How to convert Architectural drawing to Decimal drawing in AutoCAD ?

you can convert drawing with scale command

"I use decimal units as a standard. When I get an architectural-units plan to insert into a civil drawing, I convert the units using one of these two methods:

1. Scale by0. (or multiply by12, if going from civil to architectural units).
2. Scale by reference. Type scale, select object(s), pick a base point, type R for reference, then type12, press Enter, then type1 (for civil to architectural, type the reverse:1 then12)."

Regarding the conversion Factor from Architectural drawing to decimal drawing, if the Drawing is already prepared in Architecture Method so the best option is mention in Syed Shan Haider Answer and i endorse his answer. For preparing of new decimal drawings the we will use ISO setting for choosing any scale option what we need to draw the object.

Use ISO settings if you want to draw in mm or meters......

ANSI setting are for inch ... feet.. yards

When architects and engineers share AutoCAD drawings, unit conversion is necessary.Related CAD Tips

1. Set the Default Scale List
2. An Easier Way to Change the Snap Value
3. Basic Drawing Settings for Mechanical Building Services
4. Configure Units
5. Adjust Scaling between Metric and Imperial Drawings

 "I use decimal units as a standard. When I get an architectural-units plan to insert into a civil drawing, I convert the units using one of these two methods:

1. Scale by0. (or multiply by12, if going from civil to architectural units).
2. Scale by reference. Type scale, select object(s), pick a base point, type R for reference, then type12, press Enter, then type1 (for civil to architectural, type the reverse:1 then12)."

Notes from Cadalyst Tip Reviewer Brian Benton: Different industries work in different types of units. AutoCAD deals in units, but is indifferent as to what those units are — until it comes time to print or share files. In the United States, architects work in inches for units, while civil engineers work in feet, and these two groups often have to share files. In addition to the tip above, another method is to use the InsUnits system variable, which tells AutoCAD which type of units you are using in your file. If the third-party file that you need to insert into your file also has the appropriate setting set, AutoCAD will automatically scale the file for you. Use the Insert command to insert the file as you would a block. Type InsUnits on the Command line and make the appropriate setting choice. The settings range from0 (zero) to20, but you will probably only need one of these: