Measuring Screws and Bolts

Knowing how to find the diameter and length of the fasteners you need is an important first step in acquiring the correct parts.

Diameters of Screws / Bolts

To measure the diameter of screws and bolts, you measure the distance from the outer thread on one side to the outer thread on the other side. This is called the major diameter and will usually be the proper size of the bolt. You can also measure from the bottom cavity of the threads from one side to the other to find the minor diameter.

For sizes of 1/4" or lower, a number is commonly used to designate the size. This is called a nominal measurement. Here are those numbers and their decimal equivalent.

screw measurements diagram

(T) Thread Diameter. Also called major diameter.

(S) Shank Diameter

(R) Root Diameter. Also called minor diameter.

Screw Size Decimal
#0 .060
#1 .073
#2 .086
#3 .099
#4 .112
#5 .125
#6 .138
#7 .151
#8 .164
#9 .177
#10 .190
#12 .216
#14 .250

Length of Screws / Bolts

The simplest way to remember how to measure length: Length is the distance from the flat part of the head, to the tip or blunt end of the screw or bolt. Almost all screw head types will either be flat on the top or on the bottom.

Most screws are flat on the bottom of the head (pan head; round head; truss head, etc....) All of these are measured from the bottom of the head to the tip.


Flat head, countersunk screws are flat on the top of the head. They are measured from the top of the head to the tip.


Hex and hex washer head screws and bolts, although flat on the top and the bottom, are measured from the bottom of the head.


Oval head screws are an exception to the rule. The starting point for their length is at the widest point of the head. From that point to the tip is the length of the screw.


Self drilling screws are measured all the way to the end of the drill point tip.


If you have questions about the screw size you need, contact the Fastener SuperStore at (866) 688-2500, or via online chat at