The moderators might wanna use this as a Sticky thread..i found it very useful:

Measuring Tread Depth

The easiest way to specify is to have the seller measure the tread depth, using a ruler or any thin object (e.g. toothpick) on which you can draw markings and then hold it up to a ruler or tape measure to measure. Tread depth is usually measured in 32nds of an inch, but it's easy enough to convert between mm and inches. (1 inch = 25.4 mm, 1 mm = 1.26/32 inch)

It's usually, but not always, the same depth across the tread (if it's not, then measure in multiple places and say where each number is).

Calculating Percentage Left

If you really want to calculate an accurate percentage of the tread left, once you know the tread depth, here's how to do it.

First, look up the specs of the tire, either on the manufacturer's website or on the website of a retailer like the Tire Rack, to find out the tread depth when the tire is new. Not all tires come with the same tread depth when new. For example, supersticky tires like the Falken Azenis RT-615 come with 8/32" when new, many summer tires like the Yokohama ES100 come with 10/32", many all-seasons with 12/32", and many winter tires have even more tread depth when new.

All street-legal tires sold in North America have treadwear indicator bars molded into the tread. These are strips about 1/2 inch wide, going across the tread, about every foot or so around the circumference. Take a close look at any tire and you'll see them. These strips are 2/32" more shallow (less deep) than the rest of the tread. As a result, when the tread depth gets down to 2/32", the treadwear indicator bars appear flat across the tread. At this point, tires must be replaced, by law, as they are no longer street legal.

Some people replace tires before they reach 2/32" of tread depth, because when they're worn down to, say, 3/32" or 4/32", they're not as good in rain as with deeper tread. But for purposes of your computation, you can assume the tires will be replaced at 2/32".

Then, it's just a matter of interpolating tread depth between the number when new, and 2/32".

For example, let's look at the Yokohama ES100, a popular tire for Integras, in the 195/55-15 size that's stock on the GS-R. According to the specs on the Tire Rack website, it comes with 10/32" of tread depth when new. Thus, the difference between new tread depth (10/32") and replacement tread depth (2/32") is 8/32". And each 1/32" of tread depth is 1/8, or 12.5 percent, of the tread life:

If the tread depth is 9/32", it has 87.5 percent remaining.
If the tread depth is 8/32", it has 75 percent remaining.
If the tread depth is 7/32", it has 62.5 percent remaining.
If the tread depth is 6/32", it has 50 percent remaining.
If the tread depth is 5/32", it has 37.5 percent remaining.
If the tread depth is 4/32", it has 25 percent remaining.
If the tread depth is 3/32", it has 12.5 percent remaining.
If the tread depth is 2/32", it has 0 percent remaining.

 

WOW, this thread has left me speechless!

 

in a good way or bad?

 

Oh dear my brain hurts.

 

Very informative thread.

 

That's good (easy) math. Most sellers, however, will say that a tire that originally had 10/32" of tread will say "4/32" tread depth remaining, or 40%", which is absolutely unfair and inaccurate. Look on Ebay, they do it this way all the time. They include the dead zone of that last 2/32" when they are on the wear bar. As always, buyer beware!

 

Lol I'm at -12.5% then.

 

i just eye ball it..