Types of Washers Explained: Your Guide to the Washers We Stock

A washer is the small part that quietly does half the work in a good fixing. It spreads the load, protects the surface, stops a joint from rattling loose, and can extend the life of the whole connection. Get the right one and the job holds. Get the wrong one and you'll be back to tighten it — or repair it — in no time.

At Fastco we stock a broad range of washers for trades, from everyday flat washers to heavy-duty thick washers and specialist anti-loosening and insulating types. Here's the full rundown of what we stock, what each type is for, and how to pick the right one.

Quick reference — washers at a glance

Form A washers — the everyday flat washer

Form A is the standard flat round washer most people picture when someone says "washer". It sits under a bolt head or nut to spread the load over a bigger area and stop the fastener digging in or damaging the surface.

You'll use these on almost everything: bolting brackets, assembling machinery, light fabrication, general maintenance. If a job doesn't specify a particular type, Form A is usually the safe default.

What we stock: zinc plated and A2 stainless steel, from M3 right up to M30.

Shop the range: Form A Washers

Form B washers — the light flat washer

Form B is the lighter, slimmer cousin of Form A. It has a smaller outside diameter and a thinner section, so it suits tight spaces and lighter duties where a full-size Form A washer would be overkill or simply won't fit.

What we stock: zinc plated — for example an M8 Form B washer measuring 8.4mm bore x 17mm outside diameter x 1mm thick.

Form C washers — flat washer with a bigger bearing area

Form C takes the standard flat washer idea and widens the outside diameter. That extra surface area means better load spread, which matters when you're bolting into softer materials or thinner sheet that could otherwise crush or pull through.

What we stock: zinc plated and A2 stainless steel, M4–M12.

Shop the range: Form C Washers

Form G washers — the heavy-duty flat washer

Form G is the biggest and heaviest flat round washer in the standard series. The large outside diameter and thicker section make it the go-to where the joint is under real load and needs maximum strength and durability — structural connections, heavy machinery and high-tension bolting.

What we stock: zinc plated and A2 stainless steel, M5–M30.

Shop the range: Form G Washers

Heavy duty thick washers (DIN 7349)

When you need the extra thickness to resist high clamp loads and stop the washer dishing or distorting, reach for these. DIN 7349 heavy washers are noticeably thicker than a standard flat washer and are built for structural steelwork, machinery mounting and heavy fabrication where the bolt is being pulled up hard.

What we stock: zinc plated, from M6 x 17mm x 3mm up to M16 x 40mm x 6mm.

Shop the range: Heavy Duty Thick Washers DIN 7349

Penny washers (repair washers)

Penny washers — also called repair washers — have a much larger outside diameter than a standard flat washer, but stay relatively thin. That big face spreads the load over soft materials like timber, plastic or thin sheet, and it's ideal for covering worn, oversized or ragged holes.

They're a staple for fencing, decking, cladding, and any soft-wood fixing where a normal washer would simply sink in.

What we stock: zinc plated and stainless steel, from M5 up to M12, with outside diameters from 20mm to 50mm.

Shop the range: Penny Washers

Square repair washers

The square version of the repair washer. The square plate shape gives sharp, flat edges that resist spinning in timber and provide an even larger bearing area for reinforcing worn or enlarged holes, or plating over damaged material.

What we stock: zinc plated square plate washers in 40mm and 50mm sizes (M6–M20), plus heavy-duty 40mm x 40mm x 5mm and 100mm x 100mm x 10mm square washer plates with square holes designed for coach bolts.

Shop the range: Square Repair Washers

Spring washers

Spring washers are split at one point and formed into a slight helix. When you tighten the bolt down, the washer flattens under pressure and pushes back, maintaining tension on the joint — which helps stop the nut working loose under vibration.

Use them in mechanical assemblies, engines, plant equipment and anywhere movement or vibration is a risk.

What we stock: zinc plated, A2 stainless and galvanised (clearance) options, M4–M30.

Shop the range: Spring Washers

Shakeproof washers

Shakeproof washers take the anti-loosening idea further. Instead of a split, they have teeth around the inside or outside edge that bite into the bolt head and the material as the joint is tightened. That mechanical grip resists loosening and, with the external tooth type, also cuts through coatings for a reliable electrical connection.

What we stock: internal tooth and external tooth, zinc plated, from M4 up to M20 (internal) and M4–M12 (external).

Shop the range: Internal & External Shakeproof Washers

Nylon washers

Sometimes metal isn't the answer. Nylon washers are non-conductive, non-marring and corrosion-resistant, so they're perfect for electrical insulation, protecting a painted or polished surface, or reducing friction and damping vibration in assemblies where a metal washer would cause problems.

What we stock: Form A nylon washers from M3 to M20.

Shop the range: Nylon Washers

Timber connectors

Timber connectors look different to a standard washer. They're toothed plates that sit between two timber members in a bolted joint. As the bolt is tightened, the teeth bite into both faces, spreading the load, improving grip and stopping the joint twisting or shifting. They're the proper way to make a bolted timber-to-timber connection perform.

What we stock: double-sided timber connector, M12 x 50mm.

Shop the range: Timber Connectors

Screw cup washers

For the finishing touches, screw cup washers are decorative brass cups that fit under a countersunk screw head to give a clean, polished appearance. They're the kind of detail that makes the difference on cabinetry, furniture and visible ironmongery.

What we stock: solid brass in polished and nickel-plated finishes, No.6, No.8 and No.10.

Also worth knowing: bonded washers

If you're fitting roofing or cladding screws, you'll see bonded washers — a steel washer bonded to an EPDM seal. They create a weatherproof seal under the screw head. At Fastco these are supplied fitted to our range of self-drilling and roofing screws rather than sold separately, so check the screw specification for the bonded washer you need.

How to choose the right washer

If you're not sure, work through these four checks:

1. Match the size to the bolt. Choose a washer whose bore matches the bolt diameter (M8 bolt, M8 washer). A sloppy fit lets the washer wander and weakens the joint.

2. Think about the material. Zinc plated covers most indoor jobs cheaply; choose A2 stainless outdoors or in damp conditions; choose nylon where you need insulation or a soft, protective face.

3. Match the outside diameter to the surface. Soft timber or thin sheet needs a big face (penny, Form C or Form G). Hard steel is fine with a standard Form A.

4. Match the thickness to the load. Light assembly is fine on a standard washer; high-tension structural work wants Form G or DIN 7349 heavy washers.

When in doubt, keep it simple: a standard Form A washer in the right size and finish solves most jobs — and stepping up to a penny or heavy-duty washer only when the material or the load demands it.

Browse the full washer range: Washers at Fastco