ANSI Z26.1: Safety Glazing Categories, Markings, and the 70% Tint Rule

ANSI/SAE Z26.1 is the national safety standard that governs how automotive glazing is built, tested, and labeled for every motor vehicle on U.S. roads. It sorts glass and rigid plastics into 20 categories, sets the tests each must survive, and prescribes the permanent markings that identify where a piece of glazing may legally be installed. Federal Motor Vehicle Safety Standard No. 205 incorporates ANSI/SAE Z26.1-1996 by reference, which gives the standard the force of federal law and makes compliance mandatory for every manufacturer, distributor, and installer.¹

Why the Standard Has Legal Force

The regulation behind the standard is 49 CFR § 571.205. It requires all glazing in motor vehicles to conform to ANSI/SAE Z26.1-1996 unless FMVSS 205 says otherwise, and it covers passenger cars, multipurpose vehicles, trucks, buses, motorcycles, slide-in campers, pickup covers designed to carry people in motion, and low-speed vehicles.¹ The rule reaches beyond the factory. Aftermarket replacement glass must meet the same requirements that applied to the original piece or to the current version of FMVSS 205.

Every manufacturer certifies each piece by marking it with a “DOT” symbol and a manufacturer code assigned by the National Highway Traffic Safety Administration.¹ Noncompliance carries a civil penalty of up to $21,000 per violation, capped at $105 million for a related series, with each defective piece counted separately.² NHTSA can also order mandatory recalls when glazing defects create an unreasonable safety risk.

The Glazing Categories You Will Actually See

ANSI Z26.1 uses 20 “Item” numbers, each defining a set of tests and the vehicle positions the material qualifies for.³ Three of them account for nearly all the glass on a passenger vehicle.

Item 1 (AS1): Laminated Glass

Item 1 glazing is two or more layers of glass bonded together with a plastic interlayer. This is the only construction approved for windshields, because the interlayer holds the pieces together on impact instead of letting them shatter into the cabin. That behavior prevents occupant ejection and keeps the windshield structurally intact during a crash. AS1 must transmit at least 70 percent of visible light and can be used in any window position on the vehicle.⁴

Item 2 (AS2): Tempered Glass

Tempered glass is a single sheet heat-treated to much higher mechanical strength than ordinary glass. When it fails, the whole panel breaks into small, relatively dull granules rather than long shards. AS2 is approved anywhere on a vehicle except windshields, and you will find it in most side and rear windows.⁴ The windshield exclusion exists because tempered glass does not hold together on impact, which would leave the driver with no forward barrier at all.

Item 3 (AS3): Restricted-Location Glazing

Item 3 glass is approved for use anywhere except windshields and any window beside the driver used for driving visibility.⁴ On passenger cars and taxis, AS3 is further excluded from all side windows, rear windows, and interior partitions. On trucks and buses, it is excluded from the windows immediately beside the driver and from the rearmost window when that window is used for driving visibility. In practice, AS3-marked glass often appears in the rear side windows of SUVs and vans, where lower light transmission does not compromise the driver’s primary sightlines.

The remaining item numbers cover narrower applications: rigid plastics for interior partitions, folding doors, roof openings, bus standee windows, motorhome doors, and trailer windows, plus specialized glazing such as bullet-resistant panels and double-pane insulating units.⁴ The item number stamped on each piece controls where it may legally go, and installing the wrong item in the wrong opening violates federal safety standards.

What the Tests Check

Before glazing earns an item designation, it has to pass a test battery matched to that category. A few core tests recur across most of them.

Impact Resistance

The ball-drop test measures whether the material can absorb a direct hit. For tempered glass, a 227-gram steel ball is dropped from 3.1 meters to confirm the panel can resist small projectiles such as road debris.³ For laminated windshield glass, the same test asks a different question: does the plastic interlayer keep the ball out of the passenger compartment? A laminated specimen that allows penetration fails regardless of how the glass cracked.

Luminous Transmittance

Any glazing installed in a position needed for driving visibility must let through at least 70 percent of visible light.³ The threshold applies to windshields and every side window the driver depends on to see the road, pedestrians, and other traffic. Transmittance is measured before and after simulated weathering, because glass that starts clear and then hazes is no safer than glass that was never clear enough.

