How-to guide · how to scale blocks from mm to inches in autocad
How to scale blocks from mm to inches in AutoCAD
By Sumana Kumar · Published 28 Mar 2024 · Updated 31 Aug 2024
The blocks on this site are drawn full size in millimetres, which is perfect for a metric workflow — but if your template is in inches, dropping a metric block straight in gives you a piece of furniture the size of a building, because AutoCAD reads 600 (mm) as 600 inches. The fix is unit conversion, and there are two clean ways to do it: let AutoCAD convert automatically by setting your units, or apply the millimetre-to-inch scale factor by hand.
This guide covers both methods, gives you the exact conversion numbers, and explains how to avoid the classic 25.4x and 0.03937 mistakes that catch people moving between metric and imperial drawings.
The two conversion numbers you need
There are exactly 25.4 millimetres in an inch, and that single fact drives every conversion. To go from millimetres to inches you divide by 25.4, which is the same as multiplying by 0.03937 (because 1 / 25.4 = 0.03937…). To go the other way, from inches to millimetres, you multiply by 25.4.
So a 600 mm cabinet is 600 / 25.4 = 23.62 inches. A 900 mm door is 900 / 25.4 = 35.43 inches — which is why imperial door sizes like 36" map neatly to roughly 900 mm metric doors. Keep 25.4 and 0.03937 in mind and the rest follows.
Method 1 — Let INSUNITS convert automatically
The cleanest method is to never scale by hand at all. AutoCAD can convert units on insertion if both the block and the drawing declare their units. The blocks here are tagged as millimetres. In your imperial drawing, type UNITS and set the insertion scale to Inches. Now when you INSERT the millimetre block, AutoCAD sees 'block is mm, drawing is inches' and rescales it automatically — your 600 mm cabinet lands as a correct 23.62-inch cabinet with no factor typed.
This is the recommended approach because it's automatic and consistent: every metric block you insert into that inch drawing converts itself. Set INSUNITS once and forget the arithmetic.
Method 2 — Apply the 0.03937 factor on insertion
If the block's units aren't set, or you want explicit control, convert by hand. In the INSERT dialog, enter a uniform scale of 0.03937 (with Uniform Scale ticked). That multiplies the millimetre geometry down to its inch equivalent as it lands — 600 mm × 0.03937 = 23.62 inches.
Alternatively, insert the block at scale 1, then run SCALE on it with a factor of 0.03937. Same result. Use the dialog method when you know in advance you're converting, and the SCALE-after method when you've already placed a block and realised it's 25.4 times too big.
Going the other way — inches to mm
Occasionally you'll have an imperial block to drop into a metric drawing — the reverse problem. Then you multiply by 25.4 instead of 0.03937. A 36-inch door block scaled by 25.4 becomes 914.4 mm, the metric equivalent. Set it in the INSERT dialog (uniform scale 25.4) or via SCALE after placing.
As with the metric-to-imperial direction, the better long-term fix is units: if the imperial block is tagged as inches and your drawing's INSUNITS is millimetres, AutoCAD converts it automatically. Hand factors are the fallback for untagged blocks.
Fixing a block that's already inserted wrong
You'll often only notice the unit mismatch after the block is on the page, sitting there 25.4 times too big or too small. You don't have to delete and re-insert. Select the offending block, run SCALE, pick a base point near where it should sit, and apply the conversion factor directly: 0.03937 to bring a millimetre block down into an inch drawing, or 25.4 to push an inch block up into a millimetre drawing.
If several metric blocks came in oversized from the same paste or import, window-select them all and SCALE them together with one 0.03937 factor — they share the same error, so they share the same fix. After scaling, the blocks are at correct inch size, and any further blocks you insert will convert automatically once you've also set the drawing's INSUNITS. Doing both — fixing the placed blocks and setting the units — stops the problem recurring on the next insertion.
Architectural inches vs decimal — a sizing gotcha
One thing that confuses people converting to imperial: AutoCAD can display lengths as decimal inches (23.62) or as architectural feet-and-inches (1'-11 5/8"). The geometry is identical — only the display style differs, set by the UNITS dialog's 'Type'. So a converted 600 mm cabinet measuring 23.62 decimal inches is the same object whether your dimensions read 23.62" or 1'-11 5/8".
Don't try to 'fix' a converted block because the dimension reads oddly in fractions — that's just the architectural display rounding to the nearest practical fraction. The underlying size is correct; the conversion already did its job.
Pitfalls converting between mm and inches
The number-one error is the inverse factor: using 25.4 when you meant 0.03937, which makes a metric block 645 times too big (25.4 × 25.4), or vice versa. Remember the direction: mm to inches divides (× 0.03937, the small number), inches to mm multiplies (× 25.4, the big number).
The second pitfall is mixing methods — setting INSUNITS to convert and also typing a manual factor, which double-converts and gives nonsense. Pick one method per insertion. Third, after any conversion, sanity-check one known dimension: measure a door or a worktop with DIST and confirm it reads a believable inch value (a door near 35–36"). That one check catches almost every conversion slip before it propagates through the drawing.
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Questions
Frequently asked
What is the scale factor to convert mm to inches in AutoCAD?+
Multiply by 0.03937 (which is 1 ÷ 25.4, since there are 25.4 mm in an inch). So a 600 mm block scaled by 0.03937 becomes 23.62 inches. To go from inches back to millimetres, multiply by 25.4 instead.
Can AutoCAD convert a millimetre block to inches automatically?+
Yes. The blocks here are tagged as millimetres, so set your drawing's insertion units to Inches (type UNITS, set the insertion scale to Inches). AutoCAD then rescales each metric block automatically on insertion — no manual factor needed.
Why does my metric block come in 25 times too big in an inch drawing?+
AutoCAD is reading the millimetre numbers as inches. Either set INSUNITS so it converts automatically, or insert with a uniform scale of 0.03937 to convert mm to inches. Never use 25.4 in this direction — that's the inches-to-mm factor and makes it far bigger.
My converted block dimensions show as fractions — is that wrong?+
No. That's just AutoCAD's architectural display style showing feet-and-inches and rounding to the nearest fraction. The geometry is correct; switch the UNITS 'Type' to Decimal if you'd rather see 23.62" than 1'-11 5/8". The size is the same either way.
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