TL;DR
Revit formulas support +, −, *, /, and ^ (exponent) plus sqrt(), pi(), exp(), ln(), and log(). The rule that breaks most formulas: units must stay consistent — a length times a length is an area, and every numeric constant in a dimension formula needs a unit. Proportional and scaling formulas (one driving parameter, everything else calculated) are the arithmetic backbone of every intelligent family.
Key Takeaways
- ✓Five operators: + − * / and ^ for exponents. Parentheses control the order of operations.
- ✓Units multiply too: Length × Length = Area. Dividing two lengths gives a plain number.
- ✓Every numeric constant in a dimension formula must carry a unit — 50 and 50mm are not the same thing.
- ✓Proportional design means one driver parameter and many calculated followers.
- ✓sqrt(), pi(), exp(), ln(), and log() cover the scientific cases — diagonal lengths, circumferences, and curves.
- ✓If arithmetic gives you fractional millimeters, pair it with rounding functions (Chapter 4).
Every intelligent Revit family — every door that resizes its frame, every desk that spaces its legs, every curtain panel that keeps its proportions — is arithmetic underneath. Before IF statements, before lookup tables, before trigonometry, there are five operators and one golden rule about units. Get these right and everything else in the formula system builds on a solid base.
The Five Operators
| Operator | Operation | Example |
|---|---|---|
| + | Addition | Width + 2 * Frame_Thickness |
| − | Subtraction | Opening_Height - Threshold |
| * | Multiplication | Height * 0.618 |
| / | Division | Total_Width / Panel_Count |
| ^ | Exponent | Side ^ 2 |
Order of operations follows standard math: exponents first, then multiplication and division, then addition and subtraction. When in doubt, add parentheses — they cost nothing and make the formula readable six months from now.
The Golden Rule: Units Are Part of the Math
Revit does dimensional analysis on every formula. Units don't just label the result — they participate in the calculation:
This is why Width * Height can only feed an Area parameter, and why Width / Depth produces a plain ratio you can store in a Number parameter. If the units on both sides of the equal sign don't match, Revit rejects the formula with “inconsistent units.”
The same rule applies to constants. In a length formula, 50 is a number and 50mm is a length — mixing them is the single most common arithmetic error:
Proportional Dimensions: One Driver, Many Followers
The core pattern of parametric design is one driver parameter that the user controls and several followers that calculate themselves. A cabinet family where the user sets only the total width:
Resize Total_Width and every part of the cabinet follows. No manual editing, no forgotten dimensions, no broken geometry. The golden-ratio variant is a classic for façade and furniture proportions:
Scaling Formulas in Real Families
Example 1 — Evenly Spaced Legs
A table family that adds legs as it gets longer needs the spacing to recalculate:
Example 2 — Diagonal of a Panel
Pythagoras works directly in a formula with sqrt():
Note: Width ^ 2 is an area, and the square root of an area is a length — the units work out exactly like the math says they should.
Example 3 — Circumference From a Radius
The Scientific Functions
Beyond the operators, Revit's formula engine includes sqrt(), pi(), exp(), ln(), and log(). In day-to-day family building, sqrt() and pi() carry most of the weight; the logarithmic functions appear in acoustic and lighting calculations more than in geometry.
When Arithmetic Isn't Enough
Division rarely produces clean numbers — 2437mm / 3 gives 812.333…mm, which no builder can measure. That's where rounding functions come in. And when subtraction can go negative and break your geometry, you need the abs() function as a guard. Those are the next two chapters of the system.
Chapter 1 of 10
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