is intentionally grayed out in the Value column, because Revit computes it automatically. Leave the formula cell blank for any dimension you want to type manually.
Autodesk’s examples occasionally begin with an equals sign, such as =if(Length < 3000 mm, 200 mm, 300 mm). Modern Revit releases accept formulas written without the leading =. If your dialog prompts an error, add the equals sign.
Revit Formula Syntax: Operators and Functions
Revit’s formula syntax is compact and strictly typed. Below is the complete catalog of operators and functions supported natively in Revit:
Three Golden Habits to Prevent Syntax Errors:
- Parameter names are strictly case-sensitive: If your parameter is named
Width, writingwidthwill fail with an “unknown parameter” prompt. Copy names directly from the dialog. - Avoid hyphens in parameter names: Autodesk advises avoiding dashes (
-) in parameter labels because Revit reads them as minus operators. Use underscores (Frame_Width) or CamelCase. - Append explicit units to constants: Write
Width + 50 mm, notWidth + 50.
Revit Formulas for Everyday Use
These eight production formulas cover the vast majority of family modeling challenges. The receiving parameter type must match the calculated result:
Revit If Statements (Conditional Statements)
An if() statement allows a family to make automated choices. It always contains three parts separated by commas:
Example: if(Length < 3000 mm, 200 mm, 300 mm)
Translation: “If Length is less than 3000 mm, return 200 mm; otherwise return 300 mm.” Both result values must share the same data type (both lengths, both integers, or both text strings).
Combine tests with and, or, not
if(and(Width > 600 mm, Width < 1200 mm), 1, 0): Returns 1 only when both tests are true.if(or(Height > 2400 mm, Has Transom), 2, 1): Returns 2 when either condition is met.not(Has Transom): Inverts a Yes/No parameter.
“Less than or equal” does not exist: use not()
Autodesk’s conditional statements documentation notes that <= and >= are not implemented in Revit. To say “2400 mm or less”, invert the comparison: not(Height > 2400 mm).
More than two outcomes: nest the if
if(Length < 3000 mm, 200 mm, if(Length < 4000 mm, 300 mm, 500 mm))
Each opening if( requires a matching closing parenthesis ). Beyond three nested levels, configure catalog types instead to avoid fragile formulas.
Return text
An if() formula can return text when mapped to a Text parameter: if(Width > 1200 mm, "Wide", "Standard").
Yes/No Parameters in Revit Formulas
A Yes/No parameter represents a boolean checkbox in the Properties palette:
- Yes/No as the result: Type the comparison directly in the formula cell of a Yes/No parameter (e.g.
Lights > 1). Do not wrap it in anif()—the comparison itself evaluates directly to true or false. - Yes/No for visibility: Select geometry in the family, find Visible in Properties, click the small associate button, and link it to your Yes/No parameter.
- Yes/No as the input condition:
if(Has Transom, 300 mm, 0 mm)returns 300 mm when checked and 0 mm when unchecked.
Syntax Trap: The words “Yes” and “No” do not work as raw values in formulas. Never write if(Has Transom = "Yes", ...).
Revit Round Up Formula (roundup, rounddown, round)
Rounding turns continuous decimal math into discrete whole numbers (e.g., stairs, panels, shelves):
Shelf count rounding up: roundup(Height / Shelf Spacing) (Destination: Integer).
Round a length to a step, such as 50 mm
To snap dimensions to modular manufacturing steps, divide by the increment, round, and multiply back:
The Unit Rules Behind Most Formula Errors
Most broken formulas violate dimensional algebra. Autodesk sets out four foundational rules:
- Addition and subtraction: Both sides must share identical unit types (Length + Length = Length).
- Multiplication: Units multiply:
Length * Length = Area;Length * Area = Volume. - Division: Units cancel:
Length / Length = Unitless Number. - Trigonometry: Angles go in; unitless numbers come out.
Need to convert a Length into a raw Number without triggering “Inconsistent Units”? Divide it by 1 of its unit: Length / 1 mm (or Length / 1'). The units cancel out cleanly! To convert a Number back to Length, multiply by 1 unit: Raw_Number * 1 mm.
If project display units are mixed up, review our guide to units in Revit.
Using ChatGPT or Claude to Write a Revit Formula
AI tools like ChatGPT and Claude are fast at converting logic sentences into code, but they default to Microsoft Excel syntax unless constrained.
Excel habits that break in Revit
A prompt that gets a usable formula
Copy this scoped prompt into ChatGPT or Claude to prevent Excel hallucinations:
I need a formula for the Formulas column of the Family Types dialog in Autodesk Revit. CRITICAL RULES: 1. This is Revit, NOT Microsoft Excel. 2. Supported syntax ONLY: + - * / ^, comparisons < > =, if(test, a, b), and(), or(), not(), round(), roundup(), rounddown(), sqrt(), abs(), pi(). 3. <= and >= DO NOT EXIST. Invert using not() (e.g. "not(X > Y)"). 4. Append explicit units on all constants (e.g. 50 mm). 5. Parameter names are case-sensitive. MY PARAMETERS: - [Insert Parameter 1] ([Data Type, e.g. Length]) - [Insert Parameter 2] ([Data Type, e.g. Integer]) - Target Output Parameter: [Insert Parameter 3] ([Data Type]) RULE IN WORDS: [State your rule, e.g. "The number of shelves is height divided by spacing, rounded up, but never less than 2."] OUTPUT REQUIRED: Provide the single-line formula and a 3-case test verification table.
