How to Check Tire Tread Depth: 5 Methods Compared

Valentyna Zhuk

Marketing Communications Manager

Inaccurate tire tread readings put drivers at risk. During National Tire Safety Week 2026, Anyline measured US tires in the field and found 1 in 7 was unsafe. That is measured tire tread data from the TireBuddy App, not surveyed. Knowing what each method can and can’t resolve is how technicians and inspectors end up with numbers they can trust.

How to check tire tread depth

Five methods, ordered from fastest to most precise. The first four take under a minute and cost nothing or almost nothing. All four give you a pass/fail verdict, not a number.

1. The Penny Test

Insert a US penny into a main tread groove with Lincoln’s head upside down and facing you.
Look at the top of Lincoln’s head.
If you can see the top of his head, your tread is at or below 2/32 in (1.6 mm), and the tire needs replacing.

The penny test is a convention, not a measurement. NHTSA and every major manufacturer endorse it as an approximation of the 2/32 in limit. Still, no primary source publishes the actual distance from the coin’s edge to the top of the bust. Lincoln’s hair is a gradual relief, not a sharp step, so there’s no defined edge to sight against. Two people can read the same groove differently, making the penny test inconsistent.

One coin fact is exact and worth knowing: a US cent is 1.52 mm thick, which is already thinner than the 1.6 mm legal minimum across the UK and EU.

2. The Quarter Test

Insert a US quarter into a main tread groove with Washington’s head upside down and facing you.
Check whether the tread covers part of Washington’s head.
If you can see all of his head, the tread is at or below 4/32 in (3 mm) , dangerously shallow for wet roads, though still legal in most states.

The quarter test is the more useful of the two coins, because 4/32 in is where wet performance actually degrades, and 2/32 in is where you are already there. It is also the vaguer instruction; “covers part of his head” is a softer judgement than “the top of his head is visible”, making the quarter test not the most consistent method. So treat it as an early warning to get a real measurement, not as a verdict.

3. Tread Wear Indicator Bars

Look for raised rubber bars running across the bottom of the main grooves.
Compare them to the surrounding tread blocks.
When the bars sit flush with the tread surface, the tire is at the wear limit and must be replaced.

Every radial tire sold for light vehicles in the US is required to have them. FMVSS 139 (49 CFR 571.139 S5.4) specifies “not less than six treadwear indicators spaced approximately equally around the circumference” that let an inspector see visually whether the tire has worn to one sixteenth of an inch, the regulation’s own wording, which is 2/32 in, or 1.5875 mm. Tires on rims of 12 inches or smaller need at least three.

Locating them is easier than it sounds: most manufacturers mark the position on the sidewall with a small arrow and the letters TWI. Michelin uses a small Michelin Man instead. Note that this sidewall marker is a European (ECE R30) convention and manufacturer practice, not a US federal requirement; a US tire is not obliged to tell you where its wear bars are.

Wear bars are binary, and they are conservative. They tell you a tire is finished. They tell you nothing at 5/32 in, which is when you actually want to be planning.

4. Tread Depth Gauge

Zero the gauge first, press the probe fully against a flat surface and confirm it reads 0/32.
Insert the probe into the centre of a main circumferential groove, not on a tread wear indicator, tie bar or stone ejector, and push the base flat against the tread blocks.
Read the value, then repeat in at least three more places, across the tread face and around the circumference, and average them.

This is the only one of the four manual methods that produces a number. A basic mechanical gauge reads in 1/32 in increments (0.79 mm), which means it cannot distinguish 2/32 in from 2.9/32 in, a tire that is legal from one that is not.

Digital gauges display to 0.01 mm resolution. Read that carefully: it is a display resolution, not an accuracy claim. A hand probe pressed against warm, flexible rubber does not deliver hundredth-of-a-millimetre accuracy, and any spec sheet implying otherwise is quoting its screen.

5. Mobile tread scanning

Open the TireBuddy app and point the phone camera at the tread.
Hold steady while the scan completes, a few seconds per tire.
Read the depth per groove, with the result written straight into the inspection record.

