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Washer Sizes Explained – What Size Washer Do I Need for M6, M8, M10 & M12 Bolts?
What Size Washer Do I Need?
M6, M8, M10 & M12 Dimensions Explained
Choosing the correct washer size is essential for secure bolt and machine screw installations. Whether you’re fitting an M8 washer, selecting an M10 washer, or checking washer dimensions for structural fixings, understanding compatibility ensures proper clamping and load distribution.
In this guide, we explain washer sizes, bolt compatibility, Form A vs Form B differences, and when to use penny washers.
What Do Washer Sizes Mean?
Washer sizes correspond to the bolt diameter they are designed to fit.
For example:
- An M8 washer fits an M8 bolt or threaded rod.
- An M10 washer fits an M10 bolt.
- An M12 washer fits an M12 fixing.
The “M” refers to metric sizing, and the number indicates the bolt’s nominal diameter in millimetres.
On larger repair washers, you may see a format such as:
M10 x 25mm
This means:
- 10mm hole diameter
- 25mm outer diameter
This is common on penny or mudguard washers.
Metric Washer Size Compatibility Guide
Below is a general guide to common washer sizes and bolt compatibility:
| Bolt Size | Typical Washer Hole Diameter | Common Washer Types |
|---|---|---|
| M6 | 6mm | Form A, Form B |
| M8 | 8mm | Form A, Form B, Penny Washer |
| M10 | 10mm | Form A, Form B, Form C, Penny Washer |
| M12 | 12mm | Form A, Form B, Form C |
Form A vs Form B vs Form C Washers
Not all flat washers are the same. The form refers to thickness and outer diameter.
Form A Washer
- Standard thickness
- Commonly used for general clamping and spacing
- Available in zinc and A2 stainless steel
- Conforms to BS 4320 / DIN 125-A
Form B Washer
- Thinner than Form A
- Used where reduced thickness is required
- Available in zinc and A2 stainless steel
- Conforms to BS 4320 / DIN 125-B
Form C Washer
- Wider outer diameter
- Provides increased clamping area
- Zinc plated
Form C washers are useful where additional surface contact is required.
When to Use Penny (Mudguard) Washers
Penny washers, sometimes known as mudguard washers, have a significantly larger outer diameter. These are commonly used where additional surface area is required to prevent pull-through in softer materials.
Washer Sizes and Matching Bolt Sizes
As a rule, match the washer size to the bolt size. The table below shows common pairings.
| Bolt Size | Standard Washer Size | Typical Use |
|---|---|---|
| M5 | M5 washer | Light-duty fixings and small brackets |
| M6 | M6 washer | General light structural fixings |
| M8 | M8 washer | Medium-duty brackets and fittings |
| M10 | M10 washer | Heavier brackets and structural connections |
| M12 | M12 washer | Structural steelwork and heavy fixings |
| M16 | M16 washer | Heavy structural and industrial applications |
On larger repair or penny washers, you’ll often see sizes written as M10 x 25mm. The first part (M10) is still the bolt size and hole diameter. The second number (25mm) is the outer diameter of the washer, which tells you how much clamping area you’ll get.
Penny Washer Size Examples
| Washer Size | Hole Diameter | Outer Diameter | Typical Use |
|---|---|---|---|
| M10 x 25mm | 10mm | 25mm | General load spreading |
| M10 x 30mm | 10mm | 30mm | Larger clamping area |
| M10 x 40mm | 10mm | 40mm | Repair and packing out |
Zinc vs Stainless Steel Washers
Choosing the correct finish is important for corrosion resistance.
| Finish | Best For | Corrosion Resistance |
|---|---|---|
| Zinc Plated (CR3) | Internal dry environments | Moderate |
| A2 Stainless Steel | External or corrosive environments | High |
Stainless steel washers should be used where exposure to moisture or corrosive environments is expected.
What Size Washer Do I Need?
As a rule:
- Match the washer size to the bolt size.
- Choose Form A for general use.
- Use Form B where reduced thickness is required.
- Select Form C or penny washers for increased clamping area.
- Choose stainless steel for external applications.
If in doubt, check bolt diameter and required load distribution before selecting your washer.
Frequently Asked Questions
What size washer fits an M10 bolt?
An M10 washer fits an M10 bolt. The washer hole diameter matches the bolt’s nominal metric size.
What does M10 x 25mm mean on a washer?
M10 refers to the bolt size (10mm hole diameter). 25mm refers to the washer’s outer diameter.
What is the difference between Form A and Form B washers?
Form B washers are thinner than Form A. Both fit the same bolt size but differ in thickness.
When should I use a penny washer?
Use a penny washer when you need to spread the clamping load over a larger surface area or prevent pull-through.
Should I use zinc or stainless steel washers?
Use zinc for internal dry environments. Use stainless steel for external or corrosive conditions.
