A quarter in the tread groove is still the fastest check there is. If you can see the top of Washington's head, you are at or below 4/32 inch and you should be shopping.
Wear pattern matters as much as depth. Both shoulders worn means chronic underinflation. The center worn means overinflation. One shoulder worn means alignment, and no new tire fixes that on its own.
Age counts even on tires with tread left. Rubber hardens and cracks. Check the four-digit date code on the sidewall — week and year — and treat anything past six years with suspicion.