A mobile tire tread scanner uses the phone’s camera and on-device AI to build a 3D model of the tread surface, sampling around 100,000 points across the tire rather than the three or four a technician takes with a gauge. Anyline’s runs at precision down to 0.5 mm on standard hardware: iPhone SE (3rd gen) and above, Galaxy A53 and above, with no dedicated device to buy, calibrate or lose.

It is the only method on this list that produces a per-groove depth profile, a timestamped record, and a customer-shareable report in one pass. That distinction matters more than it sounds, and the next two sections are why.

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Tire tread depth chart

Depth (32nds)	Millimetres	Coin test	What it means
11/32 – 10/32	8.7 – 7.9 mm	,	Typical new passenger tire
8/32	6.4 mm	,	~25% worn. Full wet and snow performance
6/32	4.8 mm	Quarter: head well covered	Snow and hydroplaning resistance begin to degrade (Tire Rack)
5/32	4.0 mm	Quarter: head partly covered	Tire Rack’s replacement point for snow; Nokian says 4 mm for winter tires
4/32	3.2 mm	Quarter: whole head visible	Wet replacement point (Tire Rack, Consumer Reports, Michelin, Bridgestone). Also the FMCSA steer-axle limit for commercial vehicles
3/32	2.4 mm	,	Below every published wet recommendation. Legal in most states
2/32	1.6 mm	Penny: top of Lincoln’s head visible	Wear bars flush. Legal minimum in most US states, the UK and the EU. FMCSA limit for non-steer axles
1/32	0.8 mm	,	Legal minimum in California (Vehicle Code §27465(b))

Metric conversions are exact to one decimal. 2/32 in is 1.5875 mm; it is universally written as 1.6 mm, which is also the exact EU/UK figure in law.

What is the legal minimum tire tread depth?

In the US, there is no federal tread depth limit binding on passenger drivers. Limits are set state by state, and most states use 2/32 in (1.6 mm). California is the clearest low outlier at 1/32 in. In the UK and across the EU, the minimum is 1.6 mm, and it is enforced.

Two federal rules get confused with each other constantly, and the distinction matters:

FMVSS 139 (49 CFR 571.139) is a new-tire manufacturing standard. It requires tread wear indicators moulded to show one sixteenth of an inch. It does not set a wear limit for anybody driving.

49 CFR Part 570 contains a tread depth criterion, 4/32 in on front tires, 2/32 in on all others, and then explicitly disclaims itself:

This part does not in itself impose requirements on any person. It is intended to be implemented by States through the highway safety program standards.

So state law is the operative law. A peer-reviewed survey of all fifty states (Blythe & Seguin, Traffic Injury Prevention, 2006) found most states require 2/32 in, two require less, some have no statutory requirement at all, and some simply defer to the federal commercial-vehicle criterion. California’s Vehicle Code §27465(b) sets 1/32 in “in any two adjacent grooves at any location of the tire”, and specifies that measurement must not be taken where tie bars, humps or fillets are located.

The same paper argues the 2/32 in standard “lacks rational safety basis,” noting that tires below 4/32 in may lose roughly 50% of available friction in wet conditions before hydroplaning even begins.

Commercial vehicles (FMCSA)

49 CFR 393.75 is binding, and it is stricter:

Front / steer axle: at least 4/32 in, “when measured at any point on a major tread groove”
All other tires: at least 2/32 in, “when measured in a major tread groove”
Neither measured where tie bars, humps, or fillets are located

Note the phrase any point on steer tires. A single failing point is a violation. The regulation is drafted on the assumption that depth varies across a tire, otherwise, “any point” would be redundant.

One trap for fleet operators: CVSA’s out-of-service criteria are looser than FMCSA’s rule. CVSA puts a steer tire out of service at less than 2/32 in two adjacent grooves. A tire can therefore be fully compliant with roadside OOS criteria and still violate 393.75. If your inspection process is built around the OOS thresholds, it is not built around the law.

United Kingdom and EU

The minimum is 1.6 mm throughout a continuous band in the central three-quarters of the breadth of tread, and round the entire outer circumference, in force since 1 January 1992. Penalties are per tyre: up to £2,500 and 3 penalty points each, so four illegal tyres is £10,000 and an automatic ban.

