Calculate carpet needed for a room in sq ft or sq m, including underlay.
Carpet is sold by the square yard in the United States and the square metre elsewhere, which is why the same room quotes at two very different-looking numbers — a square yard is nine square feet, a square metre about 10.76. Ordering needs a waste allowance for two reasons: broadloom comes in fixed widths, usually twelve or fifteen feet, so a room narrower than the roll leaves an offcut, and a patterned carpet needs extra length to align the repeat across seams. Underlay is calculated on the bare floor area rather than the ordered amount, because it is butted and taped rather than cut to a pattern, which is why it should be priced separately instead of folded into a per-yard figure.
Carpet and underlay quantities
Area = length x width; order area = area x (1 + waste / 100); square yards = order area / 9; square metres = area x 0.0929; total = square yards x carpet price + (area / 9) x underlay price
Because broadloom comes in fixed twelve or fifteen foot widths. A room eleven feet wide still consumes the full twelve foot roll width, and that offcut is charged for even though it is unusable elsewhere.
For stretch-fitted carpet, yes — it carries most of the comfort and doubles the carpet's life by absorbing footfall. Carpet tiles and glue-down commercial carpet are laid directly on the subfloor.
Add the length of one pattern repeat for every seam, which usually works out at fifteen to twenty percent rather than ten. Ask the retailer for the repeat length before ordering.