How to Read a Hinge Specification: Load Ratings, Knuckle Count, and Finish Codes

A hinge specification sheet contains more information than most people use, and less standardisation than most people assume. Terminology varies between manufacturers, country standards differ, and the same physical product can be described in several ways depending on the market it is sold into. This guide decodes the key parameters — what each one means, why it matters, and what to look for when comparing specifications across suppliers.

Leaf Dimensions: Size, Gauge, and Radius

The primary dimension stated for a hinge is its leaf size — the height of the leaf when the hinge is open flat. A "4-inch hinge" has a 4-inch tall leaf. Width is typically stated as the leaf width per side: a "4 x 4" hinge is 4 inches high by 4 inches wide (total open width: 8 inches). A "4 x 3" hinge is 4 inches high by 3 inches wide.

Gauge refers to the thickness of the material from which the leaf is pressed or cut. Heavier-gauge hinges are more resistant to bending under load. Standard residential hinges are typically 2mm (approximately 0.079 inches). Commercial and heavy-duty hinges use 2.5mm or 3mm. The gauge is not always stated directly — if it is omitted, treat this as a red flag and request clarification.

Corner radius refers to the shape of the hinge corners. Standard options are square (0 radius), 1/4-inch radius, and 5/8-inch radius. The correct choice depends on the door and frame construction and aesthetic preference. Mis-matched corner radii between the hinge and the pre-routed door mortise will require routing adjustment.

Load Ratings: What They Mean and How to Use Them

Load rating is the maximum door weight the hinge is tested to support. This is one of the most commonly misread figures in hardware specifications — and one of the most consequential to get wrong.

Load ratings are stated per hinge, not per door installation. A hinge rated to 80kg supports 80kg when used as one of the hinges on a door. If three hinges are used, the combined rating is not simply 3 × 80kg — the load distribution between hinges is not equal, and the specification should be interpreted per the manufacturer's installation guidance.

The load rating should be a tested value, not a marketing estimate. Ask the supplier whether the load rating is derived from BS EN 1935 testing or equivalent standard. A tested rating is based on physical performance under the stated load; an estimate may simply reflect the theoretical capacity of the materials.

For specification purposes: identify the door weight, add a 30% safety margin, and verify the selected hinge exceeds that figure. A 40kg door should be specified with hinges rated to at least 52kg per hinge.

Cycle Ratings and Longevity

High-quality hinges, particularly those with ball bearings, carry a cycle rating — the number of open-close operations tested before performance degradation. For residential doors (typically 15–20 cycles per day), a 500,000-cycle rating corresponds to approximately 70 years of normal use. Commercial doors in high-traffic environments may see 200+ cycles per day, making a 1,000,000-cycle rating more relevant.

Ball-bearing hinges carry significantly higher cycle ratings than plain-bearing (loose-pin) hinges because the bearing distributes load more evenly and reduces wear. For any commercial specification, ball-bearing or concealed-bearing hinges should be the default.

Knuckle Count

The knuckle is the cylindrical barrel of the hinge that contains the pin. Knuckle count refers to the number of individual barrel sections. A 5-knuckle hinge has five cylindrical sections along the barrel length; a 4-knuckle hinge has four. Higher knuckle counts distribute pivot forces across more contact points, improving lateral stability and reducing wobble over time. 5-knuckle is standard for commercial applications; 4-knuckle is acceptable for light residential use.

Removable Pin (RP) vs Non-Removable Pin (NRP)

This is a security specification that is easy to overlook and consequential to get wrong.

On a standard removable-pin hinge, the pin can be withdrawn from the knuckle from the hinge side of the door. If a door is hung with RP hinges and the hinges are accessible from outside (i.e., the door opens outward), the door can be lifted free of the frame even when the lock is engaged, by removing the hinge pin.

A non-removable pin hinge has a set screw, a security rivet, or a fixed-head pin that cannot be withdrawn without tools. Some NRP designs use a captive ball bearing that engages when the door is closed, preventing pin removal regardless of access.

The specification rule is straightforward: any hinge on an outward-opening door that is accessible from outside should be NRP. This includes most external doors, security doors, and any entrance where the hinge barrel is visible from the non-secure side. Interior hinges where the hinge side is not externally accessible may use RP.

Finish Codes: BHMA and BS Standards

Finish codes standardise the description of surface treatments across manufacturers. Without codes, "satin" from one supplier may look different from "satin" from another. The two main standards used in architectural hardware:

• BHMA (Builders Hardware Manufacturers Association): US standard. Mirror polished stainless is BHMA 629; brushed satin stainless is BHMA 630.
• BS/EN: European standard. Equivalent codes are EN 32 (mirror polished) and EN 32D (brushed satin) for stainless steel.

When specifying hardware from multiple suppliers on the same project, always use the finish code, not the name. "Brushed satin" on a product from one supplier and "satin stainless" from another may or may not be the same grit and visual result. Finish codes provide the closest approximation to consistency.

Hand of the Hinge

Most standard hinges are non-handed — they can be installed on left-hand or right-hand doors interchangeably. Some specialised hinges (certain parliament hinges, certain concealed hinges) are handed and will only function correctly when installed on the specified handing. The specification sheet will indicate whether the product is handed (LH/RH) or non-handed (NH or "reversible").

What to Check When Comparing Specifications

• Is the load rating tested (to a named standard) or stated without test reference?
• Is the gauge stated, and is it appropriate for the application?
• Is the cycle rating provided for commercial applications?
• Is the finish code stated (BHMA or BS/EN), not just a generic description?
• Is the pin type specified (RP or NRP), appropriate for the security context?
• Are dimensional drawings available for mortise routing verification?

A supplier that provides all of the above without being asked is almost always a more reliable source than one that requires chasing for basic specification data. SSISKCON product pages include all of the above as standard. Dimensional drawings and load ratings are available for every product in the range.