# Use project parameters and expressions

Name a size once, use it in the parameters of generated furniture, and change the whole design from one field.

A project parameter is a named value, for example `wardrobeWidth = 2350`. Generated assemblies (a carcass, a stud frame, joists) can use the name in their parameters. When you change the value, every assembly that uses it is built again.

To see an example, open **File ▸ Sample projects ▸ Bookcase**. Its width comes from the parameter `bookcaseWidth`.

## Add a parameter

1. Click an empty part of the viewport, so that nothing is selected.
2. In the **Properties** panel, find the **Parameters** heading.
3. In the field with the text "name = expression", type the parameter, for example `shelfDepth = 280`.
4. Press Enter.

The name must start with a letter or `_`, and can contain only letters, digits and `_`. The **Parameters** list shows each value after `=`.

## Use it in a generated assembly

1. Select the generated assembly.
2. In the **Properties** panel, under **Parameters**, type the name in a length field, for example `shelfDepth`.
3. Press Enter.

The field keeps the name, and the value it gives shows after `=`.

## Write an expression

A parameter, or a length field of a generated assembly, can hold an expression:

| You type | Value |
| --- | --- |
| `2350` | 2350 mm |
| `2.35m` | 2350 mm |
| `90cm` | 900 mm |
| `36in` | 914.4 mm |
| `8ft` | 2438.4 mm |
| `wardrobeWidth / 3` | a third of `wardrobeWidth` |
| `(roomWidth - 20) / 2` | half of `roomWidth` less 20 mm |
| `round(wardrobeWidth / 3, 5)` | a third, rounded to the nearest 5 mm |

Numbers with no unit are mm. The operators are `+`, `-`, `*`, `/` and brackets. The functions are `min`, `max`, `round`, `floor`, `ceil`, `abs` and `sqrt`. `round`, `floor` and `ceil` take an optional step as the second value.

## Change or delete a parameter

- To change a parameter, type a new expression in its field and press Enter.
- To delete a parameter, click **✕** beside it.

You cannot delete a parameter that something still uses. If a parameter uses itself through other parameters, KerfCraft shows "Circular dependency" and the chain of names.

## Related

- [Build a cabinet, bookcase or wardrobe](/docs/how-to/furniture/build-a-carcass/)
- [Build a stud partition or a joist floor](/docs/how-to/furniture/build-a-stud-partition-or-joist-floor/)