Explainer · insunits autocad

What is the INSUNITS variable?

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By Sumana Kumar · Published 16 Sept 2022 · Updated 19 Apr 2025

If you've ever inserted a downloaded block and watched it arrive the size of a postage stamp — or so vast it swallows the whole drawing — you've met the INSUNITS variable, even if you didn't know its name. INSUNITS is the small system setting that tells AutoCAD what real-world unit one drawing unit represents, and it's the quiet mechanism behind automatic block scaling. Understanding it turns the single most common 'why is this block the wrong size' problem into a non-issue.

INSUNITS isn't glamorous, but it's one of those fundamentals that saves you from manual rescaling on every insert. This page explains what the variable is, how AutoCAD uses it to rescale blocks automatically, the related INSUNITSDEFSOURCE and INSUNITSDEFTARGET settings, and exactly how to set everything so the blocks you download land at true size every time.

What INSUNITS actually stores

INSUNITS is a system variable that holds a number representing the drawing's insertion units. 0 means Unitless, 1 means Inches, 2 means Feet, 4 means Millimeters, 5 means Centimeters, 6 means Meters, and there are codes for many others. You rarely type these numbers directly; you set them through the UNITS dialog's 'Insertion scale' dropdown, which writes the matching value into INSUNITS behind the scenes.

The key point is that this setting describes what one unit in the drawing means in the real world. A drawing with INSUNITS = Millimeters treats every unit as one millimetre, so a 5000-unit line is 5 metres. The same line in a Meters drawing (INSUNITS = 6) would be 5 kilometres. INSUNITS is how AutoCAD knows the difference.

How automatic block scaling works

Here's the clever part. A block carries its own units (set when it was created, in the BLOCK or WBLOCK dialog). When you insert that block into a drawing, AutoCAD compares the block's units against the host drawing's INSUNITS and automatically scales the block so it lands at the correct real-world size.

An example makes it concrete. Say a chair block was built in millimetres (its units = mm) and you insert it into a drawing whose INSUNITS is set to Meters. AutoCAD sees that the block thinks in millimetres but the drawing thinks in metres, so it scales the block by 1/1000 on insertion — and the 600 mm chair arrives correctly as 0.6 metres. No manual SCALE command, no guessing a factor. This automatic conversion is the whole reason INSUNITS exists, and it's why setting it correctly matters more than almost any other small setting.

Why blocks come in tiny, huge — or correct

Now the failure modes make sense. A block arrives microscopic or gigantic almost always because of a units mismatch combined with a missing INSUNITS setting.

The classic case: your drawing has INSUNITS = Unitless (0), so AutoCAD has no idea what scale to convert to and inserts the block at its raw size. If the block was drawn in millimetres and your drawing geometry is also effectively in millimetres, it'll look fine — but if your drawing was set up as metres-as-units, the millimetre block lands 1000× too big. The other classic: the block itself was saved Unitless, so even a correctly-set drawing can't auto-scale it. The cure in both cases is to give both ends real units — set the drawing's INSUNITS, and prefer blocks that carry defined units — so the automatic conversion has the information it needs.

Setting INSUNITS the right way

There are two routes. The quick one: type INSUNITS at the command line and enter the code for your unit (4 for millimetres is the most common in metric drafting). The clearer one: type UNITS to open the Drawing Units dialog and set 'Insertion scale' to your unit — Millimeters for typical metric architectural work, Inches or Feet for US imperial.

Do this once in a template (DWT) and save it, so every new drawing starts with the right insertion units baked in and you never think about it again. For the blocks on this site, which are drawn full-size in millimetres, set your drawing's INSUNITS to Millimeters and they insert at scale 1 with no rescaling. If your office works in metres, set INSUNITS to Meters and the same millimetre blocks auto-convert to 1/1000 on insertion — correct without any manual scaling.

INSUNITSDEFSOURCE and INSUNITSDEFTARGET

Two companion variables handle the awkward case of unitless content. INSUNITSDEFSOURCE sets the assumed units of a source block that was saved as Unitless, and INSUNITSDEFTARGET sets the assumed units of a target drawing that's Unitless. They're the fallback AutoCAD uses to guess when one side hasn't declared its units.

Most of the time you can leave these at their defaults, but they explain odd behaviour: if a unitless block scales unexpectedly, it's because AutoCAD applied the INSUNITSDEFSOURCE assumption. The robust fix is always to give your blocks and drawings real, explicit units rather than relying on these guesses — but knowing the fallbacks exist demystifies the cases where a 'unitless' file still seems to scale itself.

A reliable insert-at-correct-size workflow

Tie it together into a habit and the size problem disappears. First, set your drawing's INSUNITS once (via UNITS) to match how you work — millimetres for most metric drafting — ideally in a saved template. Second, prefer blocks that carry defined units; the downloads here are built in millimetres so they auto-convert cleanly into any unit-aware drawing.

Then insert normally with the INSERT command or by dragging the DWG onto the canvas. If a block still arrives the wrong size, run a two-point check: type UNITS and confirm the drawing's Insertion scale, then check the block's own units (you can see them when you re-insert via the Blocks palette). One of the two will be Unitless or mismatched. Set both to real units and re-insert, and the block lands true — no manual SCALE, no trial-and-error factors. That single discipline removes the most common beginner frustration with downloaded CAD blocks.

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Questions

Frequently asked

What does the INSUNITS variable do?+

INSUNITS sets the real-world unit that one drawing unit represents (millimetres, metres, inches, etc.). AutoCAD uses it to automatically scale inserted blocks: it compares the block's own units against the drawing's INSUNITS and resizes the block so it lands at the correct real-world size.

Why does my downloaded block insert at the wrong size?+

Almost always a units mismatch. Either the drawing's INSUNITS is Unitless (so AutoCAD can't auto-scale) or the block was saved Unitless. Set the drawing's insertion units via the UNITS command to match the block (Millimeters for the blocks here) and re-insert.

What should I set INSUNITS to?+

Match how you work. For metric architectural drafting, set it to Millimeters (INSUNITS = 4) — that's the unit the blocks on this site use. For US imperial work, set Inches or Feet. Save the setting into your template so every new drawing inherits it.

Does INSUNITS rescale existing geometry too?+

No — INSUNITS only affects how blocks (and xrefs) are scaled when they're inserted. It doesn't resize geometry already in the drawing. To change the size of existing objects you use the SCALE command; INSUNITS purely governs the automatic conversion at insertion time.

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