When Should I Use the z-index in CSS?

When building interactive web interfaces, elements often overlap — think of dropdown menus, tooltips, modals, sticky headers, and floating buttons. To manage which one appears on top, CSS gives us the z-index property.

But you might wonder:

When should I actually use z-index — and when should I avoid it?

In this blog post, we’ll cover:

✅ What z-index does
🧠 When to use it
🚫 When not to use it
⚙️ Practical use cases
🛠 Best practices

Table of Contents

✅ What Does `z-index` Do?

The z-index property in CSS controls the stacking order of elements that overlap each other. Higher z-index values bring elements “closer to the front” of the screen.

.modal {
  position: fixed;
  z-index: 1000;
}

⚠️ z-index only works on elements with a positioning context (position: relative, absolute, fixed, or sticky).

🎯 When Should You Use `z-index`?

Use z-index when two or more positioned elements overlap, and you need to control which one appears on top.

✅ 1. Dropdown Menus

Ensure dropdowns appear above other content.

.dropdown {
  position: absolute;
  z-index: 100;
}

✅ 2. Modals and Overlays

Modals should be above everything — including headers and background content.

.overlay {
  z-index: 900;
}
.modal {
  z-index: 1000;
}

✅ 3. Sticky Headers

To keep your header above scrolling content:

header {
  position: sticky;
  top: 0;
  z-index: 10;
}

✅ 4. Tooltips or Popups

Tooltips often need to float above all other UI layers.

.tooltip {
  position: absolute;
  z-index: 999;
}

✅ 5. Image or Content Overlaps

In photo galleries or featured sections, images may overlap text or each other.

.featured-image {
  position: absolute;
  z-index: 5;
}

🚫 When Not to Use `z-index`

While z-index is useful, avoid it in these scenarios:

❌ 1. When elements are not overlapping

There’s no need for z-index if nothing stacks.

❌ 2. On elements with `position: static` (the default)

z-index won’t work unless the element has a positioning context.

❌ 3. To fix layout issues caused by poor structure

If you’re using z-index to “force” elements into place, it may be time to rethink the HTML/CSS layout.

❌ 4. In isolation, without considering stacking context

Each stacking context is its own layer. A higher z-index inside a lower stacking context won’t always appear on top.

🧠 Best Practices for Using `z-index`

Tip	Why It Helps
Use consistent values	Avoid z-index conflicts across components
Pair `z-index` with `position`	Ensures it works as expected
Avoid overly high values	`z-index: 9999` everywhere causes chaos
Define a `z-index` scale	Create a design system layering guide
Keep your layout predictable	Simplifies maintenance and debugging

✅ Summary

Use Case	Use `z-index`?	Example Value
Dropdown Menus	✅ Yes	`z-index: 100`
Modals & Overlays	✅ Yes	`z-index: 1000+`
Tooltips	✅ Yes	`z-index: 999`
Static content	❌ No	—
Non-overlapping elements	❌ No	—

🚀 Final Thoughts

Use z-index when elements overlap and you need to control visual stacking order — like dropdowns, modals, and sticky headers. Avoid misusing it as a layout tool, and instead build a predictable z-index scale into your design system.

✅ What Does z-index Do?

🎯 When Should You Use z-index?