How to size a washer, screw or standoff without a second order

Three numbers describe a washer

Inner diameter is the bolt it goes over. Outer diameter is how much bearing surface it gives. Thickness is how much stand-off and how much crush resistance. A washer that is right on the inner diameter and short on the outer will dish under load and mark the surface; one that is too thick will change the grip length of your bolt.

Thread pitch is not optional

A screw is described as M4 × 0.7 × 12: diameter, pitch, length. M4 screws exist in 0.7mm and 0.5mm pitch and they are not interchangeable. If you have the old screw, count ten threads against a ruler and divide. If you do not, buy the common pitch — coarse — because that is what most consumer hardware uses.

Measure what you have before you order

A cheap digital caliper settles every one of these questions in ten seconds and is the single best tool a repair drawer can hold. Measure the shank, not the thread crest, for diameter. Measure a screw's length from under the head for a pan head and from the top of the head for a countersunk one — that catch has cost everyone an order.

Buy the sizes either side

When we are unsure on our own bench we buy the size we think we need plus the one above and the one below. Three packs of 50 washers cost less than a second delivery and a lost afternoon. The leftovers go into a sorting box and get used within a year, every time.

Material matters more in wet places

Zinc plated steel is fine indoors. Stainless A2 for a bathroom or a kitchen. Stainless A4 for anything near salt. Nylon where you need electrical isolation or where you do not want metal marking a finished surface. Brass where a fitting must not seize into another brass part.