Sources & formulas

Every calculator on PaintingCalcs rests on established paintable-area geometry plus stable painting 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.

Identities & geometry

  • Wall area: perimeter × wall height (perimeter = 2 × (length + width)).
  • Net paintable area: gross wall − Σ openings (interior door ~21 sq ft, window ~15, patio door ~40 — labeled).
  • Ceiling area: length × width.
  • Paint gallons: ceil(area × coats ÷ coverage per gallon), rounded up to whole gallons.
  • Primer gallons: ceil(area ÷ primer coverage), primer ~200–300 sq ft/gal.
  • Trim linear feet: (run − door widths) × (1 + waste).
  • Cost: (area × your $/sq ft [or gallons × your $/gallon] + labor + add-ons − discount) ×(1 + contingency); cabinet cost = (doors + drawers) × your $/unit + labor.

Convention typicals (labeled)

  • Coverage / spread rate per gallon by paint & surface: smooth drywall ~350–400, textured ~250–300, bare wood ~200–300, stucco ~150–250, unpainted brick ~100–200 sq ft/gal — labeled, confirm the can.
  • Primer coverage: ~200–300 sq ft/gal — labeled.
  • Number of coats by scenario: 1–2 like color, 2 new/primed drywall, 2–3 + primer dark→light — labeled, confirm on the product.
  • Standard door/window deductions: interior door ~21 sq ft, window ~15, patio door ~40 — labeled.
  • Cost bands, cabinet bands, story/access multipliers, contingency/waste ~5–10%: labeled planning typicals.
  • EPA RRP lead-safe rule: pre-1978 homes may have lead paint — follow the EPA Renovation, Repair and Painting rule and hire a certified firm.

Cost tools

  • Every cost tool uses the prices you enter ($/sq ft, $/gallon, labor $, add-on $, $/unit) — no material or labor price is stored, so the site needs no maintenance. Cost bands are a labeled sanity guide.
  • Quote check: derived $/sq ft = quote ÷ area, compared to the labeled band for the job — a sanity flag, not a bid.

Material and labor prices, product data-sheet values and local rules vary by place and change over time — always measure your actual surfaces and confirm coverage on the paint can, allow extra for texture, porosity, color change and waste, defer surface prep, lead-paint abatement, structural repairs and code compliance to the EPA RRP rule and a certified professional, and get itemized written quotes from a licensed, insured painter before you commit.