Deep Dive: Calculations, Assumptions, and Worked Example
This extended guide explains the logic used by the Carpet Underlay Calculator so you can audit results, adjust assumptions,
and share a consistent method with your team or customers. The calculator converts all dimensions to meters internally, sums
rectangular room areas and (optionally) stairs, applies a configurable waste factor, then optimizes rolls or tile packs.
It also estimates acoustic performance, checks combined thermal resistance (Tog) for underfloor heating (UFH), evaluates the
expected height build‑up for door clearance, and compiles a cost and accessories breakdown. Values are practical estimates
intended to streamline planning; always verify on site and consult manufacturer datasheets for certification‑grade figures.
Formulas at a Glance
| Symbol | Meaning | Formula / Reference | Notes |
|---|
A_r | Room area | L × W | Inputs can be metric or imperial; internally converted to meters |
A_s | Stair area | steps × width × tread (+ steps × width × riser if selected) | Choose whether to include risers |
A_t | Total area | ΣA_r + A_s | Before waste |
A_w | Area incl. waste | (ΣA_r)(1+w) + (A_s)(1+w+w_s) | w = waste %; w_s = extra stairs waste |
R_c | Roll coverage | rollWidth × rollLength | m² per roll |
N_r | Rolls needed | ceil(A_w / R_c) | Rounded up |
N_p | Packs needed | ceil(A_w / P_c) | P_c = pack coverage (m²) |
O | Off‑cuts / overage | N × coverage − A_w | For rolls or packs |
H | Height build‑up | carpet_thk + underlay_thk | Warn if > ~16 mm (door trimming likely) |
T_u | Underlay Tog (est.) | togPerMm(material) × underlay_thk | Illustrative; check datasheets |
T_Σ | Combined Tog | T_u + T_carpet | For UFH, target ≤ ~2.5 |
dB | Impact sound (est.) | baseDb(material) + 0.6 × max(0, thk−8) | Heuristic for comparison |
C_total | Total cost | (Underlay + Accessories + Labor) × (1 + tax%) | Accessories include grippers, tape, adhesive |
Conversions: feet to meters = ft × 0.3048; inches to meters = in × 0.0254. If your base measurements are in feet,
area in square meters can be approximated by ft² × 0.092903. The calculator manages these conversions automatically;
you only need to select your preferred unit system and supply dimensions. Waste is split so that stairs may have an extra buffer
because of tighter cuts, nosings, and more complex shapes.
Worked Example (Illustrative)
| Input | Assumed Value | Reasoning |
|---|
| Rooms | Room A 4×3 m; Room B 3.6×3.2 m | Two rectangles |
| Stairs | 12 steps, width 0.9 m, tread 0.28 m, riser included 0.18 m | Risers included for full coverage |
| Waste | 8% rooms; +10% extra on stairs | More allowance for stairs |
| Roll | 1.37 m × 11 m | Common underlay roll size |
| Material | PU foam, 10 mm | Comfortable for living/bedrooms |
| Carpet Tog | 1.2 | Typical medium‑pile |
Areas: Room A = 12.00 m²; Room B ≈ 11.52 m²; Σ rooms ≈ 23.52 m². Stairs (treads) ≈ 3.02 m²; risers add ≈ 1.94 m²;
As ≈ 4.96 m². Total before waste ≈ 28.48 m². With waste: rooms ≈ 25.40 m²; stairs with extra ≈ 5.95 m²;
Aw ≈ 31.35 m². Roll coverage Rc = 1.37 × 11 ≈ 15.07 m²; rolls needed Nr = ceil(31.35 / 15.07) = 3;
off‑cuts O ≈ 45.21 − 31.35 ≈ 13.86 m² (some of which will be useful for patches/landings). Height build‑up H = 9 + 10 = 19 mm,
so a door‑trim warning is appropriate. With PU foam: Tu ≈ 0.20 × 10 = 2.0; combined TΣ ≈ 3.2 — acceptable for
non‑UFH, but above a typical UFH target, so the calculator flags that appropriately. Acoustic estimate is around 21–22 dB, which may
or may not meet a given apartment‑by‑laws target; verify local requirements.
Costs are scenario‑dependent. If using price per m² the calculator multiplies Aw by that rate; if using price per roll/pack,
it multiplies the rounded quantity instead. Accessories combine grippers (sum of perimeters per room), tape (a seam factor heuristic),
and adhesive (equal to Aw when the method is double‑stick). Labor can be entered per m² to reflect local practice. Tax is
applied last. The suitability score blends thickness, acoustic estimate, UFH compatibility, and build‑up to create a quick comparison metric.
Disclaimer: the above is an illustrative engineering approximation intended for planning, not a substitute for professional measurement,
building regulations, or manufacturer certification. Always check datasheets for Tog/dB and adhere to local codes and warranty instructions.