Two readings, not one
A single tread depth is a snapshot with no direction in it. Two readings taken at known odometer figures give a rate, and the rate is what answers the questions people actually have: does this set survive the trip, do I buy in October or in April, is one axle going to be due six months before the other.
The arithmetic is a subtraction and a division. Depth at the first reading less depth at the second, divided by the miles between them:
Wear rate = (depth₁ − depth₂) ÷ miles between × 1,000
Worked through with the numbers this page opens on: 9/32 at 5,000 miles and 6/32 at 29,000 miles is 3/32 of tread over 24,000 miles, which is 0.125 of a 32nd per 1,000 miles. Turn that around and it is 8,000 miles per 32nd. From 6/32 down to a 4/32 limit is two more 32nds, so 16,000 miles at that rate, which at 12,000 miles a year is sixteen months.
The same rate applied from new gives the whole picture: 10/32 down to 4/32 is six 32nds of usable depth, and at 0.125 per 1,000 that is 48,000 miles of tread. On a set of four at $145 fitted, $580 spread over 48,000 miles is 1.208 cents a mile, or about $145 a year at 12,000 miles. That last figure is the one worth knowing, because tires are the wear item people are most surprised by and it is the only one where the arithmetic is this easy.
Measuring so the second reading means something
The rate is a difference between two numbers, so any inconsistency between them lands entirely in the answer. A tenth of a 32nd of sloppiness across a 3/32 difference is three percent of error; the same sloppiness across a 0.5/32 difference is twenty percent. Long gaps between readings give better rates than short ones.
Same groove, same position around the circumference, same gauge. Tread wear indicator bars sit at the bottom of the grooves and a gauge tip landing on one reads shallow by the height of the bar, which is a large error on a worn tire and a confusing one because it is repeatable. Take three readings in the groove and use the middle one rather than the first.
Depth is reported two ways and the conversion is fixed: one 32nd of an inch is 0.79375 mm, so 4/32 is 3.18 mm and 2/32 is 1.59 mm. This page shows both regardless of which you enter, because tire data sheets, gauges and rules of thumb do not agree on which unit to use.
Why wear across the tread is measured and not diagnosed
Three readings across one tire — outer shoulder, centre, inner shoulder — give a spread. The spread is a fact. What causes it is not something three numbers can settle, and this page deliberately stops at the measurement.
What the spread does change is the arithmetic, and that part is honest. If the shallowest point is 5.5/32 and the deepest is 6.5/32 on the same tire, the shallow edge reaches a 4/32 limit 8,000 miles before the deep one does. A life estimate taken from the deepest groove on the healthiest tire is optimistic by exactly that margin, which is why the conservative move is to run the numbers on the worst reading you can find rather than the best.
| Reading across one tire | Depth | Miles to a 4/32 limit at 0.125 per 1,000 |
|---|---|---|
| Outer shoulder | 5.5/32 | 12,000 |
| Centre | 6.0/32 | 16,000 |
| Inner shoulder | 6.5/32 | 20,000 |
Eight thousand miles of difference across one tire, from three numbers taken a hand-span apart. Nothing in that table is a diagnosis and none of it says whether the spread is normal for that vehicle.
Work that does not forgive a mistake quietly
Brakes, steering, suspension, wheel fasteners, airbag and restraint modules, and anything holding the car up are in a different category from checking a tread depth. A wrong torque on a control arm bolt or a caliper bracket does not announce itself in the driveway; it announces itself later, at speed, and usually to somebody else as well. If you have not been shown that work in person by somebody competent, it belongs at a shop. Nothing here is a procedure and nothing here is permission.
What the rate does not survive
A wear rate assumes the next 10,000 miles look like the last 24,000. Loads, routes, seasons and pressures all move it, and the one that moves it most is the one people control without noticing: a tire run consistently under its placard pressure wears differently and faster than the same tire run at it. The tire pressure and temperature calculator covers why a correctly set tire reads low in winter, and the tire care guide covers where the pressure figure comes from in the first place. If you are looking at a different size rather than a replacement set, tire size comparison does the diameter and speedometer arithmetic, and car ownership cost puts the per-mile figure above into the running total for the vehicle.
Questions people ask
What tread depth should I replace tires at?
This page will not tell you, and you should be suspicious of any page that does. The figure that applies to you comes from three possible places and they do not always agree: the tire manufacturer has a specification for that exact tire, the vehicle manufacturer may state something in the owner documentation, and the jurisdiction you drive in sets its own rule that changes by state and by country and gets amended. There are also separate figures in circulation for winter tires and for wet-weather performance that are recommendations rather than limits. Enter whichever one governs for you and the calculator works from that. What it does with the number is pure subtraction — it restates your measurement against your figure and never judges it.
How many miles between the two measurements before the rate is any good?
Enough that the difference between the depths is large compared to how accurately you can read the gauge. A tread gauge in careful hands is good to roughly a tenth of a 32nd, so a difference of 3/32 carries about three percent of measurement error and a difference of half a 32nd carries around twenty percent. In practice that means several thousand miles at least, and a full season or a whole year is better. Two readings a fortnight apart will produce a rate, and the rate will be mostly noise. If you want a habit rather than a one-off, write the depth and the odometer on the same page every time the oil gets changed and the arithmetic gets better every entry.
Should I measure the shallowest groove or the deepest?
Measure several and know which one you used. The shallowest groove gives the conservative answer and the one that is closest to how any inspection would look at the tire, because an inspection is interested in the worst point rather than the average. The deepest gives the optimistic answer and is the one that flatters a set you were hoping to get another winter out of. The important part is consistency between the two readings that make up a rate: a first reading from the deep groove and a second from the shallow one produces a wear rate that is partly real wear and partly you having moved the gauge, and there is no way to tell afterwards how much of each.
Does a difference across the tread mean something is wrong?
It means the tire has worn unevenly, which is a measurement. This page reports the spread and refuses to name a cause, because several mechanisms produce a difference across the tread and telling them apart needs somebody with the car in the air looking at the whole corner rather than somebody reading three numbers off a form. What the spread definitely does is shorten the useful life relative to what the deepest groove suggests, since the shallow edge reaches any limit first. If the spread is growing between measurements, that is a thing worth taking to a shop with your numbers in hand, and the numbers make the conversation shorter.
Can I use the rate from the last set on the new set?
As a starting estimate and nothing more. A different tire model with a different compound and a different tread pattern wears at a different rate on the same car and the same roads, and the difference between a hard-wearing touring tire and a soft summer tire is not small. Load, route mix, season, pressure discipline and how new suspension components are all move it as well. Use the old rate to guess what to budget, then take a real depth reading a few thousand miles into the new set and a second one a season later, and replace the guess with the measurement. The second calculation is the one worth acting on.