Fracture Pattern

When tempered glass breaks, the resulting fragments have to stay small. A test specimen is struck with a spring-loaded punch, and the heaviest fragment from the resulting shatter cannot weigh more than 4.25 grams.³ The purpose is to keep the pieces small and blunt enough to reduce serious cuts when a side or rear window breaks. A specimen that produces large, heavy shards fails the test even if the glass was otherwise strong.

Rigid plastic glazing gets an additional abrasion test, because plastics scratch more easily than glass and haze that develops with wear will eventually compromise visibility.

Reading the Markings on Your Glass

Every piece of certified safety glazing carries a permanent mark, sometimes called a bug or monogram, that functions as a readable compliance certificate. It matters most when you are replacing glass and want to confirm the new piece is legal for its position.

DOT code. The letters “DOT” followed by a number identify the manufacturer. NHTSA assigns a unique code to each certified producer, and you can look up any DOT code in NHTSA’s Manufacturer Information Database to verify the company.⁵
AS number. “AS” plus a numeral indicates the item category. AS1 is approved for any location including windshields; AS2 is approved anywhere except windshields; AS3 is restricted-location only. An arrow within the AS marking points toward the area of the glass that meets the 70 percent transmittance requirement.¹
M number. The model number identifies the specific construction: thickness, tint level, or interlayer type. This code helps match replacement glass to original equipment specifications.

The AS-1 Line on Windshields

Many windshields carry a visible line etched or printed near the top and labeled “AS-1.” That line marks the boundary between the area of the windshield that meets the 70 percent transmittance requirement and any area above it, typically the shaded band at the top, that may transmit less light.⁶ If there is no AS-1 line, the entire windshield must meet the 70 percent threshold. That line is also the reference point most states use when regulating how far down aftermarket tint may extend on a windshield.

Aftermarket Tint and the 70 Percent Rule

The 70 percent transmittance requirement applies to the factory-certified glazing in positions needed for driving visibility. Adding aftermarket film to those windows can push total light transmission below the threshold, which puts the finished window in conflict with the performance level the glass itself was certified to meet. The federal standard does not regulate aftermarket film directly; enforcement of window tint falls to individual states.

State rules vary widely. Minimum visible light transmission for front side windows ranges from roughly 20 to 70 percent depending on the state, with many jurisdictions setting the limit near 35 percent. A few states prohibit any aftermarket tint on front side windows. Nearly all allow darker tint on rear side windows and rear windows, especially on SUVs, vans, and trucks. Windshield tinting is almost universally limited to the area above the AS-1 line or the top five inches, whichever is less. Some states offer medical exemptions for documented light-sensitivity conditions, usually requiring a physician’s statement and periodic renewal, and even then the tint generally cannot extend below the AS-1 line. Check your state’s motor vehicle code before having film installed.

ADAS Calibration After Windshield Replacement

Modern vehicles increasingly mount cameras and radar sensors directly behind or near the windshield. Replacing the glass disturbs those sensors’ factory alignment, and advanced driver-assistance features such as automatic emergency braking, lane-keeping, blind-spot monitoring, and pedestrian detection may not function correctly until the sensors are recalibrated to the new glass.

No federal regulation currently mandates ADAS recalibration after windshield replacement. The requirement comes from vehicle manufacturers’ own service specifications, which repair shops are expected to follow. A miscalibrated system usually produces no dashboard warning, so the driver believes the safety systems are working when they may not respond correctly in an emergency. Several states have introduced or passed legislation requiring glass installers to follow manufacturer calibration procedures and to disclose to customers when calibration was not performed or was unsuccessful. If your vehicle has a front-facing camera behind the windshield, confirm with the installer that recalibration is part of the job and that they are using the vehicle manufacturer’s procedure rather than a generic alternative.

1
eCFR. 49 CFR 571.205 – Standard No. 205, Glazing Materials
2
Office of the Law Revision Counsel. 49 USC 30165 – Civil Penalties
3
Federal Register. Federal Motor Vehicle Safety Standards – Glazing Materials
4
SAE International. ANSI/SAE Z26.1-2007 – Safety Glazing Materials Standard
5
National Highway Traffic Safety Administration. NHTSA Product Information Catalog and Vehicle Listing (vPIC)
6
National Highway Traffic Safety Administration. NHTSA Interpretation Letter – FMVSS 205