Look up the borosilicate glass thermal expansion coefficient, and you’ll get the same answer on every page: 3.3 × 10⁻⁶ K⁻¹.
That number is real, and it explains why a borosilicate measuring cup can handle boiling water without cracking.
But it’s a single grade, not a fixed property of “borosilicate glass” as a category, and it doesn’t apply to a lot of the glass people search for this question to understand, including a large share of Pyrex.
This article gives you the actual number, where it stops applying, and what that means if you’re trying to judge a specific piece of glassware or run a real calculation.
What Is the Thermal Expansion Coefficient of Borosilicate Glass?
The thermal expansion coefficient of borosilicate glass measures how much the material grows or shrinks per degree of temperature change.
For the most common laboratory-grade borosilicate, that figure is 3.3 × 10⁻⁶ K⁻¹ — meaning the glass expands by roughly 0.00033% of its length for every 1°C rise in temperature.
The Standard Number: 3.3 × 10⁻⁶ K⁻¹
Coefficient of thermal expansion (CTE): the rate at which a material’s dimensions change with temperature, expressed per degree Kelvin or Celsius.
Borosilicate 3.3 glass, the type behind brands like DURAN and lab-grade Pyrex, carries this exact figure and is standardized under ISO 3585 and ASTM E228.
Boron trioxide (B₂O₃) is what gets it there: it forms a looser network in the glass structure than plain silica, so the material has less to expand in the first place.
That low number is also why this grade can go from ice water to a hot plate without shattering, something ordinary window glass can’t do.
Why “3.3” Is a Grade, Not a Universal Constant
“Borosilicate glass” isn’t one composition. It’s a family, and the family members don’t share a single CTE.
| Grade | Approximate CTE (× 10⁻⁶ K⁻¹) | Typical Use |
|---|---|---|
| Borosilicate 3.3 (ISO 3585 / ASTM Type I Class A) | 3.3 | Laboratory glassware, high-end bakeware, DURAN, lab-grade Pyrex |
| Low-boron borosilicate | 4.0 – 5.3 | Industrial and commercial glassware, some sight glass |
| Sealing/electrode borosilicate (matched to metals like Kovar or tungsten) | 4.6 – 7.38 | Electronics, lighting, glass-to-metal seals |
If you’re comparing “borosilicate” products and assuming they all behave the same way thermally, they don’t.
A 4.9 or 5.4 grade expands close to 60% more than the 3.3 grade for the same temperature swing, still far better than soda lime glass, but not the number most articles quote.
For a closer look at what separates these formulations from ordinary glass entirely, see borosilicate glass crystal.
Borosilicate vs Soda Lime Glass: Thermal Expansion Compared
Borosilicate glass expands roughly a third as much as soda lime glass for the same temperature change, and that gap is the entire reason one survives thermal shock and the other doesn’t.
The numbers below make the comparison concrete, and they explain why borosilicate vs regular glass keeps coming up as a buying question for bakeware and drinkware.
Side-by-Side Coefficient Comparison
| Property | Borosilicate 3.3 | Soda Lime Glass |
|---|---|---|
| Linear CTE | 3.3 × 10⁻⁶ K⁻¹ | ~9 × 10⁻⁶ K⁻¹ |
| Max temperature differential without cracking | ~166°C (330°F) | ~40°C (104°F) |
| Elastic modulus | ~9.1 × 10⁶ psi | ~10.2 × 10⁶ psi |
| Typical use | Lab glassware, bakeware, cookware | Windows, bottles, jars, and everyday drinking glasses |
Soda lime glass isn’t a worse material. It’s cheaper, easier to shape, and perfectly fine for anything that isn’t going through a rapid heat swing. It’s just the wrong choice for a dish that moves from a freezer straight into a 400°F oven.
What the Gap Means for Thermal Shock Resistance
A glass cracks under thermal shock when one part heats or cools faster than another, and the resulting stress exceeds what the material can absorb.
Borosilicate’s lower CTE means smaller stresses build up for the same temperature swing, so it can tolerate roughly four times the temperature differential soda lime can before failing.