Just like the other coin methods above, the UK’s popular “20p test” offers a quick way to check your tyres: place a 20p coin into the main tread grooves, and if the outer rim is fully hidden, your tread is safely above the 1.6 mm legal limit. If that border remains visible, safety charity TyreSafe warns your tyres “may not have sufficient depth and should be checked by a qualified specialist.” While some frequently claim the 20p coin’s outer band measures exactly 3 mm, the physical width from the coin’s edge to its raised border is actually 2.7 mm. Because of this, a visible band doesn’t mean your tyres are immediately illegal, but it does mean they are approaching the threshold where wet-weather performance drops significantly.

How to measure tire tread depth accurately

The most accurate way to measure tire tread depth in a working bay is a mobile scan. TireBuddy uses a phone camera and on-device AI to build a 3D model of the tread, sampling around 100,000 points per tire to 0.5 mm precision, against the three to six points a gauge can reach.

How TireBuddy measures tread depth

Open the app and point the camera at the tread. No dock, no jig, no calibration step. TireBuddy runs on phones you already own, iPhone SE (3rd gen) and above, Galaxy A53 and above.
Hold steady for a few seconds. On-screen guidance keeps the framing and distance right, so the scan is the same whether it’s taken by a 15-year technician or someone in their second week.
The AI reconstructs the tread surface in 3D. Rather than reading one point, it builds a depth model of the whole tread face from the camera image — roughly 100,000 sampled points per tire. The technical detail is here.
Read depth per groove. Not one number for the tire — a value for each circumferential groove, so a shoulder that’s wearing faster than the centre shows up as a gradient instead of hiding behind an average.
The result writes itself into the record. Timestamped, with the scan image attached, straight into the inspection, no clipboard, no re-keying, and something to show the customer who asks why you’re recommending four tires.

Why is TireBuddy the most accurate method in practice

Accuracy on a tire isn’t one number. It’s three things multiplied together, and every manual method fails at least one of them:

	Points sampled	Precision	Same result every operator	Creates a record
Penny/quarter	1	Incosistent	No	No
Wear bars	Visual	Pass/fail only	Roughly	No
Mechanical gauge	3–6	1/32 in (0.79 mm) steps	No	No
Digital gauge	3–6	0.01 mm display resolution	No	No
TireBuddy scan	~100,000	0.5 mm	Yes	Yes

A mechanical gauge cannot separate 2/32 in from 2.9/32 in — legal from illegal. A digital gauge fixes the readout and changes nothing about coverage: it still measures one point at a time, and the technician still chooses which points. A scan removes the choice.

What worn tire tread actually costs

AAA tested tires worn to 4/32 in. against new tires at 60 mph on wet pavement. Stopping distance rose 42% on a passenger car and 44% on a light truck. At the point where the new tires had stopped, the worn-tire vehicle was still traveling at nearly 40 mph.

Study Details: AAA (2019). Vehicles: 2017 Toyota Camry and 2017 Ford F-150 (six tire models each). Surface: Asphalt with granite aggregate, 1.0 mm average water depth. Test: Full ABS stops from 60 mph.

	New tires	Worn to 4/32 in	Change
Camry, wet stopping distance	209.44 ft	296.51 ft	+42%
F-150, wet stopping distance	197.5 ft	284.29 ft	+44%
Camry, wet lateral acceleration	,	,	−33%
F-150, wet lateral acceleration	,	,	−28%

At the point where the new-tire Camry had come to a complete stop, the worn-tire Camry was still travelling at 38.8 mph. The F-150 was doing 37.3 mph.

The counter-argument, stated fairly

Michelin publicly disputes the “replace early” consensus, and the objection is more interesting than it is usually reported.

Their argument is not that worn tires are as safe as new ones. It is that tread depth alone is a weak predictor of wet performance, because the spread between tire models is larger than the spread between new and worn. Michelin measured a 78-foot spread in stopping distance between the best and worst tires tested when worn, meaning some worn tires outperform other tires when new. They cite an EY estimate that premature replacement wastes around 400 million tires a year and costs drivers $25 billion globally, and note that replacing at 4/32 in rather than 2/32 in is a 40% increase in tire cost per mile driven.

What they advocate is not running tires to the cords. It is industry-wide performance standards for worn tires, test tires at the legal wear limit and publish the results, plus better consumer information.

