Metric & Imperial Working Conversion looks like arithmetic. It is not.

Converted dimensions are not working dimensions.

A joiner setting out from a converted number is working to a figure nobody chose. It looks arbitrary because it is, and the first instinct on a shop floor is to round it.

Rounding compounds.

A dimension rounded at the part becomes a different dimension at the assembly, and the difference lands at the junction where two components meet.

Material sizes do not convert.

Sheet goods, board and hardware come in the sizes the market supplies. A converted panel dimension can be marginally larger than a stocked sheet, which is not a rounding problem but a purchasing one.

Tolerance does not survive conversion.

A tolerance meaningful in one system becomes an odd figure in the other, and odd figures get treated as approximate.

The architectural set and the product data disagree.

Common on cross-market projects where the design team works in one system and specified products are supplied in another.

Hardware.

Frequently specified in the manufacturer's system regardless of the drawing's, which is legitimate but has to be handled deliberately.

A supplier working natively in a different system.

The most consequential source, and the reason a package should state its system rather than assume it.

Legacy details.

Reused from an earlier project in the other system, converted once, and carried forward ever since.

One system per package, stated.

The convention is confirmed at scoping and applied throughout, including schedules and part data.

Native working, not conversion.

Where a package is metric it is drawn metric from the outset. Where it is imperial it is drawn imperial. Dimensions come out as figures somebody would actually choose.

Deliberate handling where a component arrives in the other system.

Specified hardware supplied in its manufacturer's units is documented so nobody silently converts it.

Material sizes taken from what is actually stocked,

in the units they are stocked in.

Worth saying, because this page sits in a standards category.

Unit convention is not a certification, not an accreditation and not something a body approves. It is a documentation discipline. There is nothing here for anyone to hold or to be registered for.

What there is, is a common and expensive failure mode that a package either handles deliberately or discovers later.