MM to Feet Scale Factor in AutoCAD

In AutoCAD, the scale factor to convert millimeters (mm) to feet (ft) is 1/304.8, or approximately 0.00328084. This means that when you need to scale a drawing from millimeters to feet, you use the SCALE command with a factor of 0.00328084 (or its reciprocal 304.8 if going from feet to mm). This article explains why this factor is what it is, how to apply it correctly, and common pitfalls to avoid.

Formula

The conversion is based on the international definition of the foot: exactly 0.3048 meters. Since 1 meter equals 1000 millimeters, 1 foot equals 304.8 millimeters. The formula is:

Feet = Millimeters / 304.8

Where:

• Millimeters (mm) – the length in the metric unit, as used in most of the world.
• Feet (ft) – the imperial/US customary unit of length.
• 304.8 – the exact number of millimeters in one foot (0.3048 m × 1000 mm/m).

If you need the inverse (feet to mm), multiply by 304.8 instead of dividing.

Worked Example

Using the default value from the calculator above, let’s convert 1000 mm to feet.

1. Start with the length in millimeters: 1000 mm.
2. Divide by 304.8 (the number of mm in a foot): 1000 / 304.8 = 3.280839895…
3. Round to four decimal places: 3.2808 feet.

The calculator above gives the same result: 3.2808 ft. For reference, 1000 mm is also exactly 1 meter, so this confirms that 1 meter ≈ 3.28084 feet.

Why the Scale Factor Is 0.00328084

AutoCAD drawings are often created in one unit system but need to be used in another. The scale factor is simply the ratio of the target unit to the source unit. Since 1 foot = 304.8 mm, dividing any number of millimeters by 304.8 gives you the equivalent in feet. In decimal form, 1/304.8 = 0.003280839895… That is the factor you type when using the SCALE command (or the ALIGN command) to shrink a drawing from mm to ft.

Conversely, if you have a drawing in feet and need it in millimeters, the factor is 304.8 (multiply). Many users mistakenly use 25.4 (mm per inch) or 12 (inches per foot) when they actually need the combined factor. Always remember: 25.4 × 12 = 304.8.

How to Apply the Scale Factor in AutoCAD

Here are the most common methods to scale a drawing from millimeters to feet in AutoCAD:

• Using the SCALE command: Type SC or SCALE, select all objects, pick a base point (usually 0,0), then type 0.00328084 and press Enter. The drawing shrinks to feet.
• Using the ALIGN command: If you have known reference points, you can use ALIGN to scale and rotate the drawing to match known distances, avoiding manual factor entry.
• Using the INSERT command: When inserting a block or external reference, you can specify a scale factor of 0.00328084 in the X, Y, and Z fields.
• Using the UNITS command: This does not scale the drawing, but it sets the insertion scale. If you set the insertion units to millimeters, AutoCAD will automatically apply the conversion when inserting a block drawn in feet.

After scaling, always check a known dimension to verify the result. For example, a line that was 304.8 mm long should now measure exactly 1 foot.

Common Mistakes and How to Avoid Them

The most frequent error is using the wrong factor, especially confusing mm-to-inches (25.4) with mm-to-feet (304.8). Another common issue is forgetting to scale uniformly in all directions – always use the same factor for X, Y, and Z unless you intentionally want distortion. Also, beware of rounding: using 0.00328 instead of 0.00328084 introduces a small error that can become significant in large drawings. For precision, type the full factor or use the exact fraction 1/304.8 (AutoCAD accepts expressions like 1/304.8 in the SCALE command). Finally, after scaling, check the drawing units setting (UNITS command) to ensure the insertion scale matches your intended unit (feet).

When to Use the Scale Factor vs. Changing Units

Changing the drawing units (via the UNITS command) does not change the size of objects; it only changes how units are labeled and interpreted. For example, if you draw a line 1000 units long and then change the units from mm to feet, the line remains 1000 units long – it will be interpreted as 1000 feet, not 1 foot. To actually convert the physical size, you must use the SCALE command or the INSERT scale. The scale factor is necessary when you want the object to have the same real-world size in a different unit system. If you only need to change the label (e.g., for plotting), you can adjust the units without scaling.

Frequently Asked Questions

What is the exact scale factor from mm to feet in AutoCAD?

The exact scale factor is 1/304.8, which is approximately 0.003280839895. You can type 1/304.8 directly in the SCALE command, and AutoCAD will compute it precisely.

How do I convert a drawing from feet to mm in AutoCAD?

Use the SCALE command with a factor of 304.8. Select all objects, pick a base point, type 304.8, and press Enter. This multiplies every dimension by 304.8, converting feet to millimeters.

Why does my drawing not change size when I change the units?

Because the UNITS command only changes the display/interpretation of units, not the actual geometry. To resize objects, you must use the SCALE command or the INSERT scale. Changing units without scaling is like changing the label on a ruler – the physical length stays the same.

Can I use the ALIGN command instead of SCALE for mm to feet conversion?

Yes. If you know two reference points in the drawing and their real-world distances, you can use ALIGN to scale and rotate the drawing to match. This is often more reliable because it uses known geometry rather than a manual factor, but it requires accurate reference points.

What is the difference between mm to feet and mm to inches scale factor?

The mm-to-inches factor is 1/25.4 (≈0.0393701), while the mm-to-feet factor is 1/304.8 (≈0.00328084). Since 1 foot = 12 inches, the feet factor is exactly 1/12 of the inches factor. Using the wrong one will make your drawing 12 times too large or too small.

How do I check if my scale conversion was correct?

After scaling, use the DIST command to measure a known dimension. For example, if you know a wall is supposed to be 10 feet, measure it in the drawing. It should read 10.0000 (or close, depending on precision). Also, check the drawing units setting to ensure it matches your intended unit.

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in mm | Length in mm | mm | 1000 | 0
out ft | Length in feet | ft | mm / 304.8 | 4
out m | Length in meters | m | mm / 1000 | 3