Step by Step Tutorial: A rebar (short for reinforcing bar), also known as reinforcing steel, reinforcement steel, rerod, a deformed bar, reo, or reo bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression. It is usually formed from carbon steel, and is given ridges for better mechanical anchoring into the concrete.

The rebar thickness can be interpreted 2 different systems: English and metric.  The English basically consists on using one number to determine the thickness, for example, a # 4 bar or a # 8 bar. This number then needs to be divided by the number 8 to find the correct thickness in inches.  For example:
Number 4 rebar:

4/8 equals 1/2" thick rebar.
All you have to do is divide the number by 8
And the result will be the thickness of the rebar.
#3 /8 = 3/8" 
#4 /8 = 1/2" 
#5 /8 = 5/8" 
#6 /8 = 3/4" 
#7 /8 = 7/8"
#8 /8 = 1"   
Metric system:


How to Read Rebar

If you need to convert the rebar size to English system you can follow this formula:
Metric number 1 = English number
         3
For example:
A # 25 bar is equal to:
25-1 =   24 = 8
              3
A number 8 bar=1  thickness. 
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