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Paint Calculator

Find out how many gallons of paint and primer you need for walls, ceilings and trim — by coats, not guesswork.

Paint Calculator

Gallons (litres) of paint and primer for walls and ceilings, with coats, surface type and labor time.

How this is calculated

Paintable area = (perimeter × height + optional ceiling) × rooms − openings. Paint = area × coats ÷ coverage per gallon (350 ft²/gal for smooth walls; less for textured or porous surfaces).

Enter the room size, openings, number of coats and the surface type (smooth, textured, masonry). You get the exact paint needed, a rounded-up purchase size with suggested can sizes, primer, painting time and cost.

Results are estimates for planning purposes only. Real-world quantities depend on site conditions, materials and workmanship. Always verify measurements, follow local building codes and confirm quantities with your supplier or a qualified professional before ordering or building.

About the Paint Calculator

A gallon of paint looks like plenty until you are halfway through the second wall. This calculator converts room dimensions into gallons of wall paint, ceiling paint and primer, adjusted for coats and the openings you are not painting. It is made for repainting a room, pricing a whole house or comparing contractor quotes — a quote specifying one coat where two are needed is not actually cheaper.

How to use

  1. Enter the room length, width and ceiling height in feet.
  2. Enter the number of doors and windows — each door subtracts about 21 square feet, each window about 15.
  3. Choose your coats: one for a refresh in the same color, two for a new color or new drywall.
  4. Tick the ceiling box if you are painting it, and add trim for baseboards and casings.
  5. Tick primer for new drywall, bare wood or a dramatic color change — it gets its own gallon count.

Understanding your results

Wall paint is shown in whole gallons for your coat count. Ceiling paint is listed separately, since flat ceiling paint covers less on textured surfaces. Primer appears as its own line item — one coat over new drywall — so you can price it independently.

How the calculation works

Paintable wall area is twice the sum of length and width, times ceiling height, minus doors and windows. That area is divided by about 350 square feet per gallon — the standard interior latex coverage — multiplied by coats and rounded up. Ceilings use roughly 250 square feet per gallon; primer is one full coat over the wall area.

Paint Calculator FAQs

About 350 square feet per coat for interior latex on smooth walls — that is the industry standard coverage rate the calculator uses. Rough or textured surfaces, porous new drywall and dark colors can drop real coverage to 250 to 300 square feet. Always round up to whole gallons, since touching up later with a second mixed batch risks a visible color difference.
Two coats is the standard for a lasting finish, especially over new drywall or when changing color. One coat is acceptable only when refreshing the same color in good condition. Dark or vivid colors over light walls can need three coats, or one coat of tinted primer plus two topcoats. Skipping the second coat is the most common reason paint jobs look patchy.
Prime new drywall, bare wood, patched areas, stains and any dramatic color change — especially dark to light. Primer seals porous surfaces so the topcoat covers evenly instead of soaking in unevenly. Over previously painted walls in good condition with a similar color, a quality paint-and-primer product applied in two coats is usually enough.
Divide the ceiling area (length times width) by about 250 square feet per gallon — ceilings get a lower coverage rate because of texture and the difficulty of rolling overhead. A 12×12 room needs roughly one gallon for two coats. Use flat ceiling paint: it hides imperfections and roller marks far better than wall paint sheens.
Two to four hours for latex paint in normal indoor conditions — check the can, since times vary by product and humidity. Oil-based paints need 24 hours between coats. Painting the second coat too soon can pull up the first coat and leave lap marks. In humid weather, give it the full recommended time plus a margin.
Nabil Sharif, founder of CalcDune

Written by

Nabil Sharif

Founder of CalcDune — a free library of 137 online calculators for finance, health and everyday math. Every calculator shows the formula behind its result, so you can see exactly how your numbers are worked out.

More articles by Nabil →

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