What materials are used in the construction of this stool?

The stool features 7/8" O.D. 18-gauge steel tubing and a square, contoured hardboard seat with 16 rivets.

What is the weight capacity of the NPS 6318 6300 Series Heavy Duty Square Seat Steel Stool?

The NPS 6318 6300 Series Heavy Duty Square Seat Steel Stool supports up to 500 lbs.

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NPS 6318 6300 Series Heavy Duty Square Seat Steel Stool - Backless - Supports Up to 500 lb - 17.5" Seat Height - Brown Seat, Gray Base

Features a square, contoured hardboard seat with 16 rivets.
7/8" O.D. 18-gauge steel tubing.
Foot rings, 5/8 O.D. are welded to each leg by four contact points for added rigidity.
Tested by Bureau Veritas to support 500 lbs static load capacity.