Parameters & coordinates#

This is the one idea that makes Molding click. When you select an element on the Generators tab, every position / size / depth / radius / angle field shows two small buttons — V and P. That toggle is the whole coordinate concept:

Each field is V or P#

• V (value) — a fixed number you type, in millimetres for lengths or radians for angles: 45, 0, 1.5708.
• P (parameter) — the field follows a parameter you pick from the dropdown, so it changes whenever that parameter does. The − button next to it negates the parameter (mirror across the origin), e.g. HalfWidth → -HalfWidth.

That is the whole grammar: a field is either a fixed number or one parameter (optionally negated). There is one thing you will look for and not find:

There is no place to type a formula into a geometry field. A field is only ever V (a number) or P (one parameter). Any arithmetic goes into a parameter instead.

Where the math lives: parameter formulas#

To centre a rectangle at half its height, you do not compute Height / 2 on the field. You add a parameter that computes it, then point the field's P at it:

Parameter  HalfHeight   formula:  Height / 2     (Parameters tab)
Rectangle  center Y:   P -> HalfHeight            (Generators tab)

Why split it this way? Because it keeps every field trivial and identical in the browser preview and the Revit runner — both only ever ask "is this a number, or a parameter to look up?". The arithmetic lives in exactly one place (the parameter's formula), and Revit's own solver owns it.

The door panel, as parameters and fields#

This is the family you built in Getting started, written out:

Parameters tab
  Width       length   default 900     Instance
  Height      length   default 2100    Instance
  Thickness   length   default 45
  HalfHeight  length   formula  Height / 2

Generators tab
  Rectangle
    width   = P -> Width
    height  = P -> Height
    center  = [ V 0 , P -> HalfHeight ]
  Extrusion
    profile = [x] Rectangle      (ticked in the checkbox list)
    depth   = P -> Thickness

Change Width and the rectangle widens; HalfHeight updates through its formula so the panel stays centred — all from one edit. The extrusion's depth just points at Thickness because no math is needed there.

Formula operators#

Formulas (not coordinates) carry the arithmetic. They may reference other parameter keys:

Logical && || ! are not supported (Revit has no such operators) — use a conditional, e.g. count > 1 ? Spacing : 0.

Built-in helpers available in formulas: Math, Number, Boolean, isNaN, parseFloat, parseInt.

Width / 2                         // halve a length
Height * 0.5                      // scale
hasGlass ? GlassThickness : 0     // optional part
Math.max(600, Width)              // clamp to a minimum
Math.sin(Angle) * Reach           // trig (Angle is in radians)

Parameter types#

Instance vs type, and groups#

• instance parameter — editable on each placed copy in the model (e.g. a door's Width).
• type parameter — set per family type, shared by every instance of that type.
• group — which section of Revit's Properties palette it appears under: dimensions, constraints, materials…
• Shared checkbox — marks the parameter as a Revit shared parameter so it can appear in multi-category schedules and be reused across families. Most parameters do not need this.

Parameter display options#

In the expanded parameter settings panel there are four optional fields:

• Label — display name shown in Revit's Properties palette (defaults to the key if left blank).
• min / max — clamp the value in Revit's UI so users cannot enter out-of-range numbers.
• step — the increment arrow increases/decreases the value by this amount.

These are stored as meta.label, meta.min, meta.max, meta.step in the JSON.

Gotchas#

• Lengths are millimetres; angles are radians. To use degrees, make a parameter with a formula like Angle * Math.PI / 180 and point the field at it — a V field takes the raw radian number.
• A P field left on (select parameter) has no value — pick one, or switch it to V.
• To mirror, use the − button on the P field, not a formula like HalfWidth * -1.
• Parameter keys must be unique within a family; renaming a key updates every field that points at it.
• The names Math, Number, Boolean, isNaN, parseFloat, and parseInt are reserved — they are the built-in formula helpers and cannot be used as parameter or node keys.

Next: see every element you can add in the Node reference.