Stair Carpet Calculator

Flooring Mechanics & Stair Carpet Installation Methods

Carpeting a stairway requires accounting for three-dimensional geometry across every tread, riser, and curved bullnose overhang.

Primary Stair Carpet Formulas

• Length per Step: L_step = Tread Depth + Riser Height + Wrap Allowance

• Hollywood / French Cap Wrap: Adds +2.5 inches per step (carpet is stapled tightly under the curved wooden nosing to contour the bullnose profile).

• Waterfall Installation: Adds +1.0 inch per step (carpet falls straight from the nosing down to the crotch of the next tread).

• Total Continuous Linear Length:

L_total (ft) = (N_steps × L_step) + Landing Length + 18 inches extra12

• Square Yards: Sq Yds = L_total (ft) × W_carpet (ft)9

The 18-Inch Shift Trick

Professional installers leave extra carpet folded beneath the bottom riser. Over several years, when the high-traffic nose of the steps shows wear, the carpet can be unstapled and shifted down by 2 inches to reveal fresh carpet on all step nosings.

How to Use This Calculator

Enter your Number of Steps (Risers) (e.g. 13 in steps), Tread Depth / Run (Inches) (e.g. 10.5 in in), Riser Height (Inches) (e.g. 7.5 in in), Installation Style & Nosing Wrap — choose from Hollywood / French Cap (+2.5" wrapped tightly under nosing), Waterfall Style (+1.0" cascading directly to tread), Carpet / Runner Width (Inches) (e.g. 36 in in), Include Intermediate Landing? — choose from No Landing (Straight flight only), 3' × 3' Square Landing (+9 sq ft / +3 linear ft), 3' × 4' Rectangular Landing (+12 sq ft / +4 linear ft), and Carpet Price Per Square Yard () (e.g. 28 in /sq yd). Measure twice before entering dimensions — small measurement errors scale up significantly in material estimates.

The Formula

Formula Reference: Stair Carpet Calculator

Input variables:

numberOfStepsCount — Number of Steps (Risers)

treadDepthInches — Tread Depth / Run (Inches)

riserHeightInches — Riser Height (Inches)

installationStyleSelect — Installation Style & Nosing Wrap

carpetWidthInches — Carpet / Runner Width (Inches)

Computed outputs:

totalLinearFeetNeeded — Total Carpet Runner Length Needed

totalSquareYardsNeeded — Total Carpet in Square Yards

totalSquareFeetNeeded — Total Area in Square Feet

singleStepCarpetLengthInches — Carpet Length Per Single Step

estimatedTotalCarpetCostDollars — Estimated Total Carpet Cost

Mathematical relationships extracted from calculation logic:

singleStepIn = treadIn + riserIn + wrapAllowance

extraLinearIn = 18.0; // 18" safety extra for top/bottom staples and future tread shifting

totalLinearIn = (steps * singleStepIn) + extraLinearIn

totalSqFt = totalLinearFt * widthFt

totalCost = totalSqYds * priceSqYd

Threshold: landing_3x3

Threshold: landing_3x4

Reference constants:

extraLinearIn = 18.0

Standard: IRC / NAHB residential construction estimating standards

Worked Example

A standard 13-step residential staircase with 10.5" treads and 7.5" risers requires 23.7 linear feet of 36" carpet runner (7.9 sq yds) for Hollywood French-cap installation.

Understanding Your Result

Add a 10% waste factor for square-cut tile or carpet, 15% for diagonal tile installation, and 10–15% for hardwood with natural variations. Flooring is sold in boxes covering fixed square footage — always round up to full boxes. For tile: grout joint width affects coverage; wider joints reduce tile count. For carpet: roll width (typically 12 ft) determines seam placement — minimize seams in high-visibility areas.