A quick rule-of-thumb: The number of hash marks on the head +2 is the grade of bolt. | Head Markings | Grade or Class | Material | Nominal Size Range
(Inches) | Mechanical Properties | Proof Load (psi) | Minimum Yield Strength (psi) | Minimum Tensile Strength (psi) | American |  No Markings | Grade 2 | Low or Medium Carbon Steel | 1/4 thru 3/4 | 55,000 | 57,000 | 74,000 | Over 3/4 thru 1-1/2 | 33,000 | 36,000 | 60,000 | 
3 Radial Lines | Grade 5 | Medium Carbon Steel, Quenched and Tempered | 1/4 thru 1 | 85,000 | 92,000 | 120,000 | Over 1 thru 1-1/2 | 74,000 | 81,000 | 105,000 | 
6 Radial Lines | Grade 8 | Medium Carbon Alloy Steel, Quenched and Tempered | 1/4 thru 1-1/2 | 120,000 | 130,000 | 150,000 | Stainless markings vary. Most stainless is non-magnetic | 18-8 Stainless | Steel alloy with 17-19% Chromium and 8-13% Nickel | 1/4 thru 5/8 | | 80,000 – 90,000 | 100,000 – 125,000 | 3/4 thru 1 | | 45,000 – 70,000 | 100,000 | Above 1 | | 80,000 – 90,000 | Metric | 
8.8 | Class 8.8 | Medium Carbon Steel, Quenched and Tempered | All Sizes thru 1-1/2 | 85,000 | 92,000 | 120,000 | 
10.9 | Class 10.9 | Alloy Steel, Quenched and Tempered | All Sizes thru 1-1/2 | 120,000 | 130,000 | 150,000 | Stainless markings vary. Most stainless is non-magnetic | A-2 Stainless | Steel alloy with 17-19% Chromium and 8-13% Nickel | 1/4 thru 5/8 | | 80,000 – 90,000 | 100,000 – 125,000 | 3/4 thru 1 | | 45,000 – 70,000 | 100,000 | Above 1 | | 80,000 – 90,000 | Tensile Strength: The maximum load in tension (pulling apart) which a material can withstand before breaking or fracturing.
Yield Strength: The maximum load at which a material exhibits a specific permanent deformation.
Proof Load: An axial tensile load which the product must withstand without evidence of any permanent set.
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