How to Measure a Room for Paint, Flooring, Tile and Drywall

Before you start: one sketch, every number on it

Draw the room from above on a sheet of paper. It does not need to be to scale. Mark every wall, every door and window, closets, and anything built in, such as a kitchen island or a fireplace. Then measure and write each number on the sketch. Georgia-Pacific's drywall guide starts the same way: sketch the area first and use the sketch to work out how much board you need.

Measure each length from wall to wall at floor level, and the height from floor to ceiling. Write feet and inches as you read them off the tape, then convert once, at the end. The calculators do this for you: the square footage calculator has an inches box next to every length and turns 12 ft 6 in into 12 + 6 ÷ 12 = 12.5 ft. If you convert by hand, this table saves the division.

Inches	Decimal feet	Inches	Decimal feet
1 in	0.083 ft	7 in	0.583 ft
2 in	0.167 ft	8 in	0.667 ft
3 in	0.250 ft	9 in	0.750 ft
4 in	0.333 ft	10 in	0.833 ft
5 in	0.417 ft	11 in	0.917 ft
6 in	0.500 ft

The classic slip is reading 12 ft 6 in as 12.6 ft. It is 12.5 ft. On a 15 ft wide room that small error adds 1.5 square feet, and it repeats on every wall you measure that way.

Measuring the floor

Floor area is length times width. Flooring, tile, carpet and the ceiling all use this number. For anything that is not a plain rectangle, split the floor into simple pieces and add them up. The square footage calculator offers the three shapes you need:

Rectangle

length × width

Triangle

base × height ÷ 2

Circle

π × (diameter ÷ 2)²

L-shaped rooms

Cut the L into two rectangles with one imaginary line. It does not matter where you cut, as long as no part is counted twice. A 16 by 12 ft main room with a 6 by 5 ft nook in one corner is 192 + 30 = 222 square feet. Cut the other way, it is 16 by 17 ft minus a 10 by 5 ft corner that is not there: 272 − 50 = 222. Doing it both ways is a quick check on your arithmetic.

Closets, hallways and bump-outs

Measure each one as its own rectangle and add it to the room. A reach-in closet 6 ft wide and 2 ft deep adds 12 square feet of floor. The flooring calculator suggests the same: enter closets, hallways and each leg of an L-shaped room as separate rows, so nothing gets missed.

Things that stay

A kitchen island, a built-in cabinet or a hearth that will not get the new floor is a piece you subtract. In the square footage calculator, add it as an area and flip its minus switch.

Measuring the walls

Paint, drywall, wall tile and wallpaper all start from the walls. For a rectangular room, Georgia-Pacific's drywall guide gives the shortcut: length plus width, times 2, times the height. That is the perimeter of the room times the wall height.

Wall area

2 × (length + width) × height

Net wall area

wall area − doors − windows

Ceiling area

same as the floor area

An L-shaped room has six walls, but you rarely need to measure all six. When every corner is square and there is only one notch, the two short walls around the notch fill exactly the gaps in the two long walls, so the walls add up to the perimeter of the full rectangle around the room. The 16 by 12 ft room with its 6 by 5 ft nook has 2 × (16 + 17) = 66 ft of wall. Measured wall by wall: 16 + 12 + 10 + 5 + 6 + 17 = 66. For a U-shape or anything more complex, measure every wall and add them up.

Do not forget the inside of a closet if you paint it. The reach-in closet above has a back wall of 6 ft and two side walls of 2 ft. At 8 ft high that is (6 + 2 + 2) × 8 = 80 square feet. Two coats make 160 square feet, and Sherwin-Williams says a quart covers 100, so the closet alone takes two quarts.

Sloped ceilings and gable walls

Attic rooms have two extra shapes. The end wall under a pitched roof is a rectangle with a triangle on top. The sloped ceiling is a rectangle, but its width is the length along the slope, not the width of the floor below it.

Take an attic room 12 ft wide and 14 ft long, with 4 ft knee walls rising to an 8 ft peak in the middle. Each gable end is 12 × 4 = 48 square feet of rectangle plus a triangle of 12 × 4 ÷ 2 = 24, so 72 square feet. Each side of the ceiling climbs 4 ft over a run of 6 ft. Measure that slope with the tape if you can reach it. If not, the Pythagorean theorem gives it: √(6² + 4²) = 7.21 ft. Both sloped planes together are 2 × 7.21 × 14 = 202 square feet, against 168 square feet of floor. Use the floor area for a sloped ceiling and you come up 20 percent short on paint or drywall.

