Measure tire tread depth and convert 32nds to millimeters

What tread depth in 32nds means

In the US, tread depth is usually stated in thirty-seconds of an inch, written as a fraction such as 5/32 or simply "5/32nds." The number is the distance from the top of the tread surface to the bottom of a main groove. Each thirty-second is 1/32 of an inch, so a reading of 8/32 means the groove is 8 × 1/32 = 0.25 inch deep.

Because one inch is defined as exactly 25.4 mm, one thirty-second is exactly 25.4 ÷ 32 = 0.79375 mm, or about 0.79 mm. To convert, multiply the number of thirty-seconds by 0.79375 to get millimeters. To go the other way, multiply millimeters by 32 and divide by 25.4, which is about 1.2598 thirty-seconds per millimeter.

This guide covers how to take the measurement and convert it. Whether a given depth means a tire should be replaced depends on your jurisdiction's rules and the tire maker's guidance, so check those separately.

Conversion table: 32nds to millimeters

Exact values come straight from n × 0.79375. The rounded column shows those values to two decimal places for display only.

Thirty-seconds of an inch	Decimal inch	Exact millimeters	Rounded mm (2 decimals)
1/32	0.03125	0.79375	0.79
2/32	0.0625	1.5875	1.59
3/32	0.09375	2.38125	2.38
4/32	0.125	3.175	3.18
5/32	0.15625	3.96875	3.97
6/32	0.1875	4.7625	4.76
7/32	0.21875	5.55625	5.56
8/32	0.25	6.35	6.35
9/32	0.28125	7.14375	7.14
10/32	0.3125	7.9375	7.94
11/32	0.34375	8.73125	8.73
12/32	0.375	9.525	9.53
13/32	0.40625	10.31875	10.32
14/32	0.4375	11.1125	11.11
15/32	0.46875	11.90625	11.91
16/32	0.5	12.7	12.70

Conversion table: millimeters to 32nds

Millimeters	Exact thirty-seconds (mm × 32 ÷ 25.4)	Nearest whole 32nd
1 mm	1.2598	1/32
2 mm	2.5197	3/32
3 mm	3.7795	4/32
4 mm	5.0394	5/32
5 mm	6.2992	6/32
6 mm	7.5591	8/32
7 mm	8.8189	9/32
8 mm	10.0787	10/32
10 mm	12.5984	13/32

How the measurement works

A tread depth gauge has a flat foot (or shoulders) that rests on the tread surface and a probe that drops into a groove. The gauge reports how far the probe extends below the foot. If the foot sits flat across the tread and the probe reaches the groove floor, that distance is the tread depth.

Two things can throw this off. First, if the gauge was not zeroed, every reading carries the same offset. Second, if the probe lands on a wear bar, the reading stops at that bar instead of the groove bottom. Wear bars are raised rubber ridges molded across the bottom of the main grooves, so a probe resting on one reports a shallower depth than the groove actually has.

Tread rarely wears evenly. Continental's guidance is to check several locations, including any areas that look more worn, and keep the lowest reading. That lowest value is the one that describes the tire's condition, not the average.

Equipment

A tread depth gauge. The steps below follow Sealey's instructions for the VS0565 digital gauge; other models have their own zeroing and handling procedures, so follow the instructions for your gauge.
A flat, clean surface for zeroing.
A recording sheet or notes app, so each groove reading is kept with its location.
The vehicle parked on level ground with the engine off and the parking brake set before you work around the tires.