That’s not a marginal safety cushion. It’s the difference between a dish surviving an oven-to-countertop transfer and one that shatters the moment cold air hits a hot surface.
Related: Borosilicate glass composition and Properties
Is Pyrex Actually Borosilicate Glass?
Not always, and this is where most articles quoting the 3.3 figure stop being useful.
Pyrex, sold in the US since 1998, is tempered soda lime glass, not borosilicate, a reformulation made by World Kitchen after it acquired the brand from Corning.
Why US Pyrex Changed in 1998
Corning sold the Pyrex consumer brand in 1998, and the new owner switched the US-market formula from borosilicate to tempered soda lime glass, largely for cost and manufacturing reasons.
European Pyrex, made by a different licensee, kept the original borosilicate formulation. That means two dishes with the same brand name on the box can have meaningfully different CTEs — one close to 3.3 × 10⁻⁶ K⁻¹, the other close to 9 × 10⁻⁶ K⁻¹.
Someone reading the borosilicate figure and applying it to a US-bought Pyrex dish is working with a number roughly three times too low for what they actually own.
If you’re weighing specific brands against each other, Duralex vs Pyrex breaks down how these formulations diverge in practice.
How to Check What Your Bakeware Is Actually Made Of
- Check the country of manufacture and brand era printed or stamped on the dish — pre-1998 Pyrex and current European Pyrex are more likely to be borosilicate.
- Look for “tempered” or “soda lime” language on the packaging or manufacturer’s product page, which rules out true borosilicate.
- Test with a small, controlled temperature change rather than assuming safety — moving a dish from the fridge directly to a hot oven and back is the scenario where the difference actually matters.
- Cross-reference the brand and pattern against manufacturer specs, since how to identify borosilicate glass covers the physical and visual cues that separate the two materials.
Linear vs. Volumetric Thermal Expansion: Which Number Are You Actually Reading?
Most comparison charts quote a single figure without saying which kind of expansion it measures, and that omission causes real calculation errors.
The number depends on whether you’re measuring how much the glass grows in one dimension or in total volume.
Linear Coefficient (α) vs. Cubical Coefficient (γ)
Linear coefficient (α): the rate of expansion along a single dimension — length, width, or thickness. This is the 3.3 × 10⁻⁶ K⁻¹ figure everyone quotes.
Cubical (volumetric) coefficient (γ): the rate of expansion in total volume, which for an isotropic material like glass is approximately three times the linear figure.
For borosilicate 3.3, that puts γ at roughly 9.9 × 10⁻⁶ K⁻¹ — a number that looks close to soda lime’s linear CTE, which is exactly where the confusion starts.
Why Mixing These Up Changes Your Numbers by 3x
If you’re calculating how much liquid a sealed borosilicate container will displace as it heats, or estimating internal stress from a volume change rather than a length change, you need γ, not α.
Grab the wrong one and your answer is off by a factor of roughly three, enough to matter in a lab setting, in glass-to-metal seal design, or in any calculation more precise than “will this crack.”
Most consumer-facing articles never specify which coefficient they’re citing, which is fine for a kitchen decision but a real problem if you’re doing the math yourself.
What This Means for Everyday Glassware
For most kitchen use, the takeaway is direct: check what the glass actually is before assuming its behavior, don’t rely on the brand name alone.
Freezer-to-Oven and Other Rapid Temperature Changes
- True borosilicate 3.3 handles a straight freezer-to-oven transfer better than almost any other consumer glass, thanks to its ~166°C differential tolerance.
- Soda lime glass, tempered or not, should never go directly from a freezer to a hot oven — the roughly 40°C tolerance gets exceeded fast.
- Tempered soda lime glass (including modern US Pyrex) resists impact well but still has soda lime’s higher CTE, so it’s not a substitute for borosilicate when the stress is thermal rather than physical.
- A quick check of whether Duralex bowls can go in the oven is worth doing before assuming any specific brand of dish handles heat the way “glass” in general does.
