How to Measure a Room (Once, for Four Materials)

The two measurements everything needs

• Floor: length × width, in feet. Measure both dimensions in two places. Rooms are rarely perfectly square; use the larger figure.
• Walls: each wall's length × the ceiling height. Add the walls together for paint, paper and drywall.
• Note the doors and windows as you go. They come off paint and wallpaper, and they matter for drywall openings.

Flooring: area, then boxes

Hard flooring is sold by the box, and the box states its coverage. Room area ÷ box coverage, rounded up, plus a waste allowance for cuts. The Flooring Calculator does the division and the rounding. Don't forget underlayment if your planks don't have it attached: it's sold by the roll, usually 100 sq ft per roll.

Carpet: think in square yards and roll widths

Carpet is priced by the square yard, your square feet divided by 9, but it's cut from a roll, usually 12 ft wide. The room is covered in strips of that roll laid side by side, so a room wider than 12 ft always has a seam and a fill strip, and the strip layout decides the true order quantity. That's why the Carpet Calculator works out the strips, not just the area.

Paint: wall area minus the holes

Add up the wall areas, then deduct 21 sq ft per standard door and 15 sq ft per average window. A gallon covers about 350 sq ft per coat, and most rooms want two coats. The Paint Calculator handles the deductions, the coats and the ceiling, then converts the answer into whole gallons plus quarts. Papering instead? The same wall measurements feed the Wallpaper Calculator.

Drywall: sheets, then the finishing maths

A standard 4 × 8 sheet covers 32 sq ft. Wall and ceiling area ÷ 32 gives the sheet count; the finishing materials follow from the sheet area. Joint compound runs about 10 gallons per 1,000 sq ft of panels (USG's published figure), with tape and screws derived the same way. The Drywall Calculator produces the full list: sheets, mud, tape and screws.

Measure once, buy once

The pattern to notice: every material on this page is the same two tape measurements wearing different units. Get the length, width and ceiling height right, and write down the doors and windows, and the buy-lists take care of themselves.