How Much Carpet Do I Need? (Calculator + Rolls)

By RugCalc Editorial Team · Last updated: 2026-10-13 · 8 min read

Quick answer: length × width = square footage, then add ~10% waste for seams and cuts. A 12×15 ft room is 180 sq ft, so you need about 200 sq ft of carpet. Here's the full method including roll widths.

Carpet area math

Carpet is sold by the square foot. The basic figure:

Length (ft) × Width (ft) = square footage, then add waste. Because carpet comes in rolls (usually 12 ft wide), odd room shapes and seams add waste. Measure each room you're carpeting separately — the length and width of the room, not the house — and don't forget closets and any stairs.

For an L-shaped room, split it into rectangles, measure each, and add them. The installer lays carpet in one continuous piece where possible, so the math you give them needs to reflect the real layout, not just a single length × width.

Roll widths and seams

Standard carpet rolls are 12 ft wide (sometimes 15 ft). A room wider than the roll needs a seam, and seams must fall where the pattern allows. If your room is 15 ft wide, you'll waste the extra across the roll width — this is why waste can exceed 10%.

A good rule: if any dimension of the room is wider than the roll, plan on a seam and add more waste. Seams on plain carpet can be hidden better than on patterned carpet, so a patterned room that needs a seam usually costs noticeably more in both material and labor.

Worked example

• Room = 12 × 15 = 180 sq ft
• Add 10% waste = 198 sq ft → ~200 sq ft
• At 12-ft roll width, a 15-ft room needs more allowance for the seam

Round up to whole square yards when the installer quotes — carpet is priced per square yard in much of the US (9 sq ft per sq yd). 200 sq ft is about 23 sq yd before waste.

Underlayment

Order carpet padding too — about the same area as the carpet. Standard padding is sold in rolls and priced per square foot; thicker, denser padding adds cost and comfort. Padding comes in thicknesses from about 6 mm to over 11 mm, and denser is usually better for high-traffic areas and heavier carpet.

Ordering tips

Buy all of the carpet at once — dye lots differ between batches, and a seam-matched piece can't be color-matched later. Have an accurate room plan (including closets and stairs if carpeted) before you order, and add 10% for the installer's cuts.

Ask the supplier for the usable roll width and whether your layout needs a seam before you commit — that single question decides whether your waste is 10% or 20%.

Common mistakes

• Not accounting for roll width. Rooms wider than 12 ft need seams and more waste.
• Forgetting closets and stairs. They're real area.
• No waste. Installers need extra for cuts and fitting.
• Ordering padding short. Padding covers the same footprint.
• Mixing sq ft and sq yd. 9 sq ft = 1 sq yd — know which unit the installer prices in.

FAQ