The three-minute check before you trust any formula
- Paste into a spare family copy, not your production model. Click Apply in Family Types.
- Test three values: Normal size, smallest conceivable, and largest.
- Test the threshold edge: If your condition is
Width > 900 mm, test with 900 mm and 901 mm. - Flex the 3D geometry: Verify that the 3D model moves cleanly without breaking constraints.
Reading a formula someone else wrote
When inheriting a complex family with nested brackets, paste the equation into Claude or ChatGPT and ask: “Explain this Revit family formula in plain English step-by-step, and identify which parameter acts as the driver.”
Families and Parametric Design
Formulas are the final step after mastering reference plane skeletons, dimension labels, and parametric geometry.
Formulas in Revit Schedules
To use mathematical formulas in schedules, create a Calculated Value (inside Schedule Properties > Fields). Syntax is identical to family formulas, with one rule: calculated values can only reference fields already placed in that schedule. See our complete guide to schedules in Revit.
Why Is My Revit Formula Not Working?
“Inconsistent units”
An operation broke dimensional multiplication rules (e.g. adding a Length to an Area, or outputting an Area into a Length parameter). Ensure all numbers have explicit units (50 mm) and the target parameter type matches the dimensional product.
The if statement is rejected
Verify that both results share the exact same data type, count opening and closing parentheses, and confirm you haven’t typed <= or >=.
Revit says the parameter is not recognised
Check case-sensitivity and hyphens. Width ≠ width, and Frame-Width is misread as Frame minus Width.
Circular chain of references
Parameter A derives from B, and B derives from A. Map dependencies in a single direction.
The formula is accepted but nothing moves
A formula only calculates a number. That number must be assigned as a label to a dimension string or element property. See our deep dive on reference planes in Revit.
Parameters that must appear in both schedules and tag labels should be configured as shared parameters: see Revit shared parameters.
Revit Formula FAQs
Where do I write a formula in Revit?
In the Family Editor, open Family Types on the Create tab and type it in the Formulas column next to the parameter. In a schedule, use a calculated value. A normal project parameter has no formula column.
How do I write an if statement in Revit?
Use if(test, value if true, value if false), for example if(Length < 3000 mm, 200 mm, 300 mm). Combine tests with and(), or() and not(), and nest one if inside another for more than two outcomes. Both results must share the same data type.
How do I use a Yes/No parameter in a Revit formula?
To get a Yes/No result, type a comparison directly in the formula cell of the Yes/No parameter (e.g. Lights > 1). To use a Yes/No as an input test, write if(Has Transom, 300 mm, 0 mm).
Does Revit have less than or equal to in formulas?
No. Autodesk’s help states that <= and >= are not implemented. Use not() with the opposite test: not(Height > 2400 mm) means 2400 mm or less.
How do I round up in a Revit formula?
Use roundup, for example roundup(Height / Shelf Spacing) with an Integer parameter. To round a length up to a step, divide by the step, round up and multiply back: roundup(Width / 50 mm) * 50 mm.
Why does Revit say inconsistent units?
The formula breaks a unit rule: an addition mixes unit types, a multiplication does not give the expected unit, or the parameter type does not match the result. Write a unit on every constant and set the parameter type to match the result.
Can ChatGPT or Claude write Revit formulas?
Yes, but they often answer in Excel syntax. Say the formula is for Revit, list the allowed functions and your parameter names, and ask for test cases. Then test the formula in a spare family with three values before you use it.
Are Revit formulas case-sensitive?
Parameter names are. Autodesk’s help gives the example that Width must be typed with a capital W, and width is not recognised. Copy the name from the Parameters column to avoid a mistake.
Sources & Recommended Reading
- Autodesk Help Documentation: Valid Formula Syntax & Abbreviations and Conditional Statements in Formulas.
- Paul F. Aubin: Author of Renaissance Revit: Creating Classical Architecture with Modern BIM. For advanced parametric curves, right-triangle geometry, and classical orders, explore his tutorials and publications at paulaubin.com.
- Revit Forum Community: Historical community threads and peer-verified parameter test cases from the RevitForum.org family creation archives.
Mohamed Fakhry has helped thousands of architects and designers find their next project with step-by-step guidance on his blog, mashyo.com. About Mohamed
His journey began during his university years when he recognized the need for information-rich 3D models to accelerate learning and improve production processes. Now, Mohamed shares his expertise through Revit tutorials that are accessible to all.