When Soda Lime or Tempered Glass Is Still the Better Choice
- For everyday drinking glasses and room-temperature storage, soda lime costs less and holds up fine, since there’s no thermal shock scenario to guard against.
- For anything that gets dropped often, tempered soda lime’s impact resistance can matter more than its thermal performance.
- For lab work, direct stovetop heating, or anything moving between extreme temperatures, borosilicate 3.3 remains the safer material by a wide margin.
- The decision comes down to which stress the glass will actually face — thermal or physical — not which one sounds more premium.
Trying to figure out what’s actually in your kitchen cabinet? Checking the brand, country of origin, and manufacture date takes a few minutes and settles the question far better than guessing from the name on the box.
Frequently Asked Questions
What is the thermal expansion coefficient of borosilicate glass?
Standard borosilicate 3.3, the most common lab and premium bakeware grade, has a linear CTE of 3.3 × 10⁻⁶ K⁻¹. Other borosilicate grades range up to roughly 7.38 × 10⁻⁶ K⁻¹ depending on composition.
The exact figure depends on which grade you’re referencing, not “borosilicate” as a single category.
Why does borosilicate glass have low thermal expansion?
Boron trioxide (B₂O₃) forms a looser glass network than pure silica, giving the material less internal structure to expand as temperature rises.
This is what lets it absorb rapid heating and cooling without building up the internal stress that cracks ordinary glass.
Is Pyrex borosilicate or soda lime glass?
It depends on where and when it was made. Pyrex sold in the US since 1998 is tempered soda lime glass, while European Pyrex and pre-1998 US Pyrex are borosilicate.
What is the difference between borosilicate 3.3 and 5.4?
Borosilicate 3.3 has a lower CTE (3.3 × 10⁻⁶ K⁻¹) and is the standard for lab glassware under ISO 3585. Borosilicate 5.4-type grades expand more per degree and are typically used in commercial or sealing applications rather than lab work.
How much thermal shock can borosilicate glass withstand?
True borosilicate 3.3 can typically handle a temperature differential of around 166°C (330°F) without cracking. Soda lime glass, by comparison, tolerates roughly 40°C (104°F) before failing.
Is soda lime glass’s thermal expansion higher than borosilicate’s?
Yes, by close to a factor of three. Soda lime glass has a linear CTE of about 9 × 10⁻⁶ K⁻¹, compared to 3.3 × 10⁻⁶ K⁻¹ for standard borosilicate.
What does the “3.3” in borosilicate 3.3 mean?
It’s shorthand for the glass’s coefficient of thermal expansion, 3.3 × 10⁻⁶ K⁻¹, which is also the grade name used in the ISO 3585 standard. It identifies a specific formulation, not borosilicate glass generally.
Can borosilicate glass go from the freezer to the oven?
True borosilicate 3.3 can generally handle that transition, given its high tolerance for temperature differentials. Confirm the piece is genuine borosilicate first, since a soda lime dish labeled generically as “glass” cannot make the same jump safely.
What is the thermal expansion coefficient of regular glass?
Ordinary soda lime glass has a linear CTE of approximately 9 × 10⁻⁶ K⁻¹, roughly three times that of standard borosilicate. This is why soda lime glass cracks more easily under sudden temperature changes.
Why does the thermal expansion coefficient matter for glassware?
It determines how much internal stress builds up when the glass heats or cools unevenly, which is what actually causes cracking.
A lower coefficient means a wider safety margin for rapid temperature changes, whether that’s an oven, a dishwasher, or a hot drink poured into a cold glass.
Is all Pyrex made of borosilicate glass?
No. Only European Pyrex and Pyrex manufactured before 1998 in the US are borosilicate; current US-market Pyrex is tempered soda lime glass.
What is the unit for thermal expansion coefficient?
It’s typically expressed per Kelvin (K⁻¹) or per degree Celsius (°C⁻¹), since a 1-degree change is the same magnitude on both scales. Figures are usually written as a value times 10⁻⁶, reflecting how small the actual expansion is per degree.