# FREQUENTLY ASKED QUESTIONS

## SIZES

- What does SDR stand for?
- What is the SDR (SODR, SIDR)?

*What does SDR stand for?*
SDR is the Standard Dimension Ratio. Technically, it is the nominal outside diameter divided by the minimum wall thickness.
The larger the SDR, the thinner the wall thickness. See tables (in ASTM F2160, D3485, NEMA TC7 or UL) for standard SDR numbers.

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*What is the SDR (SODR, SIDR)?*
SDR is the Size to Diameter Ratio (or Standard Dimension Ratio), that is, the ratio of the inside diameter (SIDR) or
outside diameter (SODR) of the duct or innerduct to the wall thickness of the duct or innerduct. The smaller the SDR
number the thicker the wall of the duct.

Smaller SDR numbers also mean that the duct or innerduct has higher tensile strength and greater crush resistance
than a comparable product with a higher number.

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