Paint & Wallpaper: Calculators, Diagrams & Guides
3 calculators · 9 diagrams · 6 guides
Paint coverage math fails at the edges: the label says 350 to 400 square feet per gallon, but that assumes a smooth, previously painted surface and one coat — while your project has texture drinking 20 percent extra, a dark-to-light color change demanding primer plus two coats, and trim that needs its own product entirely. The calculators in this hub model those realities for interior walls and ceilings, exterior siding by material, and wallpaper by roll with the pattern-repeat penalty that turns a 56-square-foot roll into 44 usable.
The diagrams cover the takeoff and the technique: paintable-area geometry (what to subtract for openings and what not to), coverage rates by surface and texture, the primer decision tree, roll-versus-spray tradeoffs, and wallpaper pattern matching — straight, half-drop, and random — with the yield each one leaves you.
Six standalone guides back the calculators, from sheen selection through dark-to-light color strategy, textured-surface coverage, and full prep-to-cleanup interior technique. Coverage figures follow manufacturer spread rates and PDCA craftsmanship conventions. Everything here is free with no signup required — measure once, buy the right number of gallons and rolls the first time, and skip the third trip to the paint counter for one more quart.
Paint & Wallpaper calculators
- Interior Paint CalculatorInstantly calculate interior paint gallons for walls, ceilings & trim—doors and windows subtracted automatically. Free, fast, no signup needed.
- Exterior Paint CalculatorInstantly calculate exterior paint gallons for siding, trim & doors. Includes primer + 2-coat totals for vinyl, wood, stucco & brick. Free, no signup.
- Wallpaper CalculatorCalculate wallpaper bolts (double-rolls), single-roll equivalents, strips, paste & primer — with pattern repeat & half-drop math. Free, no signup.
Guides & references
- When You Need Primer: Drywall, Wood & Bare Surface Guide
- Dark to Light Color Changes: Professional Guide
- Choosing the Right Paint Sheen: Flat to Gloss
- Textured Wall Painting: Coverage Rates for Stucco & Popcorn
- Complete Interior Painting Guide: Prep to Cleanup
- How Many Rolls of Wallpaper Do I Need? Sizing Guide
Paint & Wallpaper · 9 diagrams
- Paint & Wallpaper
How paint coverage changes by surface — one gallon covers ~375 sq ft on smooth siding but only ~175 on rough stucco
One gallon spreads ~375 sq ft on smooth siding but only ~175 on rough stucco — nearly 2× difference. The surface’s texture and porosity, not the gallon math, is what throws off most paint estimates.
- Paint & Wallpaper
Why primer and why two coats — the exterior paint film from the substrate up
Skip primer on bare wood and the topcoat wicks into the pores — blotchy, paint-hungry, and stains bleed through. Primer seals the pores, blocks stains, and bonds to slick surfaces; then two thin coats hide far better than one thick one.
- Paint & Wallpaper
How to measure exterior paintable area — wall rectangle plus gable triangle, minus the big openings
Measure a gabled wall in two pieces: the rectangle (width × wall height) plus the gable triangle (½ × width × rise), then subtract the large openings. Net area ÷ coverage × coats = gallons. Add every wall and gable for the whole house.
- Paint & Wallpaper
Wallpaper match types — random, straight, and half-drop set the waste factor
How the repeat lines up at the seam is the match type, and it sets the default waste: random ≈10%, straight ≈15%, half-drop ≈20%. A bigger drop means more paper trimmed at the ceiling and floor to align the design.
- Paint & Wallpaper
Wallpaper strip yield — every strip rounds up to a whole pattern repeat
Each strip is cut to a whole number of repeats: strip length = ⌈(ceiling + 4″ trim) ÷ repeat⌉ × repeat. A 9-ft ceiling with an 18″ repeat needs a 10.5-ft strip, so a 33-ft bolt yields just 3 usable strips — that repeat rounding is why you buy more than the bare wall area.
- Paint & Wallpaper
Wallpaper bolt vs single roll — one bolt is two single rolls
Wallpaper ships as bolts (double rolls) but is priced per single roll: 1 bolt = 2 single rolls. Order in bolts and convert only to compare prices — ask for 6 bolts (12 single rolls), not 6 single rolls, or you receive half what you need.
- Paint & Wallpaper
Rolling vs spraying interior paint — spraying buys about 45% more paint
Spraying the same wall with the same coats buys ~45% more paint: a sprayer loses a third of the paint to overspray and bounce-back (×1.33 material × 1.20 waste ≈ ×1.60) where a roller is ×1.0 × 1.10. Spray wins on doors, trim, and cabinets; rolling a few walls buys less and needs no masking.
- Paint & Wallpaper
How wall texture and paint grade change interior-paint coverage
Texture is hidden surface area: smooth drywall ≈375 sq ft/gal, light/orange-peel ≈325, heavy/knockdown ≈275, stucco/popcorn ≈250. Paint grade matters too — on smooth drywall, economy 350 vs standard 375 vs premium 400 sq ft/gal.
- Paint & Wallpaper
Interior paint take-off — walls and ceiling are figured separately
Walls and ceiling are two separate surfaces: Walls = 2 × (L + W) × H − doors − windows; Ceiling = L × W. A standard door deducts 20 sq ft and a window 15 sq ft — you paint the trim, not the slab or the glass. Each surface gets its own paint, sheen, and coats.
Frequently Asked Questions
How much paint do I need for a room?
Perimeter × wall height minus openings gives paintable area; divide by 350 to 400 sq ft per gallon per coat on smooth walls, less on texture. A typical 12×12 bedroom runs a bit over a gallon for two coats on the walls alone. The interior paint calculator handles ceilings, trim, and doors as separate line items with their own rates.
When do I actually need primer?
On new drywall (its two mud-and-paper surfaces absorb differently), bare wood, stains, glossy surfaces that need adhesion, and any dramatic color change — especially covering dark or saturated colors. Paint-and-primer-in-one products handle mild repaints only. Our primer guide maps each situation to the right primer type.
How many rolls of wallpaper do I need?
Wall area ÷ usable roll yield — and the pattern repeat sets the yield. A random match keeps most of a standard roll usable; a 24-inch straight or half-drop repeat can cost 20 percent or more to matching waste. The wallpaper calculator computes strips per roll from your wall height and repeat, the way paperhangers actually estimate.
Does textured drywall really need more paint?
Yes — orange peel adds roughly 10 to 15 percent surface area, knockdown 15 to 25, and heavy popcorn can push 50. The texture also shears roller nap harder, so coverage per gallon drops even further in practice. Our textured wall guide publishes rate tables by texture so the calculator’s adjustment matches what the roller meets.