How to Measure the Necessary Size for Knobs on Doors

Diameter

• The first measurement that you will need in order to determine the right size door knob is the diameter of the hole in which the knob will fit. The diameter is the distance straight across the hole from one side to the other. Place a tape measure on one side of the hole and measure directly across the center to find the diameter in inches.

Set Back Measurement

• The next step in the process is determining the set back measurement. This is the distance from the edge of the door to the center of the hole. To find the center of the hole, divide your diameter measurement by two. Then hold the end of the tape measure at the edge of the door and measure straight across until you reach the center point of the hole. This distance is the set back measurement.

Latch Piston

• The third part of the measuring process is determining the size of the latch piston. The latch piston is the part of the door knob that extends outward from the side of the door. The latch piston is the part of the hardware that snaps into the receptacle on the door frame and keeps the door closed. Use your tape measure to measure the diameter of this hole, located on the side of the door.

Standard Measurements

• Some common industrial standards apply to door knobs. Most door knobs have a diameter between 1 7/8 inches and 2 1/8 inches. The normal set back measurements for door knobs vary between 2 3/8 inches and 2 3/4 inches. As for latch pistons, the three common diameters for this part of the door knob are 7/8 inches, 1 inch and 1 1/8 inches. As an additional bit of information, the height of the doorknob should be 36 inches from the floor. Compare your measurements with these standard figures to double check your calculations before purchasing a door knob.