The flat is the brush that does the heavy lifting in acrylic painting. It lays in backgrounds, cuts crisp edges along a horizon, paints the side of a building in one stroke and, turned on its edge, draws a surprisingly thin line. Yet the labeling on flats is confusing: some are marked with numbers, others in inches, and a size 10 from one maker can look nothing like a size 10 from another. A flat brush size chart helps you make sense of those labels and match the width to the mark you actually want on the canvas.
Quick answer: Flat brushes are labeled either by number, commonly from 0 or 2 up to 20 or more, or by width in inches, such as 1/4, 1/2, 3/4, 1 and 2 inches. Approximate numbered widths: sizes 2 to 4 are roughly 1/8 to 1/4 inch, sizes 6 to 8 roughly 5/16 to 3/8 inch, sizes 10 to 12 roughly 1/2 to 5/8 inch, and sizes 14 to 20 roughly 3/4 to 1 inch. Exact widths vary by brand. For acrylic painting on canvases up to about 16 by 20 inches, a small (2 to 4), medium (8 to 10) and large (1/2 to 1 inch) flat cover most work.
Anatomy of a Flat Brush
Understanding the parts of the brush makes the size chart easier to read, because “size” can refer to more than one measurement.
- Width: The span of the bristles across the flat face. This is the measurement most size numbers relate to.
- Length (out of ferrule): How far the bristles extend from the metal. Longer bristles hold more paint and flex more; shorter ones give tighter control.
- Thickness: How deep the bristle bundle is from front to back. Thicker flats carry more paint for long strokes.
- Chisel edge: The thin, sharp edge at the tip. A good chisel edge allows fine lines and crisp corners.
- Ferrule: The flattened metal sleeve that holds the bristles and shapes them.
Two flats with identical widths can still paint very differently if one has longer, thinner bristles and the other is short and dense. That is why experienced acrylic painters look at a brush from the side as well as from the front. Our comparison of flat brushes suited to acrylic work notes length and stiffness alongside width for exactly this reason.
Flat brush variants
| Variant | Shape | Typical sizing | Best for |
|---|---|---|---|
| Standard flat | Square tip, medium-long bristles | Numbers | General painting, edges, coverage |
| Bright (short flat) | Square tip, short bristles | Numbers | Thick paint, controlled strokes, impasto |
| Long flat | Square tip, long bristles | Numbers | Fluid strokes, more paint per stroke |
| One-stroke flat | Square tip, often soft synthetic | Inches | Decorative painting, lettering, smooth strokes |
| Wash or glaze flat | Wide, soft, thin | Inches | Backgrounds, glazes, varnish |
| Angle flat | Slanted tip | Numbers or inches | Corners, curves, leaves, edges |
Flat Brush Size Chart
The widths below are approximate and based on typical artist flats. Use them as a guide, then check the listed width of the specific brush you are considering.
| Size label | Approximate width (inches) | Approximate width (mm) | Common use |
|---|---|---|---|
| 0 to 2 | About 1/16 to 1/8 | About 2 to 3 | Fine edges, small details, tiny highlights |
| 4 | About 3/16 to 1/4 | About 5 to 6 | Small shapes, window frames, small lettering |
| 6 | About 5/16 | About 7 to 8 | Small to mid shapes, edges, foliage clusters |
| 8 | About 3/8 | About 9 to 10 | General mid-size work, buildings, rocks |
| 10 | About 7/16 to 1/2 | About 11 to 12 | Larger shapes, blocking in on medium canvases |
| 12 | About 1/2 to 5/8 | About 13 to 15 | Backgrounds on small canvases, large forms |
| 14 to 16 | About 5/8 to 3/4 | About 16 to 19 | Skies, water, broad areas |
| 18 to 20 | About 7/8 to 1 | About 22 to 25 | Large backgrounds, underpainting |
| 1/4 inch | 1/4 | About 6 | Detail and lettering in decorative painting |
| 1/2 inch | 1/2 | About 13 | All-purpose mid-size strokes |
| 3/4 inch | 3/4 | About 19 | Backgrounds, one-stroke florals |
| 1 inch | 1 | About 25 | Skies, washes, large canvases |
| 2 inch and up | 2 or more | About 50 or more | Gesso, varnish, very large backgrounds |
When comparing specific brushes for acrylic, our guide to the best flat brush for acrylic painting lists actual widths and bristle types, which matters because two “size 10” flats can differ noticeably.
