SizedBox vs Spacer in Flutter Explained

When building user interfaces in Flutter, layout management plays a critical role. Developers often face the question of whether to use SizedBox or Spacer when arranging widgets in a Row or Column. Both widgets seem similar because they create spacing, but they serve different purposes. In this article, we will break down the differences between SizedBox and Spacer, explain when to use each, and show you complete Flutter code examples.

What is SizedBox in Flutter?

SizedBox is a widget that allows you to create empty space with a fixed width, height, or both. It is commonly used to insert gaps between widgets or to constrain the size of a widget.

Basic Example of SizedBox

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('SizedBox Example')),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              const Text("Above Text"),
              const SizedBox(height: 40), // Creates fixed vertical space
              const Text("Below Text"),
            ],
          ),
        ),
      ),
    );
  }
}

In this code, the SizedBox creates a fixed 40 pixels of vertical space between the two Text widgets.

What is Spacer in Flutter?

Spacer is a widget specifically designed to occupy the remaining available space in a Row or Column. Unlike SizedBox, which sets a fixed space, Spacer is flexible and adjusts automatically based on the available free space.

Basic Example of Spacer

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Spacer Example')),
        body: Row(
          children: const [
            Text("Start"),
            Spacer(), // Takes up all free space
            Text("End"),
          ],
        ),
      ),
    );
  }
}

In this example, the Spacer pushes the second Text widget to the far right of the Row, taking up all the empty space in between.

Key Differences Between SizedBox and Spacer

Advanced Example: Using SizedBox and Spacer Together

You can combine SizedBox and Spacer to create flexible yet consistent layouts. Below is a complete example:

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('SizedBox vs Spacer')),
        body: Column(
          children: [
            const Text("Top Section"),
            const SizedBox(height: 20), // Fixed spacing
            Row(
              children: const [
                Text("Left"),
                Spacer(flex: 2), // Flexible spacing
                Text("Right"),
              ],
            ),
            const SizedBox(height: 20), // Fixed spacing
            Row(
              children: const [
                Icon(Icons.star, color: Colors.amber),
                Spacer(), // Default flex of 1
                Icon(Icons.star, color: Colors.red),
              ],
            ),
            const SizedBox(height: 20),
            const Text("Bottom Section"),
          ],
        ),
      ),
    );
  }
}

This example demonstrates how SizedBox provides consistent fixed gaps while Spacer ensures responsiveness by adjusting spacing according to screen size.

When to Use SizedBox

• When you need exact spacing in pixels.
• When constraining the size of a widget (e.g., setting a button to a specific width).
• For consistent spacing across devices.

When to Use Spacer

• When you want to fill available space in a Row or Column.
• When creating responsive layouts that adjust automatically.
• When you need proportional spacing using flex values.

Best Practices

As a rule of thumb:

• Use SizedBox for predictable, fixed gaps.
• Use Spacer for flexible layouts where screen size may vary.
• Combine both when building complex UI designs that require a mix of fixed and flexible spacing.

Conclusion

SizedBox and Spacer are both essential tools in Flutter’s layout system. SizedBox gives you precise control, while Spacer ensures flexibility and responsiveness. Understanding when to use each widget will help you build more efficient and professional Flutter applications.