Breakdown by Surface
How This Paint Calculator Works
For each room, enter the wall length, wall width, and wall height. The wall area is the perimeter (length + width x 2) multiplied by the wall height. Subtract the area of every door (about 21 square feet each) and every window (about 13 square feet each). The remaining paintable area is divided by the coverage rate for the chosen paint type, surface texture, and paint quality, then multiplied by the number of coats and the waste factor.
The ceiling panel adds a separate paint estimate for each room's floor area (length x width). The trim panel computes paint for the linear feet of baseboards, casings, and doors separately. The cost panel multiplies the total gallons by the per-gallon paint price so you see the material cost in one place.
Toggle the unit switch at the top to convert every input from feet and gallons to metres and litres. The conversion uses the standard 1 square foot per gallon equals 0.0245 litre per square metre. The 8 quick presets load realistic room dimensions for common projects (small bedroom, large bedroom, living room, kitchen, bathroom, garage, and two exterior house sizes). Add unlimited rooms using the Add Room button and rename each room so the shopping list groups paint by room.
Why This Beats Asking an AI Chatbot
AI chatbots give one coverage rate for every paint type and surface texture, forget to subtract doors and windows (which can be 10 to 25 percent of the wall area in a small room), ignore that textured drywall absorbs 30 to 50 percent more paint than smooth drywall, miss that primer counts as a separate gallon, do not apply the 10 percent waste factor for roller absorption and small spills, do not separate ceiling and trim from walls (which need different paint and different coverage), and cannot multiply by the number of coats. They give a number that feels right but is off by 20 to 50 percent on real projects, costing you either 1 to 2 gallons short or $50 to $100 in extra cans.
This calculator uses the right coverage rate for each paint type and surface texture combination, subtracts doors and windows automatically, applies the waste factor slider on top of the computed gallons, treats primer as a separate coat, and outputs wall paint, primer, ceiling paint, trim paint, and door paint as five separate line items. You can copy the result as a plain-text shopping list, print it for the paint store, and save the project to localStorage so it reloads when you come back.
Authoritative References
Paint coverage rates in this calculator come from Wikipedia - Paint (composition, coverage per gallon, sheen), Wikipedia - Painting and Decorating (surface preparation and coverage variance), and standard industry data from Sherwin-Williams paint calculator, Benjamin Moore paint calculator, and Dulux paint calculator. Surface absorption factors (smooth drywall 100%, textured 70%, wood 95%, metal 100%, masonry 60%) come from manufacturer coverage guides and trade painting handbooks.
Frequently Asked Questions
How do I calculate how much paint I need?
Measure the perimeter of the room (length + width x 2) and multiply by the wall height in feet. Then subtract 21 square feet per door and 13 square feet per window. One gallon of standard interior paint covers about 350 square feet of smooth drywall with one coat. For two coats over a primer, divide by 175. The calculator above does the math automatically when you enter your room dimensions.
How much paint do I need for a 12x12 room with 8 foot ceilings?
A 12 foot by 12 foot room with 8 foot ceilings has 384 square feet of wall area (perimeter 48 feet x 8 feet), minus 21 square feet for one door and 13 for one window, giving 350 square feet of paintable wall area. One gallon of premium paint covers 400 square feet in one coat, so you need 1 gallon for 1 coat, 2 gallons for 2 coats, or 1 gallon plus 1 primer coat. The calculator above shows the exact number for any room size.
How many square feet does one gallon of paint cover?
Coverage depends on paint quality and surface texture. Budget paint: 250 square feet per gallon. Standard paint: 350. Premium paint: 400. Textured drywall or stucco absorbs more: 200-250. Smooth primed drywall: 400. Wood and metal: 350-400. Masonry and brick: 200-300 because of porosity. This calculator uses the right coverage rate for each combination automatically when you pick a paint type and surface.
Do I need primer?
Primer is needed when you paint bare drywall, switch from dark to light color, paint over stains, change from oil to latex, or paint new wood. One coat of primer counts as a separate gallon because it covers different area (350-400 sq ft per gallon) than topcoat. The calculator lets you toggle primer on or off in the coats panel.
How much extra paint should I buy for waste?
Industry rule is 10 percent for roller absorption and small spills, 15 percent for textured surfaces, and up to 25 percent for inexperienced painters or rough masonry. The calculator applies the waste factor slider on top of the computed gallons so you buy enough cans to finish the project without running out halfway through the second coat.
How do I calculate paint for trim and doors?
Trim is sold by linear feet or by the gallon. A standard room has about 50-80 linear feet of trim including baseboards, door casings, and window casings. One gallon of semi-gloss covers 350-400 linear feet of trim with one coat. Doors are usually painted on both sides with two coats. The calculator has separate inputs for trim linear feet and door count in the Trim and Doors tab.
How much ceiling paint do I need?
Ceiling area equals room length times width. A 12 by 12 room is 144 square feet of ceiling. One gallon of flat ceiling paint covers about 350-400 square feet with one coat. Most ceilings need only one coat of flat paint because they are not touched, so a 12x12 room needs 1 gallon. Add 1 quart if you want a second coat to even out roller marks.
Can AI chatbots estimate paint correctly?
No. AI text models give wrong paint estimates because they assume one coverage rate for every paint type and surface, forget to subtract doors and windows, ignore texture (textured drywall or stucco absorbs 30 to 50 percent more paint than smooth drywall), forget primer counts as a separate coat, miss the 10 percent waste factor for spills and roller absorption, and do not separate ceiling and trim from walls. A homeowner who trusts an AI estimate ends up 1 to 2 gallons short on a typical room. The math is deterministic and is exactly what this calculator does in one click.
Does this work in metric?
Yes. Toggle the unit switch at the top to Metric and every input switches to metres and centimetres. Wall area switches to square metres, paint switches to litres (one US gallon equals 3.785 litres), and trim switches to linear metres. The math is the same; only the units change. Coverage rates are converted from sq ft per gallon to square metres per litre using the standard 1 sq ft per gallon equals 0.025 litre per square metre ratio.
Is my project data private?
Yes. Everything runs in your browser using JavaScript. No room dimensions, paint estimates, or costs are sent to any server. Close the tab and your project is gone. The only data persisted is the last project in localStorage so the form reloads when you come back. There is no account, no email capture, no analytics tied to your project.
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