The outside diameter. The bore is decided by the bolt; the outside diameter decides how much the load is spread.

Clamping force is concentrated under the head or nut. The washer's job is to distribute that force over a larger area of the material beneath, and area grows with the square of the diameter.

That is why a standard washer often fails to solve the problem it was fitted for. If the material is soft or the hole oversized, a larger outside diameter is the answer, not a thicker bolt.

Bore clearance matters in the other direction: too tight and the washer will not sit flat over a slightly misaligned hole.

Contents lists usually give the bore prominently and the outside diameter in small print, which is the wrong way round for choosing.

A kit with two or three outside diameters per bore is far more useful than one with many bores and a single outside diameter each.

Flat, spring or tooth, and at least some non-metallic - each answers a different question.

Flat washers spread load and protect the surface from the turning fastener. They are the majority of any kit and the default choice.

Spring and tooth washers resist loosening under vibration by maintaining tension or biting into the joint faces. They mark the surface, which is the trade-off.

Non-metallic washers - polymer, fibre, rubber-bonded - insulate electrically, seal against a surface, or keep dissimilar metals apart.

Sealing washers with a bonded elastomer face are a specific type worth having wherever sheet roofing or cladding is fixed.

A kit of flat washers alone is common and covers only the first of these, which is why it gets supplemented quickly.

Thick enough not to dish into the hole when the joint is properly tightened - and thin stamped washers frequently are not.

A washer that deforms under the bolt head has stopped spreading load and has become part of the joint's settlement instead, which shows up as a fastener that loosens after a short time in service.

Thin washers are adequate under light hand-tightened fixings and inadequate under torqued structural ones.

Heavy or structural-pattern washers exist precisely for this and are noticeably thicker and harder than the general commercial type.

Hardness matters alongside thickness. A soft washer under a hardened bolt head will indent regardless of how thick it is.

Where the specification names a washer, it is naming thickness and hardness as well as diameter, and a general assortment washer is not a substitute.

It can separate them, which is most of the battle - but only if it is non-conductive and if it separates them completely.

Galvanic corrosion needs two dissimilar metals in electrical contact and an electrolyte, usually just moisture. Break the contact and the mechanism stops.

A polymer or fibre washer under the head does that at the bearing face, but the bolt shank still touches the hole, so an insulating sleeve is needed as well where the problem is serious.

Choosing a metal washer close to one of the two materials reduces the driving voltage without eliminating it, which is often enough in mild environments.

Aluminium assemblies with steel fasteners are the common case, and the one where this is worth doing properly.

Keeping water out of the joint helps as much as the material choice, because without an electrolyte the couple does nothing.

No - but it is needed wherever the surface is softer than the fastener, the hole is oversized or slotted, or the fastener will be undone repeatedly.

A hardened bolt bearing directly on a machined steel face needs nothing between them, and many designed joints deliberately omit washers.

Soft materials - timber, aluminium, plastics, thin sheet - need one, because the head will otherwise crush into the surface and lose preload.

Oversized and slotted holes need one that comfortably covers the opening, or the head pulls through.

Repeated assembly is the third case: the washer takes the scuffing that would otherwise damage the component's face.

Adding a washer where the design did not allow for one changes the grip length, so the bolt may need to be a size longer.

Under whichever part turns, and under both where both turn or both bear on soft material.

The washer's protective function is about the turning face scuffing the surface, so the part that rotates during tightening is the one that needs it.

The load-spreading function applies to both ends, so soft material on both sides means washers on both sides.

Where only one is used, common practice is under the nut, since the nut is usually the part turned.

Spring and tooth washers go directly against the part they are meant to bite or bear on, and adding a flat washer under them defeats them.

Stacking multiple washers to make up length is poor practice - each interface is another place for the joint to settle.

Because two or three sizes cover most work, and the situations that empty the box are the ones needing the largest sizes.

The small common sizes are used constantly in assembly and are cheap to refill from bulk.

The oversized and heavy-pattern washers move slowly and are the ones actually worth having in a kit, because nobody stocks them otherwise.

A kit that is empty in the large sizes and full in the small ones is doing its job and needs a targeted refill.

Sealing and non-metallic washers tend to be needed in bursts, tied to a particular job rather than to general work.

As with any assortment, which compartments empty is the useful signal about what should be held in bulk.