Carriage bolts are typically used in construction projects that involve wood. These bolts come in a variety of sizes and styles. The principle measurements of a carriage bolt are generally defined by seven categories: body diameter, head diameter, head height, screw length, square width and square depth. The most common measurement generally associated with bolts is referred to as nominal size. This measurement is typically used to gauge overall size.
Nominal size is the measurement by which bolts are commonly sized. In stores bolts are identified by this measurement. Carriage bolts range from as small as a size 10 to as large as 1 1/4 inch in diameter. This is primarily a measure of the diameter of the threaded end of the bolt.
In carriage bolts, bolt length is defined as the distance from the threaded tip of the bolt to the bottom of the bolt head where it meets the surface of an object. The length of carriage bolts commonly ranges from less than an inch to 18 inches in length. Bolt length and nominal size are the two measurements that most commonly identify carriage bolts.
Head diameter measures the width of the head of the bolt. This measurement varies with the size of the bolt. Bolts with a larger nominal size typically have larger head diameters. The diameter of a carriage bolt's head typically comes to roughly twice the nominal size.
Head height is a measurement of how much the head of the carriage bolt sticks up from the surface it is fastened to. Larger bolts tend to have more head height but are rarely more than 1/2 inch. Small carriage bolts can have head heights as low as a 1/10 inch.
Carriage bolts are different from other bolts in that they possess a square neck under the head. This neck prevents the bolt from spinning inside of wooden objects. How far the square sticks down from the head is referred to as the square depth. In small carriage bolts square width can be as little as 1/10 inch. In larger bolts square depth occupies up to twice that space.
Square width is a measurement of the total width of the square. This does not measure how much the square sticks out from the shaft of the bolt. Square width ranges from 1/5 inch in small bolts to nearly 3/4 inch in larger ones.
Body diameter in carriage bolts refers to the size of the unthreaded part of the shaft between the threads and the square. This measurement varies between bolts and manufacturers but it is typically within about 1/10 inch of the nominal size of the bolt.