The drywall calculator takes rectangles, so its guide suggests entering a gable triangle as a rectangle of half its height: 12 ft wide and 2 ft high gives the same 24 square feet.

What to subtract, material by material

Not every material treats openings the same way. Here is what each calculator does with them:

Material	Doors	Windows	Other
Paint	Subtract, 21 sq ft each by default	Subtract, 15 sq ft each by default	Change the sizes for patio doors or large windows
Drywall	Subtract the measured area	Subtract the measured area	Georgia-Pacific: subtract openings for the net area, then add the overage
Wall tile	Subtract if the tile stops there	Subtract if the tile stops there	Subtract the space behind a tub
Wallpaper	One full strip per door	Not subtracted	Paper still goes above and below a window
Flooring and floor tile	Not subtracted	Not relevant	Subtract built-ins that stay, add closets

Small cutouts such as outlets and switch boxes stay in the area for every material. The drywall calculator's FAQ says the same for wallboard.

Waste allowances, and who says so

Every material is cut to fit, so you buy more than the measured area. How much more is the number most often made up, so the table shows only what the calculators use and where the figure comes from.

Material	What the calculator does	Basis
Laminate and plank flooring	10% waste, +5% on the diagonal	Swiss Krono: add 10% for cuts and waste, 20% for a tile pattern. The diagonal step is the calculator's own.
Tile	10% straight, 15% diagonal, 20% herringbone	Starting points, not a published standard; follow your installer
Drywall	10% by default, 15% as an option	Georgia-Pacific: allow 10% to 15% for cutting and piecing
Paint	No percentage; cans rounded up	Sherwin-Williams: a gallon covers about 350 to 400 sq ft; the calculator uses 350
Wallpaper	No percentage; whole strips and rolls	The pattern repeat and trim are added to every strip instead

Add the allowance once. If you type 10 percent into the square footage calculator and then enter that total into the flooring calculator with its own 10 percent, you have ordered 21 percent extra.

One room, worked end to end

Here is the L-shaped room from above, carried through every calculator. The numbers in this section come from the same code the tools run.

Main part 16 by 12 ft, nook 6 by 5 ft in one corner, walls 8 ft high.
One door, 3 by 7 ft, and two windows, 3 by 5 ft each.
Floor and ceiling: 222 square feet, or 20.6 square meters.
Walls: 66 ft × 8 ft = 528 square feet. Minus 21 for the door and 2 × 15 for the windows: 477 square feet net.

Paint

The paint calculator multiplies the area by the coats and divides by the coverage. Two coats on 477 square feet at 350 square feet per gallon is 477 × 2 ÷ 350 = 2.73 gallons, so you buy 3 gallon cans. If the paint really reaches 400 square feet per gallon, the top of the Sherwin-Williams range, it is 2.39 gallons: 2 gallon cans + 2 quarts. The 222 square foot ceiling takes 1.27 gallons for two coats: 1 gallon can + 2 quarts.

New drywall needs primer first. One coat over walls and ceiling is 699 square feet. Sherwin-Williams puts a gallon of primer at about 200 to 300 square feet, so plan on 2.3 to 3.5 gallons, more than the 2.0 you would get using the paint's coverage.

Flooring

With laminate in boxes of 22 square feet and 10 percent waste, 222 × 1.1 = 244.2 square feet, and 244.2 ÷ 22 = 11.10, so 12 boxes, covering 264 square feet. Laid on the diagonal, the calculator adds 5 percent more: 255.3 square feet, still 12 boxes, because the straight layout already left some room in the last box. Underlayment rolls of 100 square feet are counted from the floor without waste: 222 ÷ 100, rounded up to 3 rolls.

Tile instead of laminate

The tile calculator adds one grout joint to each tile before it divides. A 12 by 24 inch tile with 1/8 inch joints takes 12.125 × 24.125 ÷ 144 = 2.031 square feet. 222 ÷ 2.031 = 109.3, so 110 tiles before waste and 121 with 10 percent for a straight layout.

