In most States when they reach 2/32 of remaining tread depth. For example, a typical tire that starts with 10/32 of original tread depth has only 8/32 of useable tread depth. Its useable tread depth is calculated by subtracting a worn out tire's 2/32 from the new tire's original depth of 10/32.
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=197
Losing its grip. These photos from beneath a moving tire show how even new tread is likelier to hydroplane as speed increases. At 20 mph (left), tread has full contact with the road surface. Hydroplaning begins at 40 mph (center) and gets worse by 60 mph (right). Shallow tread grooves increase the risk of hydroplaning.
http://www.consumerreports.org/cro/2012/12/how-safe-are-worn-tires/index.htm
Another way to check tread depth is to look at the treadwear indicator bar that's molded into your tires. The bars are located at the bottom of the tread grooves in several locations around the tire. When these bars become visibly flush with the adjacent ribs the tire has no more than 2/32 of tread remaining.
https://www.justtires.com/content/content.jsp?pageName=TreadDepth
Tread depth is a vertical measurement between the top of the tread rubber to the bottom of the tire's deepest grooves. In the United States, tread depth is measured in 32nds of an inch. Because it is difficult to accurately measure tread depth with a ruler, tread depth is best measured with a tire tread depth gauge.
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=194
For example, a typical tire that starts with 10/32 of original tread depth has only 8/32 of useable tread depth. Its useable tread depth is calculated by subtracting a worn out tire's 2/32 from the new tire's original depth of 10/32.
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=194
The final 2/32 of a tire's tread depth isn't part of the equation when it comes to calculating tread depth percentages because the tire is already legally worn out with just 2/32 of remaining tread depth.. Useable tread depth is calculated by subtracting 2/32 from the tires' new tread depth.
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=197
Another way to check tread depth is to look at the treadwear indicator bar that's molded into most tires. The bars are located at the bottom of the tread grooves in several locations around the tire. When these bars become visibly flush with the adjacent ribs the tire has no more than 2/32 of tread remaining.
https://www.goodyearautoservice.com/en-US/tire-basics/tread-depth
A tire that starts with 10/32 of original tread depth has 12.5% wear for every 1/32 that is worn away, and a tire that starts with 12/32 of original tread depth, has 10% wear for every 1/32 that is worn away, etc.
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=194
There are several popular ways to check your tire tread depth. One easy way is the penny test. Simply insert a penny into your tire's tread groove with Lincoln's head upside down and facing you. If you can see all of Lincoln's head, your tread depth is less than 2/32 inch and it's time to replace your tires.
https://www.goodyearautoservice.com/en-US/tire-basics/tread-depth
Wet-weather braking and snow traction also decrease as tires wear. Tires are considered bald when one or more of their grooves reaches 2/32 of an inch deep, compared with about 10/32 of an inch for new tires (tread wear is usually measured in 1/32-inch increments).
http://www.consumerreports.org/cro/2012/12/how-safe-are-worn-tires/index.htm
what is the tire depth of a new tire
The tire depth of a new tire is 10 by 32 of an inch. The depth of a new tire is 10/32 of an inch.