Paint Cost Calculator

Background.

What does it cost to repaint a room yourself? For a 20 by 20 foot room with 8-foot walls, a door and four windows, two coats of a $42 paint over one coat of $25 primer, plus $65 of rollers, tape and drop cloths, the answer is $283. That figure is built from a short chain of steps, and seeing the chain is more useful than the total because it shows where the money goes and which assumption to question.

Paint is sold by the can, so the calculator first works out how many whole gallons you have to carry home, then prices those. In the example the walls need 3.52 gallons of finish paint once two coats and a 10% allowance are counted; you buy 4. Whatever is left in the last can is the price of buying whole cans, and it becomes the touch-up supply for the next few years.

Materials are usually the small part of a professional job. Add twelve hours of a painter's time at $45 an hour and the same room comes to $823, of which labour is about two-thirds. Contractors price prep, patching, masking, moving furniture, cutting in along ceilings, cleanup, travel and overhead, none of which a gallon count can see. So the most practical use of the calculator is comparison: DIY against a quote, one coat against two, a cheaper paint that covers less against a dearer one that covers more.

Age of the building can change the job entirely. In homes, child-care facilities and kindergartens built before 1978, US EPA rules require paid work that disturbs paint which may contain lead to be done by certified firms using lead-safe practices, and OSHA rules protect workers exposed to lead. Testing before sanding or scraping old paint costs far less than the clean-up after.

What is paint cost calculator?

A painting estimate converts a surface area into the quantities and money you need. Net paintable area is the wall, ceiling or other surface minus doors, windows and anything else that will not be painted. A coat is one complete application over that area, so two coats need twice the coverage. Coverage rate is the area one gallon covers in one coat, as printed on the can; it falls on rough, porous or textured surfaces. Primer is a separate preparatory coating that seals and evens out the surface so the finish paint dries to a uniform colour and sheen. It is usually needed on new drywall, bare wood, patched or stained areas, glossy surfaces and big colour changes, and often skipped when repainting a sound wall in a similar colour. Sheen matters to the budget too: flat finishes hide wall imperfections, eggshell, satin and semi-gloss stand up better to washing, and prices can differ between sheens of the same paint. The estimate here is material and optional labour in one currency of your choice; it is not a contractor's bid.

How to use this calculator.

1. 01Measure the walls: perimeter × height for a room, so a 20 × 20 ft room with 8 ft walls is 80 × 8 = 640 sq ft. Add the ceiling (length × width) as a separate estimate if it takes a different paint.
2. 02Enter the area of doors, windows and other unpainted openings. A 36 × 80 in door is 20 sq ft and a 3 × 5 ft window 15 sq ft; the openings must total less than the paintable area.
3. 03Set the finish coats, usually two for an even colour, and the coverage per gallon from the can's label. Lower the coverage for textured, rough or very porous surfaces. A strong change of colour, such as a deep red over white or white over navy, may need a third coat or a tinted primer.
4. 04Enter a waste allowance for roller loss, spills and touch-ups, and the price per gallon. For primer, enter its coats, coverage and price, or 0 coats if the walls do not need it.
5. 05Add supplies as one lump sum. Leave labour hours and rate at 0 for a DIY job, or enter a painter's quoted hours and hourly rate to see what share of the total is labour.

The formula.

Net area N = A − O, the paintable area less the openings. Finish paint: gallons = ⌈N × c × (1 + w) ÷ R⌉, where c is the number of finish coats, w the waste allowance as a fraction and R the coverage per gallon; the brackets mean round up to the next whole gallon. Primer: gallons = ⌈N × p ÷ Rₚ⌉, with p primer coats and Rₚ primer coverage; no waste allowance is added to primer, so if you want a margin there, enter a slightly lower primer coverage than the label states. Each cost is whole gallons × price per gallon. Labour is hours × hourly rate. Total = paint + primer + supplies + labour, and cost per square foot = total ÷ N, so it is per square foot of wall, not per square foot per coat. Because both gallon counts round up to whole cans, small changes in the inputs often make no difference: in the example the finish paint comes to 4 gallons with no waste allowance at all (1,120 ÷ 350 = 3.2) or with coverage raised to 400 sq ft per gallon, but dropping coverage to 300 for a textured wall makes it 5. The coats input has the largest effect, since it multiplies the whole finish area. The model assumes every coat covers the same area at the label rate and that paint is bought only in gallons; it does not choose between quarts, gallons and five-gallon pails, and it includes no tax, delivery or equipment hire unless you add them to supplies.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Environmental Protection Agency (2025). "Lead Renovation, Repair and Painting Program Rules." EPA.
2. [2]U.S. Environmental Protection Agency (2024). "Steps to Lead Safe Renovation, Repair and Painting." EPA.
3. [3]Occupational Safety and Health Administration (n.d.). "Lead." OSHA.
4. [4]National Institute of Standards and Technology (2026). "NIST Handbook 133, Appendix E: General Tables of Units of Measurement." U.S. Department of Commerce.
5. [5]Bureau International des Poids et Mesures (2019). "The International System of Units (SI Brochure), 9th edition." BIPM.