How to figure out how much flooring to buy
Measure the room's length and width at the widest points, in feet, and multiply to get square feet. For an L-shaped room or an open plan, split it into rectangles, add them up and enter the total as square feet directly. Include closets, the space under appliances that will be pulled out, and doorways up to the threshold. Then add a waste allowance for cuts, damaged pieces and pattern matching: 10% for a straight layout in a rectangular room, 15% for diagonal or herringbone patterns and rooms with many angles, alcoves or a lot of built-ins.
Boxed flooring is where the box size matters. Luxury vinyl plank typically comes in cartons of about 20 sq ft, laminate around 20-25 sq ft, solid and engineered hardwood 18-24 sq ft, and floor tile 10-15 sq ft, but every product is different, so read the carton and type the real figure into the box field. The tool divides the square footage including waste by the box size and rounds up to whole boxes, then tells you how many square feet that is and how much of it is spare.
Worked example: a 12 x 15 living room in vinyl plank
The default room is 12 x 15 = 180 sq ft. With 10% waste you need 198 sq ft of material. At 20 sq ft per box, 198 / 20 = 9.9, so you buy 10 boxes, which is 200 sq ft with 20 sq ft left over for cuts and a couple of future replacement planks. At $3.00 per sq ft the material comes to $600 before underlayment and trim. Switch to laminate with 24 sq ft cartons and the same room takes 9 boxes (216 sq ft). Choose a herringbone layout at 15% waste and the vinyl plank count rises to 207 sq ft, or 11 boxes.
Carpet is priced and sold by the square yard, and a square yard is 9 sq ft. The same room at 10% waste needs 198 / 9 = 22 sq yd. Broadloom carpet comes on 12-foot-wide rolls, so the installer will actually cut about 16.5 linear feet of roll (198 / 12) and plan where the seams fall; rooms wider than 12 feet always need a seam, which adds waste. Many carpet retailers quote per square foot now, so check which unit the price is in before you compare.
Underlayment, trim and other extras
Most laminate and some vinyl plank needs a foam or cork underlayment unless it has a pad attached; hardwood over a wood subfloor uses rosin paper or a synthetic underlayment; carpet needs padding. Rolls are commonly 100 sq ft, and the calculator adds 5% for overlap at the seams. You will also need transition strips at doorways and either quarter-round or shoe molding around the perimeter to cover the expansion gap, so the tool lists the room's perimeter in linear feet as a starting point for trim.
Before ordering, confirm the flooring is rated for the room (kitchens and bathrooms need a waterproof product), buy from one dye lot, and let the boxes acclimate in the room for 48 hours or as the manufacturer directs. Keep one unopened box for repairs; product lines are discontinued constantly, and an exact match a few years later is unlikely.