The practical synthesis: tread depth is a necessary check, not a sufficient one. Which is an argument for measuring it well and recording it over time, rather than for measuring it less.

How tire inspectors measure tread depth at scale

A four-tire gauge inspection means about 24 probe placements and 24 handwritten numbers. At volume, the constraint is not gauge accuracy but sampling coverage, transcription error, and the absence of a record. Optical scanning removes all three by measuring the full tread face and writing the result directly into the system.

For a shop, a dealership group or a fleet, three problems compound that a single technician with a gauge never notices:

Coverage. Six readings per tire is the defensible floor, and almost nobody does six under time pressure. Under-sampling doesn’t produce a wrong number; it produces a number that is correct at the point measured and silent about the gradient.
Transcription. Every reading taken on paper or a clipboard has to be typed into the shop management system later. The 2022 version of this article made the point about laser scanners, and it still holds: a measurement that has to be re-keyed is a measurement with a second chance to be wrong.
No record. A gauge reading exists for as long as someone remembers it. There is no timestamp, no image, no wear trend across visits, and nothing to show a customer who disputes the recommendation.

Why are more inspectors moving to digital solutions?

Digital tire inspection on a phone addresses all three in one pass. The TireBuddy app is ready to use out of the box; the TireBuddy Toolkit puts the same scanning into an existing inspection app via SDK. Both run on standard phones, iPhone SE (3rd gen) and above, Galaxy A53 and above, at precision down to 0.5 mm, with ISO/IEC 27001:2013 certification and GDPR compliance. Anyline reports a 300% boost in inspection efficiency across deployments with Michelin, Discount Tire, Bridgestone and Continental.

For commercial operators, tread depth is only half the inspection: the tire DOT code scanner captures age and identity from the sidewall in the same workflow, which is what turns a depth reading into a compliance record.

Frequently asked questions

How do I check tire tread depth at home?

Use the penny test: insert a penny into a main groove with Lincoln’s head upside down. If you can see the top of his head, the tread is at or below 2/32 in (1.6 mm) and the tire needs replacing. For an earlier warning, use a quarter, if you can see all of Washington’s head, you are at or below 4/32 in (3 mm).

Is 2.5 mm of tread OK?

It is legal; 2.5 mm is above the 1.6 mm minimum in the UK, the EU and most US states. It is below every published wet-weather recommendation. Tire Rack, Consumer Reports, Michelin and Bridgestone all put the wet replacement point at 4/32 in (3.2 mm), so 2.5 mm means you are already past the point where wet braking measurably degrades.

What tread depth do new tires have?

New passenger tires typically start at 10/32 to 11/32 in, which is about 8 to 8.7 mm. Winter tires generally start deeper. That means a tire at the 2/32 in legal limit has worn away roughly 85% of its usable tread.

Can you check tire tread with your finger?

Not reliably. You can feel whether the wear bars are flush with the tread blocks, which tells you the tire is finished, but a fingertip cannot resolve the difference between 4/32 in and 2/32 in, and that is the range where the decision actually gets made. Use a coin at minimum, a gauge for a number.

How accurate is a tire tread depth gauge?

A mechanical gauge reads in 1/32 in increments, roughly 0.79 mm; it cannot separate a legal tire from an illegal one at the margin. Digital gauges display to 0.01 mm resolution, but that is a display specification rather than a measurement accuracy. The larger issue is coverage: any gauge measures one point at a time, and tread wears unevenly.

How often should I check my tire tread?

Monthly is the common recommendation, alongside every tire rotation and pressure check. For commercial vehicles, tread depth is part of the pre-trip inspection under FMCSA rules, and steer tires must hold at least 4/32 in at any point on a major groove.

Do tires expire even with good tread?

Yes. Michelin’s technical bulletin recommends replacing any tire ten years from its date of manufacture, including the spare, “even if such tires appear serviceable and even if they have not reached the legal wear limit,” with annual specialist inspection after five years. Many vehicle manufacturers advise six years in their owner’s manuals. NHTSA has no rule of its own and reports manufacturers’ recommendations; the widely-repeated “NHTSA says six years” is not accurate. The date of manufacture is in the DOT code on the sidewall.