Number Sizes vs Inch Sizes
Why do some flats use numbers and others use inches? It largely follows tradition:
- Numbered flats come from fine-art brush making for oil and acrylic. The numbers are relative within a brand’s series and do not convert to a universal width.
- Inch-labeled flats are common for one-stroke, wash, glaze and decorative painting brushes, and for gesso and varnish brushes. The label directly states the bristle width.
If you are switching between the two systems, the inch label is the easier reference. For numbered brushes, look for a listed width in millimeters or measure the brush.
Flat Paint Brush Sizes by Canvas
| Canvas size | Large flat for blocking in | Medium flat for shapes | Small flat for edges |
|---|---|---|---|
| Up to 8 by 10 inches | Size 10 to 12 or 1/2 inch | Size 6 | Size 2 to 4 |
| 11 by 14 to 16 by 20 inches | Size 14 to 16 or 3/4 inch | Size 8 to 10 | Size 4 to 6 |
| 18 by 24 to 24 by 36 inches | 1 to 2 inch | Size 12 to 16 | Size 6 to 8 |
| Larger than 24 by 36 inches | 2 inch or wider | Size 16 to 20 or 1 inch | Size 8 to 10 |
A good rule: the largest flat should let you cover the biggest area of the painting in a few minutes. If backgrounds take a long time and look streaky, size up.
Choosing a Size by the Mark You Need
Crisp straight edges
Horizons, building edges, table tops and frames all benefit from a flat’s chisel edge. Use a medium flat sized so its width matches the edge band you want, and pull the stroke in one smooth motion. For very long edges, a larger flat with a sharp chisel holds more paint and avoids stops and starts.
Thin lines
Turn any flat onto its chisel edge and pull. A size 6 to 10 flat on edge can produce a line almost as fine as a liner brush, good for branches, masts, wires and grass. Smaller flats give finer lines but run out of paint sooner.
For the cleanest edge lines, thin the acrylic slightly with water or a flow medium so it releases evenly, load the brush on both faces, and flatten the chisel by stroking it once across the palette before touching the canvas. Pulling the stroke toward you, rather than pushing it away, tends to keep the edge steadier, and resting your little finger on a mahl stick or the canvas edge adds control on long lines.
Backgrounds and skies
Large flats, 3/4 inch and up, lay in even color quickly. Criss-cross strokes reduce streaks; finishing with long, horizontal strokes gives a smooth sky gradient.
Blocks of color and architecture
Medium flats suit walls, rooftops, rocks and geometric shapes, where you want flat planes with defined edges. Match the brush width to the plane: one stroke per plane looks cleaner than several narrow ones.
Lettering and decorative strokes
One-stroke flats in 1/4 to 3/4 inch handle lettering, borders and decorative motifs. The width of the brush sets the thickness of the letter strokes.
Glazing
Soft, wide flats apply thin, transparent layers evenly. Use the widest soft flat that fits the area so you avoid overlapping strokes, which can create darker bands in a glaze.
Bristle Type and Acrylic Paint
Acrylic paint is water-based and dries quickly, which affects which flats work best.
- Firm synthetic flats handle heavy-body acrylic, hold a sharp edge, and clean easily with water. They are a common default for acrylic painters.
- Soft synthetic flats give smooth strokes with fluid acrylic and glazes but bend under thick paint.
- Hog bristle flats are traditionally for oils, but some painters use them with heavy-body acrylic for visible texture. Water can make natural bristles limp over time.