Tile, 1/8 in joints	No waste	+10% straight	+15% diagonal	+20% herringbone
12 × 12 in	218	240	251	261
12 × 24 in	110	121	126	132
24 × 24 in	55	61	64	66

Drywall

For new walls and ceiling, the drywall calculator rounds walls and ceiling up separately, because ceilings often get a different board. With 4 by 8 ft sheets of 32 square feet and 10 percent waste: walls 477 × 1.1 ÷ 32 gives 17 sheets, ceiling 222 × 1.1 ÷ 32 gives 8, so 25 sheets in all. At 15 percent it is 26. With 4 by 12 ft sheets and 10 percent it drops to 17, with fewer seams to tape. The rest follows from the 699 square feet of board at the manufacturers' rates: about 874 screws, 350 ft of tape (2 rolls of 250 ft) and 6.3 gallons of joint compound, which USG's figure of about two 4.5 gallon pails per 1,000 square feet turns into 2 pails.

Wallpaper

The wallpaper calculator counts strips, not square feet. 66 ft of wall is 792 inches; with 20.5 inch wide double rolls that is 39 strips, one fewer for the door: 38. Each strip is cut at the wall height plus a 21 inch straight repeat plus 4 inches of trim: 96 + 21 + 4 = 121 inches. A 33 ft roll is 396 inches, so it gives 3 strips. 38 ÷ 3 rounds up to 13 double rolls. Divide the wall area by the roll area instead, 528 ÷ 56.4, and you would buy 10 and run out.

The shopping list

Material	Area used	Buy
Wall paint, 2 coats	477 sq ft	3 gallon cans
Ceiling paint, 2 coats	222 sq ft	1 gallon can + 2 quarts
Laminate, 22 sq ft boxes	222 sq ft + 10%	12 boxes, 3 underlayment rolls
Or tile, 12 × 24 in	222 sq ft + 10%	121 tiles
Drywall, 4 × 8 ft	699 sq ft + 10%	25 sheets, 2 tape rolls, 2 compound pails
Wallpaper, 20.5 in × 33 ft	38 strips	13 double rolls

Common measuring mistakes

Reading inches as decimals. 12 ft 6 in is 12.5 ft, not 12.6. Use the inches box or the table above.
Rounding each room on its own. Add all rooms of the same flooring first and round the boxes up once.
Adding waste twice. Put the allowance into one calculator only.
Using the floor area for a sloped ceiling. Measure along the slope. In the attic example that is 20 percent more area.
Counting wallpaper by area. Strips and the pattern repeat decide the rolls, and windows are not taken off.
Forgetting the closet. Its floor and, if you paint it, its walls belong in the total.

Frequently asked questions

Do you subtract doors and windows when you measure for paint?

Yes. Paint only goes on the wall, so take off each door and window. The paint calculator uses 21 square feet for a door (3 by 7 ft) and 15 for a window (3 by 5 ft). Change those sizes for a patio door or a large picture window: a 6 by 7 ft patio door is 42 square feet, twice the default.

How do I measure a room that is not a rectangle?

Split the floor into rectangles, triangles and circles, work out each piece and add them up. An L-shaped room is two rectangles. An angled corner is a rectangle with a triangle cut off: base times height divided by 2 gives the triangle. The square footage calculator takes several pieces at once and can subtract one, such as a kitchen island.

Should I include closets in the flooring measurement?

Yes, if the new floor runs into them. Measure each closet as its own rectangle and add it to the room. Add every room, closet and hallway first and round the box count up once, as the flooring calculator does, since rounding each piece separately can cost an extra box.

How much extra material should I buy?

It depends on the material, and only some of the numbers come from manufacturers. Swiss Krono says to add 10 percent to laminate for cuts and waste. Georgia-Pacific says to allow 10 to 15 percent extra drywall for cutting and piecing. The tile calculator starts at 10, 15 or 20 percent by pattern, as starting points rather than a standard. Paint and wallpaper are rounded up to whole cans and whole strips instead.

Can I measure in meters instead of feet?

Yes. Every calculator here has a metric mode. One foot is exactly 0.3048 meter, so one square foot is exactly 0.09290304 square meter. The 222 square foot room in this guide is 20.6 square meters.

Why does wallpaper need more rolls than the wall area suggests?

Because it hangs in full-height strips, and the piece left at the end of a roll is often too short for another one. In the worked room the wall area divided by the area of a roll gives 9.4 rolls, but counting strips gives 13.

Sources

Written by Tobias. Updated October 20, 2026.