Tire Tread Depth Calculator
Tread reading with replace advice in 32nds or mm.
What it does
Tread clears water so rubber meets road, and shallow grooves quit that job long before the tire looks bald. Wet grip fades from 4/32 down, while 2/32 marks the legal edge in most states.
The tool above takes a gauge reading in thirty-seconds or millimeters. It returns the converted depth with a verdict band: replace now, replace soon, winter advisory, or good tread.
The default 5/32 in reads 3.97 mm and sits in the winter advisory zone. The metric mirror (4 mm) reads about 5/32 in with the same verdict, and a worn 1/32 in (0.79 mm) fails outright.
Minimums follow NHTSA TireWise guidance with Tire Industry Association replacement notes matching: wear bars at 2/32, penny test with Lincoln head, replace when the top shows. Both name the same line.
How to use
- Pick imperial (32nds) or metric (mm).
- Gauge the major grooves, several spots.
- Enter the shallowest reading.
- Read converted depth first.
- Read the verdict band.
- Shop tires at replace soon, not at bald.
Formula
- n
- Tread depth in thirty-seconds of an inch.
- 25.4
- Millimeters per inch, exact.
- 2/32
- 1.6 mm legal minimum in most states.
- 4/32
- 3.2 mm wet grip advisory (quarter test).
New passenger tires start near 10/32 to 11/32 with winter guidance often calling 6/32 the snow line; those sit here as information while the 2/32 minimum carries the legal weight. Uneven readings across one tire point at inflation or alignment trouble.
Worked example
Example: 5/32 in tread
Defaults: imperial, 5 thirty-seconds.
- Fraction: 5/32 = 0.15625 in.
- mm: 0.15625 x 25.4 = 3.97 mm.
- Band: 5 sits 4 to 6, winter advisory.
- Tool shows 5/32 in (3.97 mm), OK watch winter.
- Worn check 1/32: 0.79 mm, replace now.
- Metric mirror 4 mm reads about 5/32, same verdict.
Reference table
| Gauge | Metric | Verdict |
|---|---|---|
| 8/32 in | 6.35 mm | Good tread |
| 5/32 in | 3.97 mm | Winter advisory |
| 3/32 in | 2.38 mm | Replace soon |
| 1/32 in | 0.79 mm | Replace now |
Limits
- Tread is one safety factor among several.
- Age cracks rubber with full tread left.
- Uneven wear needs alignment, not just tires.
- Spare age needs its own 6 to 10 year check.
Common mistakes
- Measuring shoulder instead of major grooves.
- One spot instead of several around the tire.
- Running winter rubber at 3/32.
- Mixing worn and fresh tires on one axle.
- Ignoring date codes on old full tread tires.
Try it yourself
Quick checks. Answers sit tucked under each question.
Gauge reads 3/32: verdict?
2.38 mm, replace soon, weak wet grip.
Gauge reads 8/32: verdict?
6.35 mm, good tread.
Penny vs quarter tests?
Lincoln top showing means 2/32; Washington head means 4/32.
Questions
When do tires need replacing?
At 2/32 in tread, sooner for rain or snow duty.
What is the penny test?
Lincoln head down in the groove; seeing his top means replace.
What is 5/32 in mm?
3.97 mm, in the winter advisory band.
New tire depth?
About 10/32 to 11/32 in for most passenger tires.
Winter tire limit?
Many guides call 6/32 the snow service line.
Uneven wear meaning?
Center wear hints over inflation, edges under, patches alignment.