First, let's look at the anatomy of a hinge, which all styles share to a degree. This will be important to understand when calculating the size of the hinge needed.
Hinge Types Explained:
Leaf – The flat, rectangular portion of the hinge that attaches to the door or door jamb is the leaf. A standard hinge consists of two leaves.
Knuckle/Barrel – These are the circular rolled, hollow "barrels" through which the hinge pin is inserted. They allow the hinge to bend, just like the knuckle joints on your fingers. The term knuckle and barrel are often used interchangeably. The knuckles on one leaf are offset from those on the other to allow them to mesh. The measurement from the top barrel to the bottom barrel – measured parallel to the pin – is called the barrel length.
Pin – The rod that's inserted into the knuckles to join the hinge leaves.
Leaf height – The overall length of the leaf, measured parallel to the pin is the leaf height.
Leaf width – This is the dimension measured from the center of the pin to the outer edge of the leaf, opposite the knuckle or barrel. Don't confuse this dimension with the following dimension.
Hinge/open leaf width – This is the overall width of the opened hinge, measured perpendicular to the hinge pin. This will be an important dimension to understand when calculating, ordering, or specifying a hinge or hinge set.
Hinge backset – While not part of the hinge itself, it's important to know this dimension for your hinge size calculations. This is the distance the edge of the hinge is set back from the door face.
Clearance – Clearance is best described as the gap between the door and the frame or jamb with the door in the fully open position. This is an important measurement for your calculation as it's used to keep the door from contacting trim pieces on the outer face of the frame.
How to size your hinges: Hinge Sizing Calculator
As noted, the two hinge dimensions we're concerned with are the leaf or hinge height and the overall hinge width (measured with the hinge in the fully opened position).
The hinge height required is determined by the door thickness and the door width. The width of the door is important, because it affects the weight of the door. A wider door will need more load-bearing support for stability.
For example, if the door is 1-3/8 inches thick and the width is no more than 32 inches, the hinge height is 3-1/2 inches. However, if the door is between 32 inches and 36 inches, the leaf height of the hinges must be increased to 4 inches to handle the increased weight.