Sources & formulas

Every calculator on RenovationCalcs rests on established arithmetic and geometry plus stable building conventions. Unlike topics with a time dependency, "verification" here is mathematical: each formula is tested against known values and is therefore correct for good. Here are the technical foundations by area.

Geometry & quantities

• Area: area = length × width (sq ft).
• Wall area: perimeter × height − openings (doors & windows).
• Material quantity: area ÷ coverage × (1 + waste factor); round up to whole units/boxes.
• Units: 12 in = 1 ft; 9 sq ft = 1 sq yd; countertop depth ≈ 2.083 ft (25 in).

Coverage & waste conventions (labeled planning bands)

• Paint: ~350–400 sq ft per gallon per coat (default 375) — check your product label.
• Drywall: a 4×8 sheet = 32 sq ft; sheets = ceil(area ÷ 32).
• Tile & flooring waste: ~10–15% (tile), ~7–12% (flooring) by layout.

Cost, budget & finance

• Every cost tool uses the prices you enter ($/sq ft, $/gallon, $/sheet, $/linear ft, per-unit, labor $/hour, APR) — no material price, labor rate, cost index or live loan rate is stored, so the site needs no maintenance.
• Contingency: total = subtotal × (1 + contingency%); typical band 10–20%.
• Cost per sq ft: $/sq ft = your total ÷ area (and area × $/sq ft back to a total).
• ROI: recoup% = your value added ÷ your cost × 100 — illustrative only; resale value is never guaranteed.
• Financing: standard amortization M = P·r ÷ (1 − (1+r)⁻ⁿ), r = APR ÷ 12 — illustrative, not a loan offer.

Permit rules, local code and material lead times vary by place and change over time — always confirm permits and code with your local building department.