Paint Brush Size Chart: Pick the Right Brush Width

How the paint brush size chart works

A paint brush is sized by the width of its bristles, and the right width is about a third of the surface you are painting: a brush that wide covers the surface in three strokes without dragging dry edges. The standard brush widths are 1, 1.5, 2, 2.5, 3 and 4 inches, so the rule is to divide the surface width by three and take the nearest standard size. The millimetres are the inches times 25.4, rounded to the nearest whole: 2.5 × 25.4 = 63.5, which rounds to 64 mm.

At the default — a 7.5-in surface — the raw brush size is 7.5 ÷ 3 = 2.5 in, which is already a standard size, so the brush is 2.5 in, which is 2.5 × 25.4 = 64 mm. That is the all-rounder: doors, wide trim, flat or angled. The reverse lookup works the other way: a 3-in brush covers a 3 × 3 = 9-in surface, which is 76 mm.

The worked example, line by line

Setup: surface 7.5 in; reverse brush 3 in.

Line 1 — the raw brush size. 7.5 ÷ 3 = 2.5 in.

Line 2 — the snap to standard. 2.5 in is already a standard size, so the brush is 2.5 in, with no cap and no rounding.

Line 3 — the millimetres. 2.5 × 25.4 = 63.5, which rounds to 64 mm.

Line 4 — the best use. A 2.5-in brush is best for doors, wide trim, the all-rounder, flat or angled.

Line 5 — the reverse lookup. A 3-in brush covers 3 × 3 = 9 in of surface, which is 76 mm.

Every branch, checked

4.5-in surface: the raw brush is 4.5 ÷ 3 = 1.5 in, which is already a standard size, so the brush is 1.5 in (38 mm) — window trim, baseboards, narrow moulding, angled sash.

9-in surface: the raw brush is 9 ÷ 3 = 3 in, which is already a standard size, so the brush is 3 in (76 mm) — walls, ceilings, cutting in wide edges, flat.

12-in surface: the raw brush is 12 ÷ 3 = 4.0 in, which is exactly a standard size, so the brush is 4 in (102 mm) — and this is an exact fit, NOT capped: the cap only applies when the raw size is over 4, and 4.0 is not over 4.

3-in surface: the raw brush is 3 ÷ 3 = 1 in, which is already a standard size, so the brush is 1 in (25 mm) — fine detail, cutting narrow trim, touch-ups, angled sash.

6-in surface: the raw brush is 6 ÷ 3 = 2 in, which is already a standard size, so the brush is 2 in (51 mm) — doors, cabinets, furniture, flat or angled.

15-in surface: the raw brush is 15 ÷ 3 = 5.0 in, which is over 4, so the brush is capped at 4 in (102 mm) — a 5-in brush is not a standard brush, so the 4-in brush does the job and the wall is painted in passes.

5-in surface: the raw brush is 5 ÷ 3 = 1.667 in, which is not a standard size, so the nearest standard is 1.5 in (38 mm) — 1.667 is 0.167 away from 1.5 and 0.333 away from 2, so 1.5 wins.

Reverse — the 3-in brush: a 3-in brush covers 3 × 3 = 9 in of surface, a 2-in brush covers 2 × 3 = 6 in, and a 4-in brush covers 4 × 3 = 12 in. Work backwards from the brush you already own when the surface is out of reach of the tape.

What the defaults assume

The chart is the standard brush width range: 1, 1.5, 2, 2.5, 3 and 4 inches, and the millimetres are the inches times 25.4, rounded to the nearest whole. It assumes the divide-by-three rule, which is a painter's rule of thumb, not a law: a brush a size up or down still paints, it just leaves more lap marks or takes longer. It assumes a standard brush: very cheap brushes are not true to width, so measure the bristles before you buy. The bristle note: natural bristle is for oil-based paint, because water swells natural bristle and ruins the edge; synthetic bristle is for latex and water-based paint, because oil softens synthetic bristle. This is explicitly NOT a roller or paint-sprayer chart: rollers are sized by nap and frame, and sprayers by tip size, not by brush width. Rounding convention: millimetres are rounded to the nearest whole, and all figures are computed from the inch sizes, rounded only for display.

Output · formula · meaning

Who uses this

• The homeowner painting a door: they learn a 7.5-in door panel takes a 2.5-in brush — so they buy the all-rounder, not the 4-in wall brush that leaves lap marks on the panels.
• The DIYer cutting in a ceiling: they learn a 9-in edge takes a 3-in brush — so they cut in with the 3-in flat, not the 1.5-in sash that takes all afternoon.
• The renovator painting a fence: they learn a 15-in fence board is capped at a 4-in brush — so they buy the 4-in flat and paint the boards in passes, knowing a 5-in brush does not exist.
• The hobbyist touching up trim: they learn a 3-in strip of trim takes a 1-in brush — so they grab the angled sash, not the wall brush that paints the wall as well.

Accuracy & limitations

The chart is exact for the standard brush widths: 1, 1.5, 2, 2.5, 3 and 4 inches, and the millimetres are the inches times 25.4, rounded to the nearest whole. What the chart cannot do is measure your surface for you: measure the trim, the door panel or the wall section, not the whole wall. It assumes the divide-by-three rule, which is a painter's rule of thumb: a brush a size up or down still paints, it just leaves more lap marks or takes longer. It assumes a standard brush: very cheap brushes are not true to width, so measure the bristles before you buy. The bristle note: natural bristle is for oil-based paint and synthetic bristle is for latex and water-based paint. It is not a roller or paint-sprayer chart: rollers are sized by nap and frame, and sprayers by tip size. For the paint itself, see paint cost calculator; for how much paint the job needs, see paint coverage chart.