Tempered glass is 4-5 times stronger than annealed glass and shatters into small, blunt cubes instead of dangerous shards. Annealed glass costs 2-3 times less but does not meet safety glazing requirements. This guide covers the manufacturing differences, strength ratings, break patterns, building code requirements, and which type is appropriate for each application.
Last Updated: March 2026
Performance and property comparison of tempered versus annealed glass at standard 1/4-inch (6mm) thickness.
| Feature | Tempered Glass | Annealed Glass |
|---|---|---|
| Tensile Strength | 24,000-30,000 PSI | ~6,000 PSI |
| Strength Multiplier | 4-5x standard glass | 1x (baseline) |
| Break Pattern | Small blunt cubes (~6mm) | Large sharp shards |
| Safety Glass Rating | Yes (CPSC 16 CFR 1201) | No |
| Thermal Shock Resistance | Up to 250 degrees F differential | Up to 100 degrees F differential |
| Can Be Cut After Manufacturing | No -- will shatter | Yes -- easily scored and snapped |
| Can Be Drilled After Manufacturing | No -- will shatter | Yes -- with diamond bit and coolant |
| Edge Work Options | Must be done before tempering | Can be done any time |
| Surface Compression | Over 10,000 PSI | Near zero (stress-free) |
| Manufacturing Process | Heated to 1,200 F, rapid air cooling | Slow controlled cooling in lehr |
| Relative Cost | 2-3x annealed | 1x (baseline) |
| Lead Time | 5-10 business days | 1-3 business days |
Building codes dictate which type is required in specific locations. Understanding these requirements prevents costly mistakes and code violations.
Tempered glass is approximately 4-5 times stronger than annealed glass of the same thickness. Standard 1/4-inch annealed glass has a tensile strength of approximately 6,000 PSI, while tempered glass of the same thickness reaches 24,000-30,000 PSI. This increased strength comes from the tempering process, which heats the glass to approximately 1,200 degrees Fahrenheit (620 degrees Celsius) and then rapidly cools it with air jets, creating a compressed outer surface and tensioned inner core.
When tempered glass breaks, it shatters into thousands of small, relatively blunt cubes approximately 1/4 inch (6mm) or smaller in size. This is called "dicing" and is a safety feature -- the small cubes are far less likely to cause serious lacerations. Annealed glass, by contrast, breaks into large, irregular shards with razor-sharp edges that can cause severe cuts. This break pattern difference is why building codes require tempered glass in hazardous locations such as doors, shower enclosures, and areas near floors.
The International Building Code (IBC) and International Residential Code (IRC), both adopted in Virginia, Maryland, and DC, require tempered or laminated safety glass in: all shower and bathtub enclosures, glass doors and sidelights, glass within 24 inches of a door opening, glass within 18 inches of the floor, glass near stairways and ramps, glass in guardrails and railings, and glass within 60 inches of walking surfaces in wet areas like pools. These requirements apply to both new construction and replacement glass.
No. Tempered glass cannot be cut, drilled, or edge-worked after the tempering process. Any attempt to cut or drill tempered glass will cause it to shatter completely into small fragments. All cutting, drilling, notching, and edge polishing must be completed before the glass enters the tempering furnace. This is why tempered glass must be ordered to exact dimensions -- it cannot be field-modified. If a tempered panel does not fit, a new panel must be manufactured.
Tempered glass typically costs 2-3 times more than annealed glass of the same size and thickness. The additional cost covers the tempering process (heating to 1,200 degrees F and controlled rapid cooling), the required precision cutting before tempering (since the glass cannot be modified afterward), and the quality control testing. For a standard 1/4-inch panel, annealed glass may cost a few dollars per square foot while tempered glass costs more per square foot. The price gap narrows on larger orders.
Look for a small etched or sandblasted label in one corner of the glass -- tempered glass is required to bear a permanent manufacturer mark showing the tempering standard (CPSC 16 CFR 1201 or ANSI Z97.1). You can also view the glass through polarized sunglasses: tempered glass will show dark spots or lines (strain patterns from the tempering process) that annealed glass does not have. A third method is looking at the glass at a shallow angle -- tempered glass may show a slight surface waviness (roller wave) from the tempering furnace rollers.
Heat-strengthened glass is heated and cooled similarly to tempered glass but at a slower rate, producing surface compression of 3,500-7,500 PSI (compared to over 10,000 PSI for fully tempered). It is approximately 2 times stronger than annealed glass (vs 4-5x for tempered) and breaks into larger pieces similar to annealed glass rather than dicing into small cubes. Heat-strengthened glass does not qualify as safety glass under building codes. It is used primarily in spandrel panels, insulated glass units, and areas where increased wind load resistance is needed without the safety glass requirement.
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