Washer sizing refers to the dimensional system used to define and specify a washer, covering the measurements that determine whether a given washer fits and performs correctly in a fastener assembly.
Every washer, regardless of type, is defined by three core dimensions: inside diameter (ID), outside diameter (OD), and thickness.
This guide covers SAE/USS sizing conventions for flat washers, along with the additional free height dimension used for spring washers. Engineers, procurement teams, and sourcing professionals use this guide to specify or verify washer dimensions before placing an order.
Editor's Note: The information provided in this article is intended for general informational purposes only and should not be interpreted as engineering, design, material selection, or application-specific guidance. Material requirements and specifications vary by application and should be evaluated according to individual project requirements.
How to Read a Washer Size Designation
Washer dimensions are commonly expressed as three values, in the order ID x OD x Thickness. For example, a 1/2" SAE flat washer is typically designated 0.531 x 1.062 x 0.095, with all values in inches, per ASME B18.21.1.
In some designations, the nominal bolt size is listed in place of the actual ID. "1/2 SAE," for instance, refers to a SAE washer sized for a 1/2" bolt rather than a washer with a 1/2" hole.
Military and aerospace specifications (AN, NAS, MS) use their own designation systems, where the spec number itself defines all dimensions and material requirements. Superior Washer's military washers collection covers the AN, NAS, and MS washers it stocks.
SAE vs USS Flat Washer Sizing
SAE and USS are both U.S. standard sizing systems for flat washers, but they use different outside-diameter-to-bolt-size ratios.
SAE washers, also known as Type A Narrow, have a smaller outside diameter relative to the bolt size. USS washers, also known as Type A Wide, have a larger outside diameter relative to the bolt size.
Both sizing systems are defined under ASME B18.21.1, with SAE corresponding to Type A Narrow and USS corresponding to Type A Wide. The applicable standard or design specification determines which sizing convention is used for a given application.
The chart below lists basic ID, OD, and thickness dimensions for common bolt sizes under both conventions.
| Dimension | Definition | What It Affects |
| Inside Diameter (ID) | The hole diameter; must clear the fastener shank | Fit on the fastener; typically specified slightly larger than the bolt or screw diameter |
| Outside Diameter (OD) | The outer edge diameter of the washer | Bearing surface area and load distribution. A larger OD spreads load over a wider area |
| Thickness | The depth of the washer | Load capacity, assembly stack height, and gap-filling capability |
Source: Basic nominal dimensions per ASME B18.21.1, the governing standard for Type A Narrow (SAE) and Type A Wide (USS) flat washers, as compiled and published by Brighton Best International. ASME B18.21.1 does not separately define SAE and USS for sizes below 1/4" or above 1-1/2".
Superior Washer stocks both conventions as part of its standard washers collection.
Flat Washer Dimensions: ID, OD, and Thickness
Three measurements define a flat washer's fit and function: inside diameter, outside diameter, and thickness. Each is covered below.
Inside Diameter (ID) on a Washer
Inside diameter is the hole diameter, the opening through which the fastener shank passes. It is sized to clear the fastener shank and is typically slightly larger than the nominal bolt or screw diameter. ID is expressed in inches for imperial washers.
Outside Diameter (OD) on a Washer
Outside diameter is the outer edge diameter of the washer, and it determines the bearing surface area. A larger OD provides a greater bearing surface area. OD is expressed in inches for imperial washers, and SAE and USS conventions differ primarily in OD relative to bolt size.
Washer Thickness
Thickness is the depth of the washer measured through its cross-section. It is expressed in inches for imperial washers and varies across washer types and sizing standards. Shims are available across a range of thicknesses for precision spacing applications.
Spring Washer Dimensions: ID, OD, Thickness, and Free Height
Spring washers share the same base dimensions as flat washers, inside diameter (ID), outside diameter (OD), and thickness, as described above. They also have one additional dimension specific to spring washers: free height.
Free Height
Free height is the unloaded, or uncompressed, height of the spring washer, the overall height of the washer before any load is applied. It is a defining characteristic of Belleville, curved, and three-wave spring washers, and is expressed in inches for imperial spring washers.
Free height tolerance (FREE H TOL) defines the acceptable range of variation in this dimension.
Superior Washer stocks Belleville, curved, and three-wave spring washers. Spring washer dimensions are commonly available by filtering on the product pages by ID, OD, Thickness, ID TOL, OD TOL, FREE H, and FREE H TOL.
Shim Sizing and Thickness Tolerances
Shims are a specialized washer form where thickness precision is the primary dimensional requirement, available in a wide range of thicknesses.
ID TOL, OD TOL, and thickness tolerances are defined per the applicable specification. The ID and OD of shims follow the same conventions as flat washers, but thickness is the primary specification.
Superior Washer stocks shims in stainless steel, brass, copper, aluminum, and specialty materials. Explore the shims collection for available sizes and materials.
Getting Washer Sizing Right
Flat washer sizing is defined by inside diameter, outside diameter, and thickness. Spring washer sizing uses those same three dimensions with the addition of free height.
SAE/USS are distinct sizing systems for flat washers, while spring washers generally use the same base dimensions across Belleville, curved, and three-wave configurations. Military and aerospace specifications (AN, NAS, MS) use their own dimensional designation systems entirely.
For a specific size or volume requirement, submit a quote request or contact the Superior Washer team directly.
Ready to find the right part? Browse the flat washers collection to see sizes and materials in stock.
Frequently Asked Questions
The following questions address points of confusion when specifying or verifying washer dimensions.
What do the three numbers in a washer size mean?
The three numbers refer to inside diameter, outside diameter, and thickness, often expressed in that order as ID x OD x Thickness.
What is the difference between SAE and USS washers?
SAE washers, also called Type A Narrow, have a smaller outside diameter relative to the bolt size. USS washers, also called Type A Wide, have a larger outside diameter relative to the bolt size. Both are defined under ASME B18.21.1.
What does a washer size designation mean?
A washer size designation commonly lists inside diameter, outside diameter, and thickness, though some designations reference the nominal bolt size rather than the actual ID.
What is the standard thickness for a flat washer?
Thickness varies by bolt size and sizing standard. For example, under ASME B18.21.1, a 1/2" SAE flat washer is 0.095" thick, while a 1/2" USS flat washer is 0.109" thick.
What is inside diameter and outside diameter on a washer?
Inside diameter is the hole through which the fastener passes. Outside diameter is the outer edge of the washer, which determines the bearing surface area.
What spring washer types does Superior Washer stock?
Superior Washer stocks Belleville, curved, and three-wave spring washers.
Where can I find a washer size chart?
This guide includes SAE and USS dimensional charts for typical bolt sizes. Superior Washer's product pages also allow filtering by ID, OD, and thickness for specific sizing requirements.
What is free height on a spring washer?
Free height is the unloaded, uncompressed height of a spring washer, measured before any load is applied. It is a defining dimension for Belleville, curved, and three-wave spring washers.
Do spring washers and flat washers use the same sizing system?
Spring washers share the same base dimensions as flat washers (ID, OD, and thickness) and add one additional dimension, free height, that is specific to spring washer configurations.