How to Measure a Room for Paint and Drywall

Paint and drywall estimates start with the same measurements: how big each wall and ceiling is, and how much of it is doors and windows. Measure once, carefully, record it on a single sheet, and you can feed the same numbers into the paint calculator and the drywall calculator. This guide shows how, including the shapes that trip people up: gable walls, sloped ceilings, and stairwells.

Tools and a measurement sheet

• A 25 ft tape measure. A laser measure helps for tall walls and long rooms.
• A pencil and a sheet with one line per surface: name, width, height, area, and notes.
• A sketch of the room's floor plan with each wall labeled (A, B, C, D) so you don't measure one twice.

Measure in feet and inches, then convert to decimal feet before you multiply: 6 in is 0.5 ft, 4 in is about 0.33 ft, 8 in is about 0.67 ft. One foot is exactly 12 inches, so divide the inches by 12.

Measure the walls

Measure each wall's width along the floor, corner to corner, and the wall height from floor to ceiling. Write down every wall even when two look the same. Rooms are rarely perfectly square, and a bump-out or a chimney breast adds wall you won't see on a floor plan.

If every wall is the same height, you can take a shortcut: total wall length × height. For a rectangular room, the total wall length is the perimeter, 2 × (length + width). The shortcut stops working as soon as one wall is a different height, such as a gable end or a stairwell, so list those walls separately.

Measure the ceiling

A flat ceiling has the same area as the floor: length × width. For an L-shaped room, split the ceiling into rectangles and add them. The square footage calculator adds up several shapes at once, and the irregular areas guide covers the geometry.

Doors, windows, and other openings

Measure each opening's width × height and record it on its own line. Don't subtract yet. Paint and drywall treat openings differently, so it's better to have them listed and decide later.

• For paint: subtract openings you won't paint, such as windows, doors painted separately, and built-in cabinets.
• For drywall: subtract large openings like doors and windows. Small cutouts such as outlets and switch boxes come out of full sheets and don't reduce the sheet count.

Gable walls

A gable wall is a rectangle with a triangle on top. Measure the width, the height of the straight part, and the extra height from there to the peak. The triangle's area is ½ × width × that extra height.

Rectangle = 24 × 8 = 192 ft²
Triangle  = ½ × 24 × 6 = 72 ft²
Total     = 264 ft²

Sloped ceilings

In an attic room or under a cathedral ceiling, the ceiling surface is longer than the floor beneath it. Measure the horizontal width the slope covers (along the floor), the length of the room, and the pitch, which you can find with the methods in the roof pitch guide. The sloped width is the horizontal width × the slope factor, which the roof pitch calculator shows for any pitch.

Slope factor = √(1 + (8 ÷ 12)²) = 1.2019
Sloped width = 6 × 1.2019 = 7.21 ft
Area         = 7.21 × 14 = 101 ft²

You can also measure the sloped width directly with a tape if you can reach it safely.

Stairwell walls

The wall beside a staircase is often taller at the bottom of the stairs than at the top, so it's a trapezoid (in the diagram, the stairs climb toward the short end). Measure its length and the height at each end, then average the heights.

Area = (8 + 17) ÷ 2 × 12 = 150 ft²

Worked example: one room, start to finish

A 14 × 12 ft bedroom with 8 ft walls, one 36 × 80 in door, and two 3 × 4 ft windows. All dimensions are example inputs.

Walls 416 ft² · ceiling 168 ft² · openings 44 ft².

Turning measurements into paint and drywall

Paint

Paint needed = net area × coats ÷ coverage per gallon, with the coverage taken from your paint's label or data sheet. As an example input only, the arithmetic below uses 400 ft² per gallon. That is the low end of the 400 to 450 ft² per gallon spread rate Benjamin Moore publishes for Regal Select Eggshell (N549), which it says depends on surface texture and porosity.

Net wall area = 416 − 44 = 372 ft²
Paint         = 372 × 2 coats ÷ 400 ft²/gal = 1.86 gal → 2 gallons

Drywall

Drywall needed = (walls + ceiling − large openings) plus an allowance for cuts, divided by the area of one sheet. Sheet sizes vary by product. USG lists its Sheetrock UltraLight 1/2 in panels in 4 ft or 54 in widths and lengths from 8 to 16 ft. The example uses 4 × 8 ft sheets and a 10% allowance.

Net area  = 416 + 168 − 44 = 540 ft²
+ 10%     = 594 ft²
Sheets    = 594 ÷ 32 ft² = 18.56 → 19 sheets

Enter the same figures in the paint calculator and drywall calculator to try other coverages, coats, sheet sizes, and allowances. Both round purchases up, so you won't come up short.