Tempered glass: the default choice, and the five specs that separate good from worthless

A tempered glass protector is not simply a thin sheet of window glass. It is chemically strengthened: the glass is bathed in a molten potassium salt, and the larger potassium ions swap places with smaller sodium ions near the surface. Those oversized ions crowd the surface layer into permanent compression.

That compressive layer is what does the work. A crack has to overcome the compression before it can propagate, so the glass takes a meaningful impact and then — when it finally fails — fails all at once, spreading the energy across the whole sheet instead of concentrating it on your display. A protector that has shattered into a spiderweb has just done exactly what you paid for.

It is also why a tempered protector cannot be cut or trimmed to fit. Cutting breaches the compression layer and the sheet falls apart. Buy the exact model, or do not buy.

Thickness you can feel. A glass protector adds 0.25 to 0.33 mm. On a phone that is barely noticeable; under a stylus, or on a device with edge-swipe gestures, it is.

It is consumable. Glass takes the scratch instead of your screen, which means it accumulates damage and needs replacing. Budget for that rather than treating it as a one-time purchase.

It fights curved edges. On a phone with meaningfully curved glass, a flat tempered sheet either stops short of the edge or lifts along it. Flexible film is the better answer there — see our glass vs film comparison.

Since every listing claims 9H, ignore it and compare these instead.

• Thickness. 0.25–0.33 mm is the sensible band. Thicker absorbs more impact; thinner feels better and is essential if your phone has an ultrasonic fingerprint reader.
• Full adhesive, not edge-only. The single most important spec. Edge-glued protectors leave an air gap in the middle that produces a visible rainbow shimmer and stops under-display fingerprint sensors working. Full adhesive costs a little more and fails far less.
• Oleophobic coating. The layer that makes your finger glide and lets fingerprints wipe away. It is also the first thing to wear out — and after three months it is the clearest difference between a known brand and an unbranded three-pack.
• 2.5D rounded edges. A squared-off edge catches on cases and chips on side impacts. Rounded edges cost nothing extra and last longer.
• An installation frame in the box. Worth more than any hardness claim, because the most common failure is not the glass — it is a crooked or dusty fitting that no specification can rescue.

Every product in this category advertises “9H hardness”, which makes it useless for choosing between them. It is also widely misunderstood: 9H is a pencil-hardness grade, not a position on the Mohs scale. It means the surface resists the hardest graphite pencil in a standard test set.

On the Mohs scale that actually predicts scratching, this glass sits around 6. Quartz — the main ingredient of sand and a good share of household dust — sits at 7. That is why beach trips scratch protectors, and why keys and coins, which are far softer, never did.

We unpack this properly in the 9H hardness guide.

A foldable’s inner screen. Never. The inner panel is flexible plastic; glass cracks on the first fold and can take the display’s top layer with it. Flexible film only.

A watch with sapphire crystal. Sapphire is far harder than protector glass. Covering it swaps a highly scratch-resistant surface for a less resistant one.

An E Ink reader. A glossy sheet over a matte, low-contrast display undermines the readability that is the entire point of the device.