Steps: measuring with the Sealey VS0565

1. Prepare the gauge. Sealey says to keep the gauge clean and dry and to use it at reasonably constant temperatures around 20°C (68°F). A gauge moved between very different temperatures may drift, so let it settle before use.
2. Choose the display unit. The VS0565 can show decimal millimeters or decimal inches. It does not display fractions, so a reading in thirty-seconds always requires a calculation.
3. Zero the gauge. With the gauge on, place its foot on a flat surface, bring the probe level with the foot, and press ZERO. Confirm the display reads 0.00. If it does not, repeat the zeroing before measuring anything; a gauge that will not zero should not be used for readings.
4. Position the gauge on the tire. Bridge the tread with the foot so it rests across the tread blocks, and place the probe over a main groove. Continental describes the same principle for any gauge: probe in the groove, shoulders flush against the tread.
5. Lower the probe. Let the probe down until it stops moving, then read the display. Use uniform, constant pressure and do not press hard; pushing the foot into the rubber changes the reading.
6. Check the endpoint. Make sure the probe is on the groove floor, not on top of a wear bar or a stone lodged in the groove. If it is on a wear bar, move along the groove and measure again.
7. Repeat across the tire. Measure several grooves (for example inner, center and outer) and at several points around the circumference, including any visibly worn spots. Record each reading with its location.
8. Completion check. You are done when every location is recorded in the gauge's native unit and you have identified the lowest reading for each tire.

Converting VS0565 readings to 32nds

If the gauge is set to inches, multiply the decimal-inch reading by 32 to get thirty-seconds. If it is set to millimeters, multiply by 32 and divide by 25.4. Do the arithmetic on the raw reading and round only the final displayed number.

thirty-seconds = inches × 32
thirty-seconds = mm × 32 / 25.4
mm             = thirty-seconds × 25.4 / 32   (= thirty-seconds × 0.79375)

Illustrative example (a conversion exercise, not a measured tire): 5/32 inch equals 5 × 0.79375 = 3.96875 mm, which displays as 3.97 mm to two decimal places. Going the other way, a reading of 3 mm equals about 3.7795 thirty-seconds. Rounded to a whole thirty-second that becomes 4/32, but converting 4/32 back gives 3.175 mm, not 3 mm. Record the original 3 mm reading alongside the rounded 4/32 so the rounding is visible and reversible.

Resolution, accuracy and rounding are different things

Sealey lists the VS0565 with a range of 0–25 mm (0–1 inch), a resolution of 0.01 mm / 0.0005 inch, and an accuracy of ±0.02 mm / ±0.001 inch. These numbers describe different properties and should not be mixed up.

Resolution is the smallest step the display can show. A 0.01 mm display increment does not mean the reading is correct to 0.01 mm.
Accuracy is how close the reading is to the true depth under the stated conditions. On this gauge it is twice the resolution in millimeters (±0.02 mm versus 0.01 mm steps).
Rounding is a choice you make when you express the result. Converting a reading does not make it more precise: 3.97 mm converted to 5.0016 thirty-seconds has four decimals, but the measurement is still only as good as the gauge's stated accuracy.
Scale matters for the decision. ±0.02 mm is about ±0.025 of a thirty-second, while rounding to the nearest whole thirty-second can move a value by up to half a thirty-second (about 0.40 mm). When you report in whole 32nds, rounding, not the gauge, is the larger source of difference.

A recording worksheet

Keeping every reading, rather than just a final number, lets you see uneven wear and recheck conversions later. A simple layout:

Field	What to record
Tire	Position, e.g. front left
Location	Groove (inner, center, outer) and point around the tire
Endpoint	Groove floor confirmed, not a wear bar
Gauge	Model, e.g. Sealey VS0565
Native unit	mm or decimal inch, as displayed
Raw reading	Exactly as shown on the display
Resolution	0.01 mm / 0.0005 inch for the VS0565
Accuracy	±0.02 mm / ±0.001 inch for the VS0565
Converted value	Calculated from the raw reading
Rounding rule	e.g. two decimals in mm, or nearest whole 32nd

Workflow summary

1. Zero and position the gauge following its own instructions.
2. Record each groove reading in the gauge's native unit, with its location.
3. Identify the lowest reading for each tire.
4. Convert using the exact factor (25.4 mm per inch, 0.79375 mm per thirty-second).
5. Round only the displayed result, and keep the raw value beside it.
6. Compare the lowest value against the applicable legal requirement and tire manufacturer guidance as a separate step.