U-bolt is a bolt in the shape of the letter U with screw threads on both ends.

U-bolts have primarily been used to support pipework, pipes through which fluids and gasses pass. As such, U-bolts were measured using pipe-work engineering speak. A U-bolt would be described by the size of pipe it was supporting. U-bolts are also used to hold ropes together.

As U-bolts are now being used by a much wider audience to clamp any kind of tubing / round bar, then a more convenient measurement system needs to be used.

Four elements uniquely define any U-bolt:

Material type (for example: bright zinc-plated mild steel)
Thread dimensions (for example: M12 * 50 mm)
Inside diameter (for example: 50 mm – the distance between the legs)
Inside height (for example: 120 mm)

U-bolt is a bolt in the shape of the letter U with screw threads on both ends.

U-bolts have primarily been used to support pipework, pipes through which fluids and gasses pass. As such, U-bolts were measured using pipe-work engineering speak. A U-bolt would be described by the size of pipe it was supporting. U-bolts are also used to hold ropes together.

As U-bolts are now being used by a much wider audience to clamp any kind of tubing / round bar, then a more convenient measurement system needs to be used.

Four elements uniquely define any U-bolt:

Material type (for example: bright zinc-plated mild steel)
Thread dimensions (for example: M12 * 50 mm)
Inside diameter (for example: 50 mm – the distance between the legs)
Inside height (for example: 120 mm)