This pavement structural number calculator finds the AASHTO structural number (SN) of a flexible pavement from each layer’s coefficient, thickness, and drainage factor. Enter the layers above.
Formula
SN = a₁D₁ + a₂D₂m₂ + a₃D₃m₃
- a — layer coefficient, D — thickness (in), m — drainage factor
Example
a₁=0.44, D₁=4″; a₂=0.14, D₂=6″; a₃=0.11, D₃=8″ (m=1): SN = 1.76 + 0.84 + 0.88 = 3.48.
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Frequently Asked Questions
How do I calculate the structural number?
Sum each layer’s coefficient times its thickness times its drainage factor, per the AASHTO flexible pavement method.
What is a layer coefficient?
A measure of a material’s relative strength — about 0.44 for asphalt, 0.14 for base, and 0.11 for subbase.
What is the drainage factor?
A modifier (m) for how well unbound layers drain; good drainage gives a value near or above 1.