Explainer · what is a block lookup table
What is a block lookup table, explained
By Sumana Kumar · Published 27 Aug 2023 · Updated 1 Feb 2025
A block lookup table turns a dynamic block's raw numbers into a friendly menu. Instead of asking the user to type a width of 1000 and a height of 2100, a lookup table lets them pick 'Door Type 6' from a dropdown, and the block sets all the underlying parameters at once to match. It is the feature that makes a complex multi-parameter block feel as simple as choosing from a catalogue.
This explainer covers what a lookup table is, how the Lookup parameter and Lookup action work together, the difference between forward lookups (name sets values) and reverse lookups (values report a name), and how authors use them to package standard sizes cleanly. If you have ever picked a named size from a block's grip menu, a lookup table was doing the work behind it.
Mapping inputs to outputs
At its core a lookup table is a grid that maps combinations of input parameter values to an output value, usually a readable label. Each row says 'when Width is 900 and Height is 2100, the name is Type 1; when Width is 1000 and Height is 2100, the name is Type 6.' The lookup grip on the inserted block lets the user pick the label, and the block reads the row backwards to set the parameters.
The inputs are typically other dynamic-block parameters — Linear, Polar or visibility values — and the output is a Lookup parameter that holds the chosen label. This is what lets a block with several independent dimensions be controlled through one simple named menu instead of three separate grips.
The Lookup parameter and Lookup action
Building a lookup table needs two pieces. First, a Lookup parameter, which holds the result and provides the dropdown grip on the inserted block. Second, a Lookup action, which is where you actually build the table. Double-clicking the Lookup action opens the Property Lookup Table dialog — the spreadsheet-like grid where you add input columns (other parameters) and the output lookup column.
This is one of the rare cases where the action carries the data rather than just a transform. The Lookup action does not move geometry; it stores the mapping and applies it. Add the parameter, add the action, open the table, and start filling in rows that pair input values with the labels you want users to see.
Forward and reverse lookups
A lookup column can work in two directions. In a forward (read-only display) lookup, the table reports a name based on the current parameter values — drag the width grip to 1000 and the lookup field automatically shows 'Type 6', a handy live label. In a reverse lookup, picking the name from the dropdown drives the parameters — choose 'Type 6' and the block sets width and height to that row's values.
You control this with the 'Allow reverse lookup' setting in the table dialog. Reverse lookup is what most people want for a size selector because it lets the user choose by name and have everything snap into place. Forward-only is useful when you want a read-out that reflects whatever the user dialled in manually.
Why lookup tables beat raw grips
Consider a door block with independent width, height and frame-depth parameters. Exposed as three separate grips, it is fiddly and easy to set to a nonsensical combination. Wrapped in a lookup table of standard door types, it becomes a single dropdown of real, buildable sizes — every combination in the table is one a specification would actually use.
That is the practical payoff: lookup tables encode your standards. They prevent invalid combinations, they speak the language of the catalogue ('Type 6', 'FD30', '900 leaf') rather than bare numbers, and they make a sophisticated multi-parameter block usable by someone who knows nothing about how it was built.
Building a lookup table step by step
In the Block Editor, first create the input parameters you want to drive — say a Linear Width and a Linear Height, each with sensible value sets. Then add a Lookup parameter and a Lookup action. Double-click the action to open the Property Lookup Table, click 'Add Properties' to bring in Width and Height as input columns, and add a Lookup column for the names.
Fill the grid: one row per standard size, with the exact input values and the label for each. Tick 'Allow reverse lookup' on the lookup column so selecting a name drives the geometry. Close the dialog, exit the editor, and test by picking names from the lookup grip — each one should snap the block to the matching size.
Lookup tables versus visibility states
Both features give the user a named dropdown, so it helps to know which to reach for. Use visibility states when the variants differ in which geometry is shown — a plan view versus an elevation, a door with or without a swing arc. Use a lookup table when the variants differ in parameter values — sizes, depths, angles — and the geometry is essentially the same shape scaled or stretched.
The two combine well. A door block might use visibility states to switch between plan and elevation and a lookup table within each to pick the standard width. One handles 'which version of the drawing', the other handles 'which size', and together they make a single block cover a whole product range.
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Questions
Frequently asked
What is a block lookup table in AutoCAD?+
A grid inside a dynamic block that maps combinations of parameter values to friendly names. Users pick a name from a dropdown grip and the block sets the underlying parameters to the matching values.
What is the difference between forward and reverse lookup?+
Forward lookup reports a name based on current parameter values as a live read-out. Reverse lookup lets the user pick the name and drives the parameters to match. Enable reverse lookup for a size selector.
What parameter and action do I need?+
A Lookup parameter to hold the result and provide the dropdown grip, plus a Lookup action whose Property Lookup Table dialog is where you build the rows mapping inputs to names.
When should I use a lookup table instead of visibility states?+
Use a lookup table when variants differ by parameter values like size or depth. Use visibility states when variants differ by which geometry is shown, such as a plan versus an elevation view.
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