Measuring & coverage
Measuring a room for paint (and what to subtract)
Ten minutes with a tape measure is all a paint estimate needs, if you measure the right things and subtract only what is worth subtracting.
What you are measuring
A paint estimate needs one number per product: the area of wall, the area of ceiling, and the length of trim. Everything else, the coats, the coverage and the cans, follows from those. The goal is not an architect’s survey. Paint is sold in quarts and gallons, so an error of a few square feet disappears in the rounding. An error of a whole wall does not.
You need a tape measure of at least 25 ft, a pencil and paper, and for high walls a laser measure helps. Measure in feet and inches and round to the nearest half foot, or in metres to the nearest 5 cm.
Walls: perimeter times height
For a plain rectangular room, measure the length and width along the floor, add them, double the sum for the perimeter, and multiply by the wall height.
Measure the height in a corner, from the floor to the ceiling. If the room has a deep crown moulding that will be painted with trim enamel, measure to the underside of the crown.
Rooms that are not rectangles are easier wall by wall. Measure each wall’s width and height and add the areas. The square footage calculator takes up to six walls, subtracts the openings and adds the ceiling if you want it.
What to subtract, and what to leave
Subtract the things you will not paint with wall paint, but only the big ones:
| Opening | Subtract | Why |
|---|---|---|
| Door | 20 sq ft | A standard 36 × 80 in door |
| Window | 15 sq ft | About 3 × 5 ft |
| Patio door, wide archway | 40 sq ft, or measure it | About two standard doors |
| Fireplace opening, built-in bookcase | Measure it | Varies |
| Tiled backsplash or shower surround | Measure it | Not painted |
| Outlets, switches, vents, thermostats | Nothing | Too small to matter |
Do not subtract the trim around openings separately; it is part of the 20 and 15 sq ft. And do not over-subtract. Cutting in around every opening uses paint and time, so a room with many small windows needs more paint than its net area suggests. When in doubt, subtract less. The standard sizes are listed in the standard sizes reference.
Ceilings: length times width
The ceiling is the floor area: length times width. A 12 × 14 ft room has a 168 sq ft ceiling. Measure the floor, which is easier than the ceiling, and ignore closets unless you are painting their ceilings too.
A sloped or vaulted ceiling is larger than the floor. For a ceiling that slopes on both sides, measure along the slope from wall to peak and multiply by the room length for each side. For a single slope, the sloped length times the room length.
Awkward shapes
Gable or triangular walls. A wall that rises to a peak is a rectangle plus a triangle. The triangle is half the base times the height from the top of the rectangle to the peak. A 12 ft wide gable rising 4 ft above the 8 ft wall line adds ½ × 12 × 4 = 24 sq ft.
Walls under a sloped ceiling. Use the average height: the height at the low side plus the height at the high side, divided by two, times the width.
Stairwells. The walls are tall and often stepped. Treat each wall as a rectangle to its average height, or split it into a rectangle and a triangle along the stair line. The stairwell calculator takes up to four walls with two-storey heights and warns when you will need a scaffold rather than a ladder.
Alcoves, chimney breasts and bay windows. Measure around them. A chimney breast that projects 18 in adds two 18 in returns to the wall length.
Closets. Add the closet walls if you are painting them. Even a small reach-in closet has three walls of full height, so it is worth measuring rather than guessing.
Trim: linear feet
Baseboard is the room perimeter minus the doorways, about 3 ft each. Door and window casings are counted rather than measured. Crown moulding and chair rail are the perimeter again. The trim paint calculator turns the lengths and casing counts into area, and trim, doors and windows covers the quart arithmetic.
A worked room
A 12 × 14 ft living room, walls and ceiling
8 ft walls, one door, two windows, ceiling included.
- Walls: 12 + 14 + 12 + 14 = 52 ft of perimeter × 8 ft = 416 sq ft
- Openings: 1 door × 20 + 2 windows × 15 = 50 sq ft
- Paintable walls: 416 − 50 = 366 sq ft (34 m²)
- Ceiling: 12 × 14 = 168 sq ft
- Walls and ceiling: 534 sq ft
- Wall paint at 350 sq ft/gal, two coats + 5%: 2.2 gallons
The figures are from the square footage calculator. The wall and ceiling areas go into different calculators, because ceiling paint is a different product with a different coverage; the wall paint calculator and the ceiling paint calculator take them from here.
A whole house without measuring every room
For a whole-house repaint, measuring every room is slower than it needs to be. Count the rooms by size instead: small (about 10 × 10 ft), medium (about 12 × 14), large (about 15 × 18), bathrooms and the length of hallway. On a typical house the errors between the template and the real rooms cancel out.
A house by room counts
Two small rooms, three medium rooms, one large room, two bathrooms and 30 ft of hallway with three doors, 8 ft ceilings.
- Walls: 2,924 sq ft, from 285 sq ft per small room, 366 per medium, 458 for the large room, 189 per bathroom and 420 for the hallway
- Ceilings: 1,175 sq ft
- Trim: 471 sq ft
- Wall paint, two coats + 5%: 17.54 gallons, buy three 5-gallon pails, two gallons and three quarts
These figures are from the whole-house estimator. Measure the one or two rooms that are clearly unlike the templates, such as a double-height living room, and enter them separately.
Writing it down so it is useful later
Sketch the room as a rectangle on paper and write each wall’s length on its side, the height in the middle, and a mark for each door and window. It takes a minute and it catches the forgotten wall. Note which surfaces are being painted and in which product: walls, ceiling, trim, doors. If you are working in metric, measure in metres to two decimals and the calculators will give square metres and litres; mixing feet and metres on one sketch is the one error the arithmetic cannot catch.
Keep the sketch with the paint receipt. When you repaint in five years, or need to order a touch-up quart, the measurements and the colour code are already there. For a whole house, one sheet per floor is enough.
Common mistakes
- Measuring the floor and calling it the wall area. A 12 × 14 room has 168 sq ft of floor and 416 of wall.
- Forgetting a wall. Count the walls as you measure them; write each one down.
- Subtracting every small opening. Only doors, windows and big built-ins come out.
- Using the ceiling height of the wrong room. Older houses often have different heights upstairs and down.
- Measuring a sloped ceiling as flat. It is larger than the floor below it.
- Putting walls and ceilings into one figure. They take different paints.
The next step is the coats and the coverage, which how much paint do I need covers in full.