About Carpet Calculator (Broadloom Strips, Seams & Waste)
Carpet does not sell by the square foot of your floor — it sells as broadloom, a roll of fixed width, almost always 12 ft and sometimes 15 ft in the US. A room is covered by whole strips cut from that roll, running the length of the room, so a 14-ft-wide room on a 12-ft roll needs two strips and buys nearly twice its width in carpet whether you like it or not. That is why this calculator has no "waste percentage" input: the waste is computed, not assumed, as the difference between the carpet you must buy (whole strips) and the carpet that actually lands on the floor.
Enter each room as length × width — the length being the wall you want the strips (and their seams) to run parallel to — and pick the roll width your product sells in. The tool reports strips, bought and laid square footage, and seams per room, the emergent waste percentage across all rooms, the pad square footage, and the tack strip length from the room perimeters with an editable per-room doorway deduction. An optional stairs card adds a straight flight from honest step geometry: steps × width × (tread + riser + a 1-inch nosing wrap).
How It Works
- Add each room as length × width in feet. The model runs the strips along the room length, so enter the wall you want the seams parallel to as the length — rotating a room in the inputs can change its strip count and waste.
- Pick the roll width (12 ft is standard; 15 ft where offered). Each room buys ceil(width ÷ roll width) whole strips of length × roll width; seams between strips are strips − 1.
- The totals compare carpet bought against carpet laid: the difference is the waste, and the waste percentage is (bought − used) ÷ used — emergent from the roll arithmetic, not a flat allowance.
- Pad is counted on the laid area (plus the stairs when included), and tack strip is the sum of room perimeters minus an editable doorway allowance per room (floored at zero — a big allowance can never drive a small room negative).
- Optionally add a straight stair flight: steps × width × (tread + riser + 1″ nose) ÷ 12 gives the stair carpet in ft², reported separately and added to the carpet grand total and the pad.
Worked Example
Two rooms on a 12-ft roll, both 16 ft long: one 10 ft wide and one 14 ft wide. The 10-ft room needs ceil(10/12) = 1 strip — 12 × 16 = 192 ft² bought for 160 ft² laid, no seams. The 14-ft room needs ceil(14/12) = 2 strips — 384 ft² bought for 224 ft² laid, one seam. Together: 576 ft² bought, 384 ft² laid, so the emergent waste is (576 − 384) / 384 = 50%, from geometry alone. Pad covers the laid area, 384 ft². Tack strip is the perimeters, 52 + 60 = 112 LF, minus the 3-ft doorway allowance per room: 106 LF. On a 15-ft roll the same rooms take one strip each — 480 ft² bought and 25% waste — the roll width, not a table, decides the waste.
Formulas
- Strips per room
strips = ceil( W / W_roll )- Carpet bought vs laid per room
bought = strips × L × W_roll ; laid = L × W ; seams = strips − 1- Emergent waste
waste% = ( Σbought − Σlaid ) / Σlaid × 100- Pad and tack strip
pad = Σlaid + stairs ; tack = Σ max(0, 2(L + W) − allowance)- Stair carpet (waterfall wrap)
stairs = steps × w × ( tread + riser + 1″ nose ) / 12
Standards & References
- Broadloom roll arithmetic — 12- and 15-ft US roll widths; no external standard governs the takeoff
- Representative stair profile — 10″ tread, 7.75″ riser (IRC maximum), 1″ nosing wrap; stated as typical, editable
Frequently Asked Questions
Why does the calculator have no waste percentage input?
Because broadloom waste is geometric, not statistical. Carpet sells in fixed-width rolls, so a room buys whole strips: a 14-ft-wide room on a 12-ft roll buys two full strips whether it uses the second one or not. The waste is the difference between the strips you must buy and the floor you cover, and this calculator computes exactly that. A flat 10% allowance would understate a bad room-to-roll fit and overstate a perfect one.
Should I choose a 12-ft or a 15-ft roll?
Twelve feet is the standard US broadloom width and nearly every product sells in it; 15-ft rolls exist in a smaller selection of products. When your rooms are 12 to 15 ft wide, a 15-ft roll can eliminate a seam and cut waste dramatically — the worked example drops from 50% to 25% waste — so it is worth asking whether your carpet comes in 15 ft. Toggle the roll width here and compare the bought totals before you shop.
Which way do the strips run, and does room orientation matter?
This model runs the strips along the room length, so the strip count comes from the room width. Orientation matters a lot: a 14 × 16 ft room is 2 strips one way and 2 the other on a 12-ft roll, but a 10 × 16 ft room is 1 strip across the 10-ft width and 2 across the 16-ft width. Enter the wall you want the seams parallel to as the length. Installers also keep the pile direction consistent, which can force an orientation regardless of what wastes less.
Where should carpet seams go?
Away from traffic and light. Practice is to run seams parallel to the main light source (windows rake light across a seam and reveal it), keep them out of doorways and pivot points where traffic wears them open, and place them under or behind furniture when possible. Every strip beyond the first adds one seam in this calculator — if a room shows a seam badge, that is a conversation to have about where the installer will hide it.
Does patterned carpet need extra?
Yes. A pattern must match across every seam, so each strip after the first is cut long by up to one pattern repeat, and large repeats can add several feet per strip. This calculator models solid or random-texture broadloom; for a patterned product, take the strip counts from here and ask the retailer to figure the repeat allowance for your specific pattern length.
How do I measure stairs for carpet?
Count the steps and measure one step: the tread (the flat part you stand on), the riser (the vertical face), and add about an inch for the nosing wrap — the calculator uses tread + riser + 1″ per step, at defaults of a 10″ tread and 7.75″ riser that you should replace with your own measurements. Multiply by the carpeted width. Stair carpet is reported separately here because installers usually cut it from the strip offcuts the rooms already generated; the pad quantity includes the stairs.