The formula everyone quotes — 2 × (L + W) × H — only works because most rooms are boxes. The moment your room is an L-shape, has a vaulted ceiling, or flows into the kitchen, the formula bends. The good news: the fix is always the same. Stop thinking of rooms; start thinking of walls.

The rectangle, done right

For a normal rectangular room: measure floor length and width once (wall-to-wall, not furniture-to-furniture), then wall height floor-to-ceiling. Wall area = 2 × (length + width) × height. A 12 × 14 ft room at 9 ft: 468 sq ft gross.

Then deduct openings, because paint doesn’t go on glass and door slabs: ~20 sq ft per standard door (3 ft × 6.8 ft ≈ 21), ~15 sq ft per standard window (3 × 5 ft). French doors run ~40, picture windows 25–40, skylights whatever the tape says — measure them if they’re bigger than standard. What stays: closets you won’t paint, floor-to-ceiling bookcases, shower enclosures.

Odd rooms, wall by wall

• L-shaped rooms. Two rectangles on the floor plan still means straight walls — walk the full perimeter, multiply each wall run by height, sum. Or take a shortcut: perimeter × height works for any polygonal room, not just rectangles. Measure the perimeter (it’ll be more than 2(L+W)) and multiply.
• Vaulted / cathedral ceilings. Split the wall at the plate line: rectangle below + triangle above. Gable wall = (width × peak rise) ÷ 2. A 20 ft wall rising from 8 ft plate to 14 ft peak adds (20 × 6) ÷ 2 = 60 sq ft per end wall — and the ceiling itself is now a bigger, sloped rectangle.
• Shed or single-slope walls. Average height × length: a wall that’s 8 ft at one end and 12 ft at the other paints as (8 + 12) ÷ 2 × length.
• Open-plan spaces. Decide where the color stops — usually a corner or a bulkhead — and measure only the walls getting paint. Two zones in different colors are two calculator runs, which is also the honest way to price them.
• Closets, nooks, dormers. Inside a closet you’ll paint: side walls + back wall at closet height, typically 60–90 sq ft for a 6 ft reach-in. Worth a quart, not a gallon.

Stairwells: the area is easy, the access isn’t

Below the stair slope: a triangle, (run × rise) ÷ 2. Beside the slope: a parallelogram the same size. Above and around: ordinary walls, just very tall — stairwell walls commonly reach 16–18 ft. Measure it like any wall and the calculator does the rest; the real budget line on a stairwell is the ladder or scaffold day-rental, not the paint.

Numbers to keep for the calculator

When you finish, you should have: total wall area (or the room’s L × W × H if it is a simple box), door count and window count with any non-standard sizes, ceiling area if the ceiling gets paint (floor area for flat ceilings — the ceiling is the floor plan), and trim length for enamel math — roughly the room’s perimeter of baseboard plus ~17 ft of casing per door and ~15 ft per window. Feed them in here and the rounding to cans is automatic. For why openings matter more than people think, see the overbuy list.

Frequently asked questions

What's the formula for wall area of a room?

For a rectangular room: 2 × (length + width) × height. A 12 × 14 ft room with 9 ft ceilings gives 2 × 26 × 9 = 468 sq ft gross. Then subtract ~20 sq ft per door and ~15 sq ft per window for the net paintable area. Same formula in metres — just make sure every measurement is in the same unit before you multiply.

How do I measure an L-shaped or non-rectangular room?

Don't fight it — measure wall by wall. Walk the perimeter with a tape, multiply each straight wall's length by its height, and add the results. Every room, however odd the floor plan, is just a list of flat rectangles when you go wall by wall. The calculator's 'I know the wall area' mode takes that total directly.

How do I measure a vaulted or sloped ceiling wall?

A triangular gable wall is (width × peak height) ÷ 2 on top of the rectangular part below the plate line: e.g. walls to 8 ft then a peak at 14 ft on a 20 ft wall = 20×8 + (20×6÷2) = 160 + 60 = 220 sq ft. For a shed-style single slope, average the two wall heights: (h1 + h2) ÷ 2 × length.

Should I measure the stairwell walls differently?

A stairwell is two stacked problems. The walls below the slope are triangles/parallelograms — measure them as (length × height) ÷ 2 shapes. The rest are ordinary walls. The trickier question is access, not area: stairwell walls run 12–18 ft high, which is scaffolding territory, and paint quantity is the least of that job.

Do I measure behind radiators, cabinets and built-ins?

Paint goes where you can reach — behind a radiator, yes (that's a mini-roller job); behind fixed cabinets and floor-to-ceiling built-ins, no. Subtract big permanent fixtures the way you subtract windows: a 6 ft run of kitchen base plus wall cabinets is ~30–40 sq ft you'll never touch.