Trim & Baseboard Paint Calculator

Understanding your result

The headline is the trim enamel for the coats you chose, shown in quarts when it is under a gallon because that is how trim paint is bought. The sub-line is the can list. The three figures show the total trim area, the paint for a single coat and the cans again.

The breakdown shows each kind of trim as its own line: the baseboard, the crown or rail, and the casings with the counts. That is deliberate. If one line looks wrong, it is usually a measurement rather than the arithmetic, and the casings line is the one to check because it uses fixed areas per opening rather than a tape measure.

Trim enamel is a different product from wall paint, harder and glossier, and it is bought in different cans, which is why this calculator stands apart from the wall paint calculator. The door paint calculator and the window frame calculator use the same enamel; if you are doing all three, add the quantities before you buy, because one gallon is cheaper than four quarts.

How we calculate this

Strips of trim are long and thin, so their area is simply length times height. The division by 12 turns feet times inches into square feet; in metric mode the calculator takes metres and centimetres and does the equivalent division by 100.

The casing figures are derived rather than measured. A door opening of 36 × 80 in has about 196 in of casing on each face; at 3.5 in wide that is 686 sq in, or 4.8 sq ft, per face. The jamb between them is 4.5 in deep on the same 196 in, about 6.1 sq ft. Two faces plus the jamb is 15.7 sq ft, rounded to 16. A 3 × 5 ft window has 156 in of casing (sides and head) at 3.5 in, about 3.8 sq ft, a stool and apron of about 1.5 sq ft and a jamb of about 4.9 sq ft: roughly 10 sq ft. These and the other standard sizes are on the standard sizes page, and the trim and doors guide shows how to adjust them for wide colonial casings or deep jambs.

Coverage for trim enamel is typically 350 to 400 sq ft per gallon; the coverage rates table has the published figures. The allowance is 10% rather than the 5% used for walls, because brushing wastes more.

The assumptions behind the numbers

Assumptions last reviewed October 7, 2026.

The calculator does not model bare or new wood, which should be primed first (the primer calculator uses the same area), nor a change from dark stained trim to white, which often needs a bonding primer and three coats. It assumes a brush or a small roller; sprayed trim uses far more paint for a better finish, and the room painting guide explains when that trade is worth it.

Two worked examples

Where to find your inputs

Baseboard length. Walk the room with a tape along the baseboard and subtract the doorways, or take the room perimeter and subtract 3 ft per door. Closets count if you are painting inside them.

Baseboard and crown height. The face you see from the room, measured with a ruler. A 1 × 6 board is 5.5 in tall; a profiled baseboard is sold by its nominal height. Crown moulding is installed at an angle, so measure the sloping face, not the width of the board, and ignore the small bottom edge.

Casings. Count doorways and windows, not doors: a double door is one casing, a cased opening with no door is still a casing. Count both sides of a doorway only if you are painting both rooms, and remember that the second room’s calculation would otherwise count it again.

Coverage. From the can, lower end of the range. Alkyd and waterborne alkyd enamels brush out thinner than latex and sometimes cover a little more; the data sheet figure is the one to use.

Common mistakes

• Measuring baseboard width instead of height. A 1 × 6 is 5.5 in tall and 3/4 in thick; the tall face is what you paint.
• Forgetting the jambs. The inside of a door opening is the largest part of its casing area, and it is what the 16 sq ft includes.
• Buying a gallon for 60 sq ft. Two quarts cover a room’s trim twice. A gallon is for three rooms or more.
• Using wall paint on trim. It stays soft, marks and does not clean. Trim enamel is harder and glossier for a reason.
• One coat over old gloss. Sand or de-gloss first, then two coats; one coat over gloss peels at the edges within a year.
• Counting doorways twice. A doorway between two rooms you are both painting has one casing per side; count each side once.

Questions people ask

How much paint does the trim in one room take?

A 12 × 12 room with 48 ft of 5 in baseboard, two door casings and two window casings has about 72 sq ft of trim. Two coats of enamel at 400 sq ft per gallon with a 10% brushing allowance come to 1.58 quarts, so buy two quarts.

How do I turn a baseboard length into square feet?

Length in feet times height in inches, divided by 12. Fifty feet of 5 in baseboard is 50 × 5 ÷ 12 = 20.8 sq ft. Crown moulding and chair rail work the same way using the face height you see from the room, not the width of the board.

How much is a door casing in square feet?

The calculator uses 16 sq ft per door, made of 3.5 in casing on both faces of a 36 × 80 in opening (about 4.8 sq ft each) plus the 4.5 in jamb (about 6 sq ft). A window casing with its stool and apron is about 10 sq ft. Wider or deeper trim adds proportionally.

Is a quart enough for the trim in one room?

Rarely for two coats. A quart covers about 100 sq ft per coat, and an average room has 60 to 90 sq ft of trim, so two coats need just under two quarts. Buy two, or a gallon if you are doing more than two rooms.

Why is the allowance 10% rather than 5%?

Brushing narrow profiles wastes more than rolling a wall. Paint dries in the brush and the cup, it gets scraped off the edges of the tape, and the first coat sinks into any bare wood or filler. Ten percent is a round figure; sprayed trim should use 30% or more.

Should I count the doors here?

No. The door slabs are in the door paint calculator, which counts their style and both faces, and window sashes are in the window frame calculator. This calculator covers the trim that is nailed to the wall, meaning baseboard, crown, rails and casings.