No - a larger outside diameter spreads load. Thickness stops the washer deforming while it does so.

Bearing area grows with the square of the outside diameter, so a modest increase in diameter changes the pressure on the joint face substantially.

Thickness contributes nothing to area. What it does is keep the washer flat under the bolt, so the area it presents is the area it actually delivers.

A thin washer of large diameter curls at its rim and gives most of that advantage straight back.

So the two dimensions answer different questions: diameter for how much area, thickness for whether the washer holds its shape.

Stacking two thin washers is not equivalent to one thick one, because each interface is another place for the joint to settle.

Where the bearing face is at least as hard as the fastener, the hole is a close fit, and the joint is not repeatedly dismantled.

A hardened bolt bearing on a machined steel face is well matched, and many designed joints deliberately omit washers so that the grip length and preload are predictable.

Some connection designs specifically prohibit washers because they change the relationship between torque and tension.

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

Oversized or slotted holes need one large enough to cover the opening.

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

It is thicker and harder, so it does not indent under a high-strength bolt at full preload.

High-strength bolts are tightened to a substantial fraction of their yield, and the pressure under the head is enough to emboss a commercial washer.

An indented washer means preload has been lost into deformation rather than into clamping the joint.

Structural washers also have controlled dimensions and, in some systems, a specified surface condition because they affect the torque-tension relationship.

They are usually identifiable by thickness, by markings, or by supply as part of an assembly with the bolt and nut.

Where a connection is designed, the washer arrives with the bolt in the specification, and swapping in a commercial one changes what was designed.

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

The protective function concerns the rotating face scuffing the component, so the part that turns 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, because the nut is usually the part turned.

Some structural systems specify washer position explicitly, including which face carries a hardened washer.

Where a bolt passes through a slotted or oversized hole, the washer goes on that side regardless of which part turns.

Enough to pass the bolt easily and sit flat, without leaving a gap that lets the washer offset.

Standard washer series pair a bore with a nominal bolt size, and that pairing generally gives sensible clearance.

Too tight a bore catches on the thread or the shank radius and stops the washer seating flat, which loses the bearing it was fitted for.

Too loose and the washer can shift far enough to leave part of an oversized hole uncovered.

The radius under a bolt head is a specific trap: a washer with a sharp small bore can sit on that radius rather than on the underside of the head.

Where holes are oversized deliberately, the washer's outside diameter matters more than its bore.

One compatible with both the fastener and the component - the washer is in contact with both, so it can create a galvanic couple as easily as prevent one.

Zinc plating is the indoor default and matches the majority of general fasteners.

Hot-dip galvanised washers accompany galvanised bolts and are correspondingly thicker in coating.

Outdoors and in washdown areas the washer goes stainless, in whatever grade the fastener already is - not in a grade chosen on its own.

Introducing a third metal into a joint is a real risk - a plated steel washer between a stainless bolt and an aluminium bracket makes the problem worse.

Where dissimilar metals must be joined, a non-metallic washer is the deliberate answer rather than a compromise finish.

By what the joint face looks like when it is dismantled - and by whether the fastener stayed tight.

A washer that has embossed itself into the component means the bearing area was too small for the load.

A washer that has dished means it was too thin or too soft for the fastener above it.

Circular scuff marks on the component rather than on the washer mean the washer turned with the fastener instead of the fastener turning on the washer.

A fastener that has needed repeated retightening usually indicates the joint settled into a deformed washer.

None of these is a reason to omit the washer; they are reasons to size it properly.