How to Choose Washers for Wood, Metal, and Slotted Holes
Washers do different jobs depending on the material and the hole shape, from spreading load to keeping fasteners from pulling through. I’ll help you sort out when a washer is doing useful work and when it’s just extra metal.
Choosing the right washer is less about collecting hardware and more about matching the fastener to the job. In many projects, a washer spreads load, protects a surface, or helps a bolt sit properly in a hole. In others, it adds nothing except another part to drop under the workbench.
The simplest way to think about washers is this: the washer should solve a problem the fastener alone can’t. If the material is soft, thin, slotted, or prone to crushing, a washer usually earns its keep. If the joint is already strong and the hardware is designed to clamp directly against a flat surface, the washer may be optional or unnecessary.
What washers actually do
A washer increases the contact area under a nut or bolt head. That matters when the material underneath can dent, split, or pull through. Wood is the classic example. A bolt bearing directly on a soft wood surface can bite in unevenly, especially if the grain is weak or the load is repetitive. A washer helps spread that pressure across a wider area, which makes the connection more stable.
Washers also help when holes are larger than the fastener shank or when you need a little forgiveness during assembly. They can bridge small surface imperfections and give the hardware a flatter seat. In some cases, they reduce wear from vibration or repeated tightening, though they won’t magically make a poor joint reliable.
Washers for wood
Wood usually benefits from washers when a screw or bolt is clamping one piece to another, especially if the wood is soft, thin, or near an edge. A plain washer under the head or nut helps prevent the fastener from crushing into the fibers. That becomes more important with softer woods, plywood, and lumber that may compress over time.
For structural or heavily loaded joints, a larger-diameter washer is often better than a tiny one. The bigger footprint gives the load somewhere to go. If you’re fastening through a slot in wood, a washer can also help keep the hardware from pulling through the opening, as long as the washer is wide enough to cover the slot with some margin.
That said, not every wood screw needs a washer. If you’re driving a screw with a trim head, pocket-hole style fastener, or a purpose-made washer head, the hardware itself may already provide the needed bearing area. Adding a washer on top can interfere with the intended fit or leave the head sitting awkwardly.
Washers for metal
Metal is less forgiving about concentrated load, especially when the sheet is thin. A washer helps prevent the fastener from deforming the metal around the hole. That’s one reason washers are common on sheet metal, brackets, and thin steel or aluminum parts. They keep the force from turning a neat round hole into a slightly alarming tear-shaped one.
With metal, the fit matters. A washer that’s too small won’t cover enough area to help much. A washer that’s too large may interfere with nearby hardware, edges, or moving parts. If the hole is already reinforced with a built-in flange, captive nut, or welded tab, a separate washer may not be needed.
Metal-to-metal joints can also benefit from washers when the surfaces don’t mate perfectly flat. In those cases, a washer gives the fastener a cleaner bearing surface and can improve clamping consistency. That doesn’t mean every bolt in a steel bracket needs one; it means the washer should support the joint, not just decorate it.
Slotted holes need extra attention
Slotted holes are where washers become more than optional accessories. A slot allows movement for alignment, expansion, or adjustment, but that same opening gives the fastener less material to hold onto. A washer with enough diameter to cover the slot is usually the difference between a secure clamp and a fastener that slowly walks its way into trouble.
The key is coverage. The washer must span the slot fully so the bolt head or nut doesn’t bear directly on the edges. If the slot is long and the washer is too small, the hardware can slip into the opening under load. If you’re dealing with a slotted metal bracket or adjustable woodworking hardware, choose a washer that covers the widest part of the slot with room to spare.
Sometimes a fender washer is a better choice than a standard flat washer because it has a wider outer diameter. That extra width helps it bridge a slot or spread force over a larger area. In other cases, a specialty washer or plate may be the right answer if the slot is carrying significant load.
When a washer is unnecessary
A washer isn't automatically an upgrade. If the fastener already has a wide bearing surface, if the material is hard enough to resist crushing, and if the hole is properly sized, extra hardware may not improve anything. In some assemblies, it can even make the joint stack up incorrectly or reduce how much thread engagement you have left.
There’s also a temptation to use washers as a fix for mismatched parts. If a bolt is loose in a hole, a washer might hide the symptom, but it doesn’t always solve the real problem. A better-fitting fastener, a bushing, or a properly sized bracket may be the cleaner repair. Washers are useful, but they’re not a substitute for the right hardware.
Check the bearing surface before you tighten Before you commit to a washer, look at what the fastener is actually pressing against. If the material is soft, thin, or slotted, the washer should spread the load; if the head already has enough surface area, you may not need one at all.
Choosing the right washer size
The main dimensions to watch are inner diameter and outer diameter. The inner diameter should fit the bolt or screw without excessive slop, while the outer diameter should be large enough to do the job. For wood and slotted holes, outer diameter matters a lot because that’s what prevents pull-through and spreads pressure.
Thickness matters too, especially in metal work or high-load joints. A thin washer may be fine for light assembly, but it can deform under heavy clamp force. A thicker washer holds its shape better and gives the fastener a more stable bearing surface. If the joint will be tightened firmly, the washer should be sturdy enough to stay flat instead of cupping like a tiny metal bowl.
Material can matter as well. Most everyday projects use steel washers, often zinc-plated for general corrosion resistance. For outdoor or damp locations, you’ll want to think about the surrounding hardware and environment so the washer doesn’t become the first part to rust. Matching materials can help avoid corrosion issues, especially where dissimilar metals may react over time.
A simple decision rule
Start with the hole and the material, not the washer bin. If the hole is round, the material is strong, and the fastener head bears well, a washer may be optional. If the material is soft, thin, or slotted, a washer is usually the smarter choice. If you need a wider footprint, choose a larger washer rather than hoping a standard one will do heroic work.
When you’re unsure, ask what failure you’re trying to prevent. Pull-through, crushing, and poor load distribution each point to a washer, but not always the same one. That’s the useful habit here: match the washer to the weakness in the joint, then keep the hardware as simple as the job allows.
If you’re standing in the aisle trying to decide, bring the bolt or screw with you and compare the sizes directly. That quick check often saves a second trip, and it’s usually easier than learning the hard way after the piece starts shifting, sinking, or loosening later.