Tire Tread Depth Converter
Convert tire tread depth between 32nds of an inch and millimeters.
Includes wear stage indicators and replacement guidelines.
Type in either field — the other updates instantly.
Understanding Tire Tread Depth
Tire tread depth is the vertical measurement from 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. In most other countries, it is measured in millimeters. Tread depth directly affects a tire’s ability to grip the road, especially on wet or snowy surfaces.
The Conversion:
1/32 inch = 0.794 mm
1 mm = 1.26 thirty-seconds of an inch
Depth (mm) = Depth (32nds) × 0.794
Depth (32nds) = Depth (mm) / 0.794
Tread Depth Wear Stages:
| 32nds of inch | Millimeters | Condition | Action |
|---|---|---|---|
| 10-12/32" | 7.9-9.5 mm | New tire | Full tread life ahead |
| 8-9/32" | 6.4-7.1 mm | Good | 70-80% life remaining |
| 6-7/32" | 4.8-5.6 mm | Adequate | 50-60% life remaining |
| 5/32" | 4.0 mm | Start monitoring | Plan for replacement soon |
| 4/32" | 3.2 mm | Marginal | Reduced wet traction, consider replacing |
| 3/32" | 2.4 mm | Worn | Replace soon, poor wet grip |
| 2/32" | 1.6 mm | Legal minimum (US) | Replace immediately |
| 1/32" or less | 0.8 mm or less | Unsafe / Bald | Dangerous — replace now |
Legal Minimums by Region:
Different jurisdictions have different legal minimum tread depths:
- United States: 2/32" (1.6 mm) — most states
- Canada: 2/32" (1.6 mm) — summer; some provinces require 3.5 mm for winter tires
- European Union: 1.6 mm (approximately 2/32")
- Germany winter tires: 4 mm recommended, 1.6 mm legal minimum
- United Kingdom: 1.6 mm across the central 3/4 of the tread
Built-in Wear Indicators:
All modern tires manufactured after 1968 have built-in tread wear indicator bars (TWI). These are small raised rubber bars molded into the grooves at the 2/32" (1.6 mm) level. When the tread surface wears down flush with these bars, the tire has reached the legal minimum and must be replaced.
Look for small triangles or “TWI” letters on the sidewall to locate the indicator positions.
The Penny and Quarter Tests:
Two quick ways to check tread depth without a gauge:
- Penny test: Insert a penny with Lincoln’s head pointing down into a groove. If you can see the top of Lincoln’s head, the tread is below 2/32" — time to replace.
- Quarter test: Insert a quarter with Washington’s head down. If you can see the top of Washington’s head, the tread is below 4/32" — start planning a replacement.
Why Tread Depth Matters for Safety:
Stopping distance on wet roads increases dramatically as tread wears:
- New tires (10/32"): Baseline stopping distance
- Half-worn (5/32"): Stopping distance increases by approximately 20-30%
- At 2/32": Stopping distance can increase by 80-100% compared to new tires
- Hydroplaning risk increases significantly below 4/32" (3.2 mm)