How to Use the Bucket Fill Tool in GIMP

The Bucket Fill tool looks simple - click an area, fill it with color. But the threshold and sample settings change what gets filled dramatically. Without understanding these two settings, the bucket fill either floods too much or leaves annoying unfilled patches. This guide covers everything you need to use it well.

Basic Usage

Press Shift+B to activate Bucket Fill. Click anywhere on the canvas to flood fill that area with your foreground color.

The fill spreads to all connected pixels that are similar in color to the pixel you clicked. Where it stops is controlled by the Threshold setting.

Threshold - The Most Important Setting

Threshold is in the Tool Options panel. It controls how similar adjacent pixels need to be to get filled.

Start at 30 for most fills. If the fill leaves small unfilled patches near the edges, increase it. If it bleeds into areas it should not touch, decrease it. Anti-aliased edges (common on text and smooth shapes) often require a threshold of 20-40 to fill cleanly.

Sample Merged

By default, the Bucket Fill only considers pixels on the active layer when deciding what to fill. If you have a line art layer above a coloring layer, this means clicking on the coloring layer with Sample Merged off will ignore the line boundaries and flood fill over them.

Check Sample Merged in Tool Options to make the bucket tool respect color boundaries from all visible layers. Now it will stop at the edges of your line art even when filling a layer below it. This is the key setting for coloring inside line art on a separate layer.

Fill Whole Selection

Check Fill whole selection in Tool Options and the bucket fill behaves very differently: it fills the entire active selection area with a flat color, completely ignoring the threshold and color-matching logic.

This is equivalent to Shift+Backspace (Edit > Fill with Foreground Color). Use it when you want to fill a shape you have selected, not a color region you clicked on. Covered in more depth in the fill with color guide.

Filling with Patterns

Change the Fill Type in Tool Options from "Foreground color" to "Pattern." Then click the pattern thumbnail to pick from GIMP's built-in patterns or any custom patterns you have installed.

The pattern tiles automatically to fill the entire area. Patterns respect the same threshold and selection settings as color fills. For web textures, checker patterns, or repeating backgrounds, this is much faster than manually tiling a small image.