One 32nd of an inch is exactly 0.79375 mm, so tread depth converts with no rounding at all: 2/32 in is 1.5875 mm and 4/32 in is 3.175 mm. Those two rows are the ones that matter. Federal rules for commercial vehicles set 2/32 in as the minimum major-groove depth on most tires and 4/32 in on the front wheels of buses, trucks and truck tractors.
Why tread is measured in 32nds
A depth gauge marked in 32nds reads a tire directly, with no arithmetic between the measurement and the limit. That is the entire reason the unit survives: the numbers people have to act on — the wear bars, the regulation, the point a tire stops clearing water — are all whole 32nds, and expressing them in decimal inches or millimetres turns a clean threshold into a judgement call.
Tread wear indicator bars are moulded into the major grooves at the minimum depth. When the surrounding tread has worn down level with them, the tire is at the limit — no gauge needed, though a gauge tells you how much is left before you get there, which is the more useful question.
Tread depth conversion table
| 32nds | Decimal inch | Millimetres | Note |
|---|---|---|---|
| 1/32 | 0.03125 | 0.79375 | |
| 2/32 | 0.0625 | 1.5875 | Federal minimum, most tires |
| 3/32 | 0.09375 | 2.38125 | |
| 4/32 | 0.125 | 3.175 | Federal minimum, front wheels of trucks and buses |
| 5/32 | 0.15625 | 3.96875 | |
| 6/32 | 0.1875 | 4.7625 | |
| 7/32 | 0.21875 | 5.55625 | |
| 8/32 | 0.25 | 6.35 | |
| 9/32 | 0.28125 | 7.14375 | |
| 10/32 | 0.3125 | 7.9375 | |
| 11/32 | 0.34375 | 8.73125 | |
| 12/32 | 0.375 | 9.525 | |
| 13/32 | 0.40625 | 10.31875 | |
| 14/32 | 0.4375 | 11.1125 | |
| 15/32 | 0.46875 | 11.90625 | |
| 16/32 | 0.5 | 12.7 | |
| 17/32 | 0.53125 | 13.49375 | |
| 18/32 | 0.5625 | 14.2875 | |
| 19/32 | 0.59375 | 15.08125 | |
| 20/32 | 0.625 | 15.875 |
Method and precision
The inch is defined as exactly 25.4 mm, so one 32nd is exactly 0.79375 mm and every row above is an exact decimal rather than a rounded one. Nothing here is truncated for display. The two flagged rows are 49 CFR 393.75, which sets a minimum major-groove depth of 2/32 in on most tires and 4/32 in on the front wheels of buses, trucks and truck tractors; the rest of the table carries no legal claim. Requirements for passenger cars are set by individual states and should be checked locally.
Reading a gauge marked in millimetres
A metric gauge and a 32nds gauge do not disagree, but they do land awkwardly against each other. The federal 2/32 in minimum is 1.5875 mm, so a metric gauge reading 1.6 mm is at the same point for practical purposes and one reading 1.5 mm is already past it. At the other end, 4/32 in reads 3.175 mm, which a gauge graduated in half millimetres will show as somewhere between 3 and 3.5 — worth knowing before you decide a tire is fine.
Measure in a major groove, away from the wear bars and away from any moulded hump or fillet, and take several readings across the width. A tire that measures well on one shoulder and badly on the other is telling you about alignment or pressure, not just about wear.
Depth gauges and calipers are in measuring equipment, and the rest of the vehicle range is in automotive tools.