What Is the Highest z-index Value in CSS?

The z-index property in CSS controls the stacking order of elements—essentially determining which elements appear in front of or behind others when they overlap. But a common question that arises is:

“What is the maximum or highest z-index value you can set in CSS?”

Let’s dive into what the z-index limit really is, how browsers handle it, and best practices for using high z-index values.

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🎯 Short Answer

Technically, there is no defined maximum z-index value in the CSS specification. However, browsers implement their own limits based on the size of integers they support.

📌 In Practice:

Most modern browsers support z-index values up to:

2,147,483,647 (2^31 - 1)

That’s over 2 billion, which is more than enough for almost any realistic use case.

🔍 Tip: Setting a z-index of 9999 or 999999 is common for modals, popups, and overlays.

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💡 Why So Many Digits?

People often use large z-index values to “make sure” their element appears on top. Examples include:

.modal {
  position: fixed;
  z-index: 9999;
}

or even:

.super-modal {
  z-index: 2147483647;
}

While this works, using arbitrarily high values can cause more harm than good—especially when multiple components or third-party libraries all fight to be “on top.”

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❌ Potential Problems with High z-index

• Conflicts between libraries or components using extreme z-index values.
• Difficulty in debugging and maintaining stacking order.
• CSS getting messy with values like z-index: 99999999.

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✅ Best Practices

Best Practice	Why It Matters
Use layered increments	e.g., 100, 200, 300 for clarity
Reserve high values	For overlays, modals, etc.
Define a z-index scale	Use a design system or CSS variables
Avoid extreme values	Unless absolutely necessary

📦 Example: Scalable z-index map

:root {
  --z-base: 1;
  --z-dropdown: 100;
  --z-modal: 1000;
  --z-toast: 1100;
}

.toast {
  z-index: var(--z-toast);
}

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🧠 Fun Fact: Why 2,147,483,647?

That’s the maximum value for a signed 32-bit integer, a data type widely used in computing.

• 2^31 - 1 = 2,147,483,647
• Some browsers use 32-bit integers internally to represent z-index

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📝 Conclusion

The z-index value can theoretically go up to 2,147,483,647, but in most practical web design and development scenarios, you should avoid extreme values. Instead, use a structured, scalable approach to layering your elements with clarity and consistency.

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🔑 Recap

Question	Answer
Highest z-index	2,147,483,647 (browser limit)
CSS Spec Limit	No defined limit
Best practice value	Stay within a logical scale (e.g., 1–1000)
Avoid	Arbitrary high values like 99999999