- Taklon and similar synthetic blends are popular for one-stroke and decorative work because they keep a clean chisel edge.
Flat Brush Sizes in Other Media
The same chart applies to oil and watercolor, but painters in those media tend to favor different parts of it. Oil painters often use long-handled hog or firm synthetic flats in the medium and large range, and many keep several brushes of the same size so each holds a different value. Watercolorists lean toward soft, inch-labeled flats, especially 1/2 inch to 1 inch, for washes and crisp-edged shapes, with short handles for close work. Gouache painters often prefer small to medium soft flats for flat, even color blocks. Whatever the medium, the logic stays the same: choose the width that fills the shape in as few strokes as possible.
A Practical Flat Brush Set
You do not need every size. A focused set covers most acrylic paintings on common canvas sizes:
- Small flat (size 2 to 4): edges, details, small shapes.
- Medium flat (size 8 to 10): the everyday workhorse for shapes and edges.
- Large flat (size 14 to 16 or 3/4 inch): blocking in and backgrounds.
- Wide wash flat (1 to 2 inch): skies, gesso, varnish and big canvases.
Optional additions are a bright of the same size as your medium flat for thick paint, and an angle flat for corners and leaves.
How to Test a Flat Brush Size Before Committing
If you already own several flats, a five-minute test sheet shows what each one really does:
- Load each brush with the same paint consistency.
- Make a single full-width stroke and note how far it travels before running dry.
- Turn the brush on edge and pull a line.
- Press and lift to see the stamp shape.
- Label each set of marks with the brush size and brand.
Keep the sheet near your palette. When you need a specific mark, you can choose the brush at a glance instead of guessing.
Common Flat Brush Size Mistakes
- Using a small flat for a big background. It leaves streaks and takes far longer. Size up.
- Buying by number across brands. Compare actual widths.
- Ignoring bristle length. Long flats and brights of the same width behave very differently.
- Letting acrylic dry in the ferrule. This splays the bristles and ruins the chisel edge, effectively making the brush wider and less precise.
- Pressing too hard. Flats give their crispest edges with light to moderate pressure.
Caring for Flat Brushes
- Rinse often during acrylic sessions so paint never dries in the bristles.
- Wash with brush soap at the end of the session, working paint out of the ferrule.
- Reshape the chisel edge while damp.
- Dry flat or bristles down.
- Store so the bristles are not bent against anything.
FAQs
What are the standard flat paint brush sizes?
Flats are commonly numbered from about 0 or 2 up to 20 or more, or labeled in inches such as 1/4, 1/2, 3/4, 1 and 2 inches. Numbered sizes vary between brands.
What size flat brush should a beginner buy?
A small (size 2 to 4), a medium (size 8 to 10), and a large flat around 3/4 inch cover most beginner acrylic paintings on canvases up to 16 by 20 inches.
How wide is a size 10 flat brush?
Often around 7/16 to 1/2 inch, or about 11 to 12 mm, though it varies by brand and series.
What is a flat brush used for?
Coverage, crisp edges, straight lines, blocks of color, backgrounds, and thin lines when used on edge.
What is the difference between a flat and a bright brush?
Both have square tips, but a bright has shorter bristles. Brights give more control and handle thick paint; flats hold more paint and make longer strokes.
Do I need a wash brush if I have large flats?
Not necessarily. A soft 1 to 2 inch flat can serve as a wash brush for skies, glazes and varnish. A separate brush for gesso and varnish keeps your painting brushes cleaner.
Final Thoughts
A flat brush size chart translates numbers and inch labels into real widths, but the right size depends on your canvas and the mark you need: wide flats for backgrounds, medium flats for shapes and edges, small flats and the chisel edge for detail. Compare actual widths across brands and keep a small, focused set rather than every size. When you are ready to pick specific brushes for acrylic, see our list of top flat brushes for acrylic painters.
