Why do I need more carpet than the square footage of my room?

Because carpet comes off a roll of a fixed width, and every piece is the full width of the roll whether the room needs it or not. A 15' × 13' room is 21.67 sq yd of floor, but on a 12' roll it takes two drops either way: 40.00 sq yd laid along the 15' length, or 34.67 sq yd laid across it, before any trim. The strip left over from the last drop is paid for and thrown away.

What widths does carpet come in?

12 feet for most broadloom, with some ranges made 13' 6" or 15' wide. It matters most for a room just wider than 12': a 15' × 13' room laid along its length takes two drops and one seam on a 12' roll, and one drop with no seam on a 13' 6" roll. Ask what width the carpet you like comes in before you work anything out.

How much carpet do stairs take?

Each step takes a piece the width of the stair and as long as the tread plus the riser plus the wrap over the nose, cut with the pile running down the stair. Those pieces sit side by side across the roll, so a 12' roll gives four pieces 36" wide per row. Thirteen steps with a 10" tread, a 7 1/2" riser and 2" of wrap is four rows of 19 1/2" — 6.5 ft off the roll and 8.67 sq yd.

Where should the seams go?

Where they are walked across least and lit least from the side. Keep them out of doorways and away from the path through the room, and run them toward the main window rather than across its light, which shows every seam. All the drops must run the same way, too: pile leans, and a piece turned ninety degrees reads as a different colour beside its neighbour.

How much carpet pad do I need?

The floor area of the room, plus each stair's tread and the wrap over its nose — pad does not go up the riser. Pad has no pile to match, so its offcuts piece in anywhere and it wastes far less than the carpet does; the calculator adds 5% for trimming, which is our assumption rather than a published figure, and rounds up to the whole square yard pad is sold in.

Carpet calculator: square yards, seams and stairs

Carpet comes off a roll of a fixed width, so a room is laid out in drops rather than divided by area. This lays them out, marks the seams, adds the stairs and the pad, and gives the square yards to order.

Room length

= 15 ft

Room width

= 13 ft

Roll width

12' is most broadloom; ask the store what the carpet you like comes in

Drops run

Pile should lean away from the main door or window; fix the direction if it matters

Stair steps

0 for no stairs. Count the treads, landing excluded

Advanced settings6" trim a cut · 36" stair, 10" tread, 7 1/2" riser, 2" wrap · 5% pad waste

Trim allowance per drop

Both ends together; 3" at each wall is usual

Stair width

Nose wrap

Extra per step to wrap and tuck

Tread depth

Riser height

= 7.5"

Pad waste

Pad has no pile and pieces in anywhere; 5% is our assumption

Carpet to buy

44.67sq yd

Off the roll

33.5 ft of 12'

Seams

Pad

28 sq yd

How it is cut

Room: 2 dropsRunning along the width, 15' to cover across a 12' roll; 9' of the last drop is offcut: 2 × 13' 6"
SeamMeasured across the room from the wall the first drop starts at: 12'
Stairs: 13 pieces4 pieces 36" wide side by side across the roll, pile running down the stair: 4 rows of 19 1/2"
Room floor area21.67 sq yd if carpet came in any width: 195.0 sq ft
Bought beyond what is coveredRoll width you pay for and trim off — the cost of a fixed-width roll: 36%

Both ways round, on a 12' roll

Drops along the length2 × 15' 6", 1 seam: 41.33 sq yd
Drops across the width2 × 13' 6", 1 seam — shown: 36.00 sq yd

Room only, before the stairs. A wider roll can remove a seam outright, so try the other widths above before settling on one.

Before you order

Seams go where they are walked across least and lit from the side least — not across a doorway, and parallel to the light from the main window where you can. If the drop direction above puts a seam somewhere bad, choose the other one; the yards change, the room does not.

Order the room and the stairs from one roll in one dye lot. A second order of the same carpet is close, not identical, and the stairs sit right next to the landing.

Where to buy this

44.67 sq yd of 12' carpetHome Depot Lowe's
28 sq yd of padHome Depot Lowe's

This searches the material, not a size we worked out for you: carpet is cut from the roll in store, so take the linear feet and the roll width with you.

Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.

Worked example: a 15' × 13' room and 13 steps

On a 12' roll the room takes two drops of 13' 6" running across its 15', with one seam: 36.00 sq yd for 21.67 sq yd of floor. Laid the other way it would take 41.33 sq yd.

The 13 steps are cut four to a row across the roll, each 19 1/2" long, which is four rows and 8.67 sq yd more.

Order 44.67 sq yd — 33.5 ft off the roll — and 28 sq yd of pad.

Carpet: 44.67 sq yd
Off the roll: 33.5 ft of 12'
Seams in the room: 1
Pad: 28 sq yd

These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.

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About carpet

Why do I need more carpet than the square footage of my room?: Because carpet comes off a roll of a fixed width, and every piece is the full width of the roll whether the room needs it or not. A 15' × 13' room is 21.67 sq yd of floor, but on a 12' roll it takes two drops either way: 40.00 sq yd laid along the 15' length, or 34.67 sq yd laid across it, before any trim. The strip left over from the last drop is paid for and thrown away.
What widths does carpet come in?: 12 feet for most broadloom, with some ranges made 13' 6" or 15' wide. It matters most for a room just wider than 12': a 15' × 13' room laid along its length takes two drops and one seam on a 12' roll, and one drop with no seam on a 13' 6" roll. Ask what width the carpet you like comes in before you work anything out.
How much carpet do stairs take?: Each step takes a piece the width of the stair and as long as the tread plus the riser plus the wrap over the nose, cut with the pile running down the stair. Those pieces sit side by side across the roll, so a 12' roll gives four pieces 36" wide per row. Thirteen steps with a 10" tread, a 7 1/2" riser and 2" of wrap is four rows of 19 1/2" — 6.5 ft off the roll and 8.67 sq yd.
Where should the seams go?: Where they are walked across least and lit least from the side. Keep them out of doorways and away from the path through the room, and run them toward the main window rather than across its light, which shows every seam. All the drops must run the same way, too: pile leans, and a piece turned ninety degrees reads as a different colour beside its neighbour.
How much carpet pad do I need?: The floor area of the room, plus each stair's tread and the wrap over its nose — pad does not go up the riser. Pad has no pile to match, so its offcuts piece in anywhere and it wastes far less than the carpet does; the calculator adds 5% for trimming, which is our assumption rather than a published figure, and rounds up to the whole square yard pad is sold in.