Stair carpet calculator

A runner only needs a length, since its width is fixed by the product. Full-width carpet needs to be estimated as an area, because you're covering the entire tread and riser surface edge to edge, and carpet is sold and cut from a roll of a given width, commonly 12 ft.

The area for one step is its surface length (tread + riser + nosing wrap) times the stair width. Multiply by the number of treads to get the total area for the flight, before adding landings or waste.

Per-step surface length = tread depth + riser height + nosing wrap. For 10.5 in tread, 7.5 in riser, 1 in nosing wrap, that's 19 in. Total surface area (in²) = treads × per-step length × stair width. With 13 treads and a 36 in stair width: 13 × 19 × 36 = 8,892 in².

Convert to square feet by dividing by 144: 8,892 ÷ 144 ≈ 61.75 ft². Add roughly 10 percent waste for cutting and pattern alignment: 61.75 × 1.10 ≈ 67.9 ft². Convert to square yards by dividing by 9: 67.9 ÷ 9 ≈ 7.55 yd².

On an open-sided stair (no wall on one or both sides), installers commonly wrap a small amount of extra carpet, roughly 2 in per open side (hedged, varies by installer and finishing method), down the stringer face for a finished edge rather than leaving a raw cut visible.

Landings are calculated separately as their own rectangle: landing depth × stair width, converted to square feet the same way, then added to the flight's total area before applying waste.

Residential carpet commonly comes on 12 ft wide rolls. For a stair section that's 36 in (3 ft) wide, a cutter can usually get four stair-width strips across a 12 ft roll width, so ordering is often discussed in linear feet of roll consumed rather than raw square footage.

Ask your carpet retailer how they cut stair sections from their roll stock; some quote by square yard, others by linear foot of a specific roll width, and the conversion between the two depends on how efficiently your stair width divides into their roll width.

Stair pad is typically ordered to match the carpeted area, sometimes in a firmer, thinner density made specifically for stairs to keep the nosing edge crisp rather than spongy. Use the same area figure calculated for the carpet.

Waste on full-width carpet is commonly cited around 10 percent for straight runs of stairs with a simple pattern or solid color, rising for stairs with winders, unusual widths, or a pattern that needs to be centered on each tread.

• Estimating carpet like a runner (length only) and forgetting to multiply by stair width for a full-width job.
• Leaving out landings, which are easy to forget when focused on the sloped part of the flight.
• Skipping the open-side wrap allowance on a stringer that will be visible after installation.
• Assuming every retailer cuts and quotes stairs the same way; confirm whether their number is per square yard or per linear foot of a given roll width.