Chapter 02Paint · Know-how
How much paint for a ceiling, trim and doors
Treat each part as a flat shape. The ceiling is the floor length times the width. A door is its width times its height, once for each face you paint. Trim is its length in feet times its face width in inches, divided by 12. Add the parts you are painting in the same color, multiply by the coats and divide by the coverage on the label. Sherwin-Williams' calculator uses 400 sq ft per gallon for ceilings, trim and doors, one coat.
Source: Sherwin-Williams paint calculator, checked October 10, 2026.
- Ceiling of a 12 x 14 ft bedroom168 sq ft Arithmetic from the figures on this page
- Two coats on that ceiling at 400 sq ft per gallon0.84 gal Sherwin-Williams paint calculator, October 10, 2026
- Two coats on its trim and one door, both faces2 quarts Sherwin-Williams paint calculator, October 10, 2026; Bob Vila standard door sizes, October 6, 2026
Why the ceiling, trim and doors get their own sums
Walls, ceilings and trim are often painted in different products, colors or sheens, so a single total for the room does not tell you what to buy. The Sherwin-Williams calculator works them out one by one and gives ceilings, trim and doors their own coverage figure: 400 sq ft per gallon against 350 for walls, both for one coat.
Source: Sherwin-Williams paint calculator, checked October 10, 2026.
The other reason is size. Trim and doors are small areas, and the arithmetic often lands between a quart and a gallon. Working them out stops you buying a gallon of trim paint for one room, or a quart that runs out on the last door.
Measure each part, step by step
Ceiling: length times width
Measure the floor along two walls; the ceiling is the same shape. A 12 x 14 ft room has a 168 sq ft ceiling. For an L-shaped room, split it into two rectangles and add them.
Doors: width times height, per face
Measure the door slab, not the opening. A 32 x 80 in interior door is 2,560 sq in, or 17.8 sq ft a face. Painting both faces doubles it to 35.6 sq ft.
Baseboard: length along the floor
Add up the wall lengths and take out the doorways. The 12 x 14 ft room has 52 ft of wall; one 32 in doorway leaves about 49 ft of baseboard.
Casings: up, across, down
Run the tape around the outside edge of each door and window casing and write the length. This room has 17 ft around the door and 15 ft around each of two windows, 47 ft in all.
Turn trim into square feet
Multiply each length in feet by the face width in inches and divide by 12. Baseboard 49 x 3.5 / 12 = 14.3 sq ft. Casings 47 x 2.5 / 12 = 9.8 sq ft.
Coats, coverage, round up
Add the parts in the same paint, multiply by the coats and divide by the coverage on the label. Trim plus the door is 59.6 sq ft; two coats make 119.3 sq ft, or 0.30 gallons at 400 sq ft a gallon. Round up to the next quart: 2 quarts.
Source: Sherwin-Williams paint calculator, checked October 10, 2026; Bob Vila standard door sizes, checked October 6, 2026.
Ceiling, trim and doors in the worked bedroom
- Ceiling: 168 sq ft, two coats 336 sq ft, 336 / 400 = 0.84 gallons. Buy 1 gallon.
- Trim and the door, same paint: 14.3 + 9.8 + 35.6 = 59.6 sq ft, two coats 119.3 sq ft, 0.30 gallons. Buy 2 quarts.
- Walls for comparison: the same room has 372 sq ft of wall once the door and two windows come out, so walls take most of the paint.
Source: Sherwin-Williams paint calculator, checked October 10, 2026.
Read your own label
The 400 sq ft figure is the calculator's default. Trim enamels, ceiling paints and primers each list their own coverage on the can or the product data sheet; when the label gives a figure, use it.
Source: Sherwin-Williams paint calculator, checked October 10, 2026.
Where these sums come out wrong
- Paneled doors have more surface than their flat size, because the edges of every panel get paint too. The flat size undercounts them, so round up.
- Forgetting the door edges and the closet doors. A house has more doors than you remember until you count them room by room.
- Measuring the opening instead of the slab. The slab is the part you paint.
- Leaving the ceiling out when it shares the wall color. It goes on the same shopping total, worked out at the ceiling coverage figure.
Checklist to print
Fill this in before you buy the second and third cans:
- Ceiling: floor length x width, written down
- Doors counted room by room, with closets
- Each door slab measured, faces to paint noted
- Baseboard length, doorways taken out
- Casing lengths around every door and window
- Face width of the baseboard and casings, in inches
- Parts grouped by product and color
- Coverage figure read from each label
- Coats decided, total rounded up to the quart
- Each product written on the paint record
Tips and tricks
- Tip 01
Measure trim once for the whole house
Baseboard and casing widths rarely change from room to room in one house. Measure them once and reuse the numbers.
- Tip 02
Count doors on a floor plan
Walk the house with a sketch and tick each door, closets included. Doors are the easiest part to miss.
- Tip 03
Keep the ceiling apart
If the ceiling paint differs from the wall paint, give it its own line on the record so the next touch-up uses the right can.
- Tip 04
Quarts add up
Once the trim total passes three quarts, compare the price of four quarts with one gallon of the same paint.
What we would do
Work each paint out on its own line
Measure the walls with the calculator, then add a line for the ceiling and one for trim and doors, each with the coverage from its own label. Buy every can of one product on the same trip, and round trim up to the next quart rather than the next gallon.
Questions
How much paint do I need for a ceiling?
Multiply the floor length by the width, then by the coats, and divide by the label coverage. At Sherwin-Williams' 400 sq ft per gallon, two coats on a 12 x 14 ft ceiling come to 0.84 gallons.
Source: Sherwin-Williams paint calculator, checked October 10, 2026.
How do I work out trim in square feet?
Multiply the length in feet by the face width in inches and divide by 12. 49 ft of 3.5 in baseboard is 14.3 sq ft.
Is a quart enough for the trim in one room?
Not always. In the 12 x 14 ft bedroom on this page, two coats on the baseboard, the casings and one door, both faces, come to 0.30 gallons, a little more than one quart.
Source: Sherwin-Williams paint calculator, checked October 10, 2026.
How big is an interior door for paint?
Measure your own. Bob Vila lists 32 x 80 in as the standard interior door, which is 17.8 sq ft a face.
Source: Bob Vila standard door sizes, checked October 6, 2026.
Do I count both sides of a door?
Count each face you will paint. A bedroom door is often painted on both faces; a closet door is often painted only on the room side.
Part of a system
This page is one part of a bigger job. The system page puts the parts in order.
The tool and more guides
Chapter 02Paint coverage calculatorWork out how many gallons of paint a room needs from its size, doors, windows and coats, using coverage figures published by paint makers.Open the tool
Problem-solving guideHow much paint do I really need?Work out gallons from wall area, coats and the coverage on the label, and see why a can runs out before the wall is done.Read the guide
Know-howOne coat or two? Reading the coverage line on a paint canThe coverage on a can is square feet per gallon for one coat. Two coats double the paint. When one coat can do, and what a second costs in a bedroom.Read the guide- Chapter 02All paint toolsPaint quantity tools built on coverage figures published by paint makers, plus a printable record of colors, codes and leftover cans.Open the chapter
Sources
- Paint Calculator, Sherwin-Williams. Checked October 10, 2026.
- Standard Door Sizes, Interior and Exterior Dimensions, Bob Vila. Checked October 6, 2026.
Every outside number on this page names its source and the date we checked it. See them all on the sources page. Spotted an error? Tell us.