20". The range is about 20" - 25"

What is the diameter of the seat?

I just measured it with a ruler. It's 15 inches across (12 + 3).

what is the min and max seat height?

My height range is only 28-34". I don't know why mine is 2" shorter than the other respondent, but I sure could use an extra inch or two. Note there's another question where several people report max heights of only 34" or so.

How do you know exactly what replacement with locks for wheels will work for these dangerous wheels?

I have not replaced mine but this what you can do: Remove 1 caster (it should pull or knock out) and measure the stem diameter. Then go on the internet to Mapp Caster, who has industrial quality wheels, pick a set of locking casters.

ASIN	B072Y2S76T
Assembly Required	No
Best Sellers Rank	#3,840 in Our Brands ( See Top 100 in Our Brands ) #36 in Office Drafting Chairs
Brand	Amazon Basics
Color	Black
Date First Available	February 6, 2019
Furniture Finish	Chrome
Global Trade Identification Number	00841710174106
Item Weight	23.5 pounds
Item model number	GF-60118-1
Leg Style	Arched
Maximum Weight Recommendation	275 Pounds
Number of Items	1
Number of Pieces	1
Product Care Instructions	Wipe with Dry Cloth
Product Dimensions	23.4"D x 23.4"W x 45.6"H
Seat Height	33.9 Inches
Size	23.4"D x 23.4"W x 45.6"H
Style	Drafting Stool with Foot Rest
UPC	841710174106

Amazon Basics Multi-Purpose Adjustable Drafting Spa Bar Stool with Sturdy Foot Rest, Back Support Cushion, 360° Swivel and Wheels, Black

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