TL;DR
Revit formulas are expressions attached to family parameters that calculate values automatically. They support arithmetic (+, −, *, /, ^), functions like abs(), sqrt(), and pi(), rounding, trigonometry, conditional logic with IF/AND/OR/NOT, formula-driven text and Yes/No parameters, and CSV lookup tables via size_lookup(). Formulas are written in the Family Types dialog, parameter names are case-sensitive, and units must stay consistent. This guide maps the complete system in 10 chapters.
Key Takeaways
- ✓Formulas live in the Family Types dialog — one formula per parameter, recalculated on every change.
- ✓Parameter names are case-sensitive; units are part of the math and must stay consistent.
- ✓Four capability tiers: arithmetic → conditions → text and visibility control → lookup tables.
- ✓A comparison like Width > 900mm is itself a Yes/No value — the simplest automation there is.
- ✓Nested IFs max out at 3 levels of maintainability; CSV lookup tables take over from there.
- ✓Almost every formula failure is one of six known error patterns — all learnable.
A Revit family without formulas is a drawing that happens to be 3D. A family with formulas is a small machine: change one input and frames resize, hardware upgrades itself, labels rewrite themselves, and parts appear or disappear — all correctly, all at once. This guide is the map of that entire system: what formulas can do, the rules they all obey, and where to learn each part in depth.
What a Revit Formula Is
In the Family Editor, open Family Types. Every parameter has a Formula column — an expression written there calculates the parameter's value instead of accepting manual input:
The moment any referenced parameter changes, every formula that depends on it recalculates, and the geometry follows. That is the whole trick — and the rest is knowing the vocabulary and the rules.
The Three Rules Every Formula Obeys
1 — Names are case-sensitive. A parameter named Width must be written exactly Width. The “can't find parameter” error is almost always a casing or spelling mismatch.
2 — Units participate in the math. Length × Length is an area, Length ÷ Length is a plain number, and every numeric constant in a dimension formula needs its unit: 50mm, not 50.
3 — Types must match. A formula must return the same type as its parameter, and both branches of an IF must return the same type as each other.
The Complete Function Reference
| Category | Functions | Chapter |
|---|---|---|
| Arithmetic | + − * / ^ sqrt() pi() exp() ln() log() | 1 |
| Absolute value | abs() | 2 |
| Rounding | round() roundup() rounddown() | 4 |
| Conditional logic | IF() AND() OR() NOT() | 5 |
| Comparisons | < > = <= >= | 5, 7 |
| Trigonometry | sin() cos() tan() asin() acos() atan() | 8 |
| Table lookup | size_lookup() | 9 |
What Formulas Can Drive
Every parameter type is fair game: lengths and numbers (dimensions, counts, angles), Yes/No parameters that automate geometry visibility and switch nested families, and text parameters that select complete label phrases by condition. The two practical limits: materials cannot be assigned by formula (use the two-geometry visibility workaround), and strings cannot be concatenated.
The 10-Chapter Learning Path
Each chapter below is a full article. Read them in order and you cover the complete system — or jump straight to the problem in front of you:
Where Most People Get Stuck
Three walls, in order: the units wall (“inconsistent units” on the first real formula — Chapter 1 breaks it down), the nesting wall (IF chains that grow until nobody can read them — Chapter 9's lookup tables are the way out), and the debugging wall (a red formula bar and a vague message — Chapter 10's isolation method turns hours of guessing into minutes of process).
Frequently Asked Questions
The complete system
All 10 chapters, worked examples, and the HD Video Vault
Mastering Revit Family Formulas — ebook + HD Video Vault + Formula Cheat Sheet. 40% off at $18.