Introduction

When buying a screen protector, you will often see terms like 9H, 11D, 21D, 5D, 6D, 9D and many more. These labels can be confusing because most people don’t know what they actually mean. The truth is: 9H and 11D/21D measure different things.

This guide explains the real difference in simple Indian English so you can choose the right tempered glass for your smartphone.

Difference in One Line

1. What Is 9H Tempered Glass?

9H refers to the hardness level of the tempered glass. It comes from the pencil hardness test — 9H means it can resist scratches from a 9H pencil, which is very hard.

Advantages of 9H Glass

• High scratch resistance.
• Good protection against drops.
• Clear visibility and smooth touch.
• Strong front protection for flat displays.

Disadvantages

• Normal 9H glass may not cover curved edges fully.
• Edges can lift on curved smartphones.

2. What Are 11D and 21D Tempered Glass?

11D and 21D do not represent hardness. These numbers are mainly marketing terms used to describe:

• glass curvature (flat, 2.5D, 5D, 9D, 11D, 21D, etc.)
• edge coverage (how much of the screen it covers)
• thickness and frame style

They show how well the protector fits your screen design, especially if your phone has curved edges.

Advantages of 11D / 21D Glass

• Better edge coverage compared to normal glass.
• May include a coloured or black border for alignment.
• More flexible frame for curved screens.

Disadvantages

• Still may not stick well on fully curved edges.
• Most 11D or 21D glasses are edge-glue only, not full-adhesive.
• Not as strong as UV tempered glass.

3. Why So Many “D” Numbers? (5D, 6D, 9D, 11D, 21D)

The “D” numbers are mainly marketing. In general:

• Bigger number = more curved edge / wider coverage
• But not more hardness or strength

Example:

• 5D → Slight curvature
• 9D → More curved, better fit
• 21D → Full-coverage border glass

4. Which One Protects Best?

5. Key Differences Explained Simply

• 9H = Hardness (material strength)
• 11D / 21D = Design style (shape + coverage)
• Higher D number = more curved coverage, not stronger protection
• Strength mostly depends on glass quality, not D number

6. Which One Should You Choose?

• Flat screen: 9H tempered glass
• Mild curved edges: 11D or 21D
• Strong curves / premium phones: UV tempered glass (best fit & protection)
• Budget option: 9D or 11D normal glass

Conclusion

9H, 11D and 21D serve different purposes. 9H tells you the hardness of the glass, while 11D and 21D describe the design and coverage style.

If your priority is maximum protection, choose 9H or UV tempered glass. If your phone has curved edges and you want better coverage, go for 11D or 21D. For the best overall performance on curved displays, UV glass remains the top choice.

Final advice: Don’t choose based on numbers alone — choose based on your phone’s display shape and your protection needs.