Proof Load vs Yield vs Tensile Strength
Three strength numbers appear on fastener data. Proof load is the highest test load with no permanent stretch; yield is where permanent stretch begins; tensile is the breaking point.
Example: 1/2-13 Grade 5 (stress area 0.1419 sq in)
| Property | Stress | Load |
|---|---|---|
| Proof | 85 ksi | 12,060 lbf |
| Yield (min) | 92 ksi | 13,050 lbf |
| Tensile (min) | 120 ksi | 17,030 lbf |
Order
| Always | |
|---|---|
| Proof < Yield < Tensile | Proof is the safe test value; tensile is failure |
These tables carry the same figures as the printed sheet below.
The printed sheet
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Text version of this sheet
Proof load, yield strength and tensile strength
What it is
FIELD IDENTIFICATION
Three strength numbers appear on fastener data. Proof load is the highest test load with no permanent
stretch; yield is where permanent stretch begins; tensile is the breaking point.
Tensile
Yield
Proof
load
stretch
Proof < yield < tensile
Example: 1/2-13 Grade 5 (stress area 0.1419 sq in)
Property
Stress
Load
Proof
85 ksi
12,060 lbf
Yield (min)
92 ksi
13,050 lbf
Tensile (min)
120 ksi
17,030 lbf
Order
Always
Proof < Yield < Tensile
Proof is the safe test value; tensile is failure
Key points
Working loads are set by the engineer with a safety factor, never taken straight from tensile.
Governing standards: SAE J429 (2014); ISO 898-1:2013. Editions shown are the latest confirmed as of September 2026. The drawing and the governing standard always take
precedence over this sheet.
torquing any fastener. Verify the current governing standard, project documents, OEM requirements, size, thread, coating and lot certification. To the fullest extent permitted by law, The Fastener App disclaims liability
arising from reliance on this reference.
Fastener Guide reference FG-024ABE · thefastenerapp.com/fastener-guide/technical-reference/proof-load-vs-yield-vs-tensile-strength · © The Fastener App. Reference only — verify the current governing standard, drawing and manufacturer data before specifying or installing.
