Measuring your board in inches for a rack specified in millimetres

US boards get specified in inches. Most rack dimensions in this industry are published in millimetres. The conversion is simple arithmetic and it still causes returns, because the risk is not in the conversion, it is in the rounding.

The numbers worth memorising

One inch is 25.4 mm exactly. That figure is exact by definition, not approximate, so there is no error introduced by the conversion itself.

Useful reference points: 6 in is 152.4 mm. 8 in is 203.2 mm. 12 in is 304.8 mm. 18 in is 457.2 mm. 20 in is 508 mm.

A tenth of an inch is 2.54 mm, which is worth holding on to because component pitch in through-hole work is frequently 0.1 in.

Where the error comes in

Nobody gets 25.4 wrong. What goes wrong is rounding in the direction that removes clearance.

A board measured at 7.9 in converts to 200.66 mm. Rounded to 201 mm it is fine. Rounded to 200 mm it is now smaller on paper than it is in reality, and if the rack was chosen against that 200 mm figure with a nominal clearance, the clearance has been quietly spent.

The rule is to round board dimensions up and rack capacity down. Always. That way every rounding error you make is one that gives you more room rather than less.

Measure the board you have, not the board on the drawing

The drawing gives you the nominal outline. What goes in the rack is the finished panel or board, and finished is not always nominal.

Routed edges carry a tolerance. Depanelled edges can leave a small amount of material at the tab positions. A board that has been through a break-out operation is often very slightly larger at those points than the drawing says.

If your boards arrive depanelled, measure across the tab positions, because that is where the widest point will be. Our note on choosing a depaneling method covers how much edge each method leaves behind.

The measurement people forget

Width and length decide whether the board fits the slot. Neither of them decides whether the board fits the rack.

What decides that is the height of the tallest thing standing on the board, measured from the board surface, because that is what has to clear the pitch between one slot and the next. Populated boards are three dimensional objects and slot pitch is the dimension that gets checked last, usually after delivery.

Measure the tallest component, in the same units as everything else, and check it against the published pitch before you order.

A short checklist

Convert at 25.4 exactly. Round board dimensions up, rack capacity down. Measure the finished board rather than the drawing, across the tab positions if it has been depanelled. Measure the tallest component from the board surface. Then compare against the published rack figures.

Every product in the universal racks and magazine racks ranges lists dimensions and slot pitch on its page, and how to size a rack to your boards works through a full example. Everything here ships from the US.