| Item | Value |
|---|---|
| Drop per step | — |
| Effective coverage width | — |
| Area per step | — |
| Steps area | — |
| Landings area | — |
| Raw total area | — |
| Waste allowance | — |
| Order area after waste | — |
| Linear length (cut method) | — |
| Linear length (area ÷ roll width) | — |
The calculator estimates fabric using two complementary approaches and selects the larger value for safety:
- Cut method: Linear length equals (tread + riser + nosing) × steps, plus landing lengths as applicable.
- Area method: Total area divided by roll width.
For steps, area per step is (tread + riser + nosing) × effective width. Landings can be full width or match the runner. Add your own waste allowance to account for trimming and pattern matching.
A stair carpet calculator helps homeowners, installers, and property managers quickly determine the amount of carpet required for a staircase. Because stairs combine horizontal treads, vertical risers, nosings, and potential turns, estimating by guesswork often results in costly over‑ordering or mid‑project shortages. This article explains the inputs a calculator uses, the formulas behind typical outputs, practical allowances for waste, and a worked example you can mirror. You will also find reference tables you can print or copy into your own estimating workflow.
What the Calculator Estimates
A well‑designed stair carpet calculator returns three core outputs: total linear length of the carpet run, required carpet width, and recommended overage for pattern matching, cuts, and installation contingencies. For straight flights, the math is relatively direct; for turning stairs with winders or landings, the calculator adds allowances to preserve a consistent pattern flow and to accommodate angled cuts.
Key Measurements to Collect
- Tread depth (D): horizontal depth from riser to nosing edge.
- Riser height (R): vertical rise from tread to tread.
- Nosing wrap (N): allowance for the carpet to wrap the nosing; many installers treat this as 20–40 mm (approx. 3/4–1 1/2 in).
- Number of steps (S): count all visible steps in the flight.
- Stair width (W): clear width to be covered; for a stair runner, use runner width instead.
- Landings: quantity and sizes (length × width) for each landing to be carpeted.
- Carpet roll width (CRW): common widths include 4.0 m (13'2″) and 5.0 m (16'4″).
- Pattern repeat (PR): length required to repeat a woven or printed pattern; set to 0 for plain styles.
Formulas Used by a Stair Carpet Calculator
For a standard straight flight with uniform steps and no intermediate landing, the per‑step run is:
PerStepRun = D + R + N.
Total linear length for S steps is:
TotalStairRun = S × (D + R + N).
If there are landings, add their dimensions (converted to linear length based on carpet roll width or runner width). For patterned carpet, round TotalStairRun up to the next multiple of PR to protect pattern continuity:
PatternAdjusted = ceil(TotalStairRun / PR) × PR.
Finally, include a sensible waste factor (WF). For plain carpet on straight stairs, 5–10% is typical. For patterned goods, turns, or winders, 10–20% is safer:
OrderLength = (PatternAdjusted or TotalStairRun) × (1 + WF).
Typical Waste Factors by Stair Type
| Stair configuration | Pattern repeat | Suggested waste factor (WF) | Notes |
|---|---|---|---|
| Straight flight, no landing | None | 5–8% | Uniform cutting; minimal offcuts. |
| Straight flight with landing | None | 8–12% | Landing cuts can increase offcuts. |
| Quarter‑turn with winders | None | 12–16% | Angled treads require larger blanks. |
| Any layout | Patterned (PR > 0) | 15–20% | Aligning motifs needs extra length. |
Worked Example
Suppose you have 14 uniform steps with the following average measurements: tread depth D = 260 mm, riser height R = 185 mm, nosing wrap N = 30 mm, and stair width W = 900 mm. There is one small top landing measuring 900 mm × 900 mm. The carpet is plain (PR = 0) and available on a 4.0 m roll. You plan a full‑width installation (not a runner).
- Per step run: D + R + N = 260 + 185 + 30 = 475 mm.
- Total stair run: 14 × 475 = 6,650 mm = 6.65 m.
- Landing conversion to linear length: Landing area / roll width = (0.9 × 0.9) m² / 4.0 m = 0.2025 m.
- Subtotal length: 6.65 + 0.2025 = 6.8525 m.
- Waste factor: choose 10% for a plain carpet with one landing.
- Order length: 6.8525 × 1.10 ≈ 7.54 m. Round up to 7.6 m to be safe.
Because W = 900 mm is well within the 4.0 m roll width, the required carpet width is satisfied by the standard roll. If you were using a stair runner, ensure the runner width plus side reveals meet your design and building code requirements.
Example Inputs & Outputs (for your records)
| Parameter | Value | Unit | Comment |
|---|---|---|---|
| Number of steps (S) | 14 | count | Uniform steps |
| Tread depth (D) | 260 | mm | Measured at centerline |
| Riser height (R) | 185 | mm | Check several steps |
| Nosing wrap (N) | 30 | mm | Allows a clean bullnose |
| Stair width (W) | 900 | mm | Full‑width carpet |
| Landing size | 900 × 900 | mm | Top landing |
| Roll width (CRW) | 4.0 | m | Standard wide goods |
| Pattern repeat (PR) | 0 | m | Plain carpet |
| Waste factor (WF) | 10% | — | Chosen for layout |
| Order length | 7.6 | m | Rounded up |
Choosing Width: Full‑Width vs Runner
A full‑width installation simplifies planning because the roll typically exceeds the stair width and edges are hidden under stringers. Runners introduce two added checks: stair nosing visibility and equal reveals along both sides. If your runner must center across winders or turns, expect trimming losses and consider increasing the waste factor or ordering an extra pattern repeat.
Pattern Matching and Visual Continuity
For stripes, plaids, or bold motifs, uninterrupted visual flow across risers is crucial. Ensure that the PR divides evenly into your per‑step run or plan to “drop” repeats on risers to keep lines level. The calculator’s rounding to the next multiple of PR prevents awkward half‑motif cuts but slightly increases order length. Always review a physical pattern book before committing.
Advanced Cases: Winders and Quarter‑Turns
Winder treads are triangular or trapezoidal, so the blank you cut must fit the widest diagonal dimension rather than the average depth. Estimators often model a winder set as three oversized rectangles to be trimmed on site. If your stair transitions at a landing and changes direction, measure the landing separately and treat each flight independently, then sum the lengths and apply a global waste factor.
Quick Reference: Minimum Recommended Blanks
| Step style | Blank depth to cut | Blank width to cut | Why |
|---|---|---|---|
| Standard rectangular step | D + R + N | W | Covers tread, riser, and nosing wrap. |
| Open‑sided step with return | D + R + N | W + 80–120 mm | Extra width for the visible return. |
| Winder (outer radius) | Max diagonal depth + trim | W + trim | Diagonal cut preserves grain/pattern. |
Installation Considerations That Affect Quantity
- Underlay thickness: Thicker underlay can slightly increase nosing wrap; re‑measure if changing spec.
- Gripper placement: Leave the usual gap to tuck carpet; overly large gaps may need extra length.
- Seaming strategy: For landings larger than roll width, plan seams parallel to traffic with adequate allowances.
- Safety and codes: Keep edges secure and free from trip points; confirm local code requirements.
How to Use a Stair Carpet Calculator Effectively
- Gather measurements for steps, landings, and total stair width using a rigid tape.
- Choose carpet type (plain vs patterned) and note the pattern repeat, if any.
- Enter D, R, N, S, and landing dimensions.
- Confirm roll width or runner width to validate coverage.
- Select a waste factor from the table above that matches your stair layout.
- Review the calculator’s recommended order length and round up to the nearest practical cut length from your supplier.