How we measure tread depth of tires exactly?

today i was in canadian tire for service my car. examiner said that i have car tires with tread depth 4. what is mean by this number. i have to change tires now as he said. what is tread depth number for a new tire. can any body tell me what tread depth atleast for a tire if i bought a used tire.How we measure tread depth of tires exactly?It means that you have 4/32 of an inch left on your tire.if you take a penny and put Abe's head down into the grove of the tire and you can see the top of his head you need new tiresHow we measure tread depth of tires exactly?The examiner had a depth indicator.. We here in the states use the good old American Penney..You insert the Penney in the tread head first... if you can see the top edge of Lincoln's head you need new tires..How we measure tread depth of tires exactly?Buy a tread depth gauge. Minimum safe depth is 2/32 of an inch, or if you have a penny stick it in the groves between the treads with the top of Abe's head towards the tire, if you can see the space on the penny between the top of Abe's a head and the edge of the penny where it is touching the tire, you do not have enough tread left.How we measure tread depth of tires exactly?tread depth is measured by how many thousands of an inch deep it is. look at the tread, on about 4-6 diferent places around the tire there will be tread wear bars. if your tires are worn down the the depth bars (almost bald) then they need replaced. you can buy a depth gauge (very cheap) or use the picture on a penny. Hold it upside down, stick it in the tread, if you can see abe's forehead, you need tires.How we measure tread depth of tires exactly?well being in canada, i would guess that he meant 4milimeters and not 4/32ths of an inch. but its the same concept...new tires range from about 7mm on up to 20ish for off road tires. 4mm is quite a bit...i work at a tire shop and we dont recomend replacing tires till 2/32 of an inch..or actually about 2mm...that is, unless the customer is needing more traction for the snow.How we measure tread depth of tires exactly?Depending on wether he was reading metric or SAE 4 represents 4/32%26quot; , or 4 mm's. on your tire inbetween the %26quot;lugs%26quot; in the tread there are wear indicators wich represent 2/32 of tread remaining, by hiway traffic act this is when your tires are officially worn out (if they want to be sticky) at 4 your traction and handling is compromised and should concider shopping for new tires. With this being done at the time of a regular service your tires might not last till your next service so they are suggesting a recommended repair.