Paint & interior · updated 2026
Paint Calculator
Measure the paintable surface, subtract openings, and divide by the coverage rate times your coats. This paint calculator handles interior rooms, exterior wall area, and ceilings, with a five-room interior takeoff, optional primer, DIY or pro labor, and editable local prices. Coverage defaults are planning assumptions, so verify the paint label before buying.
21 sq ft deduction each — editable assumption in formula
15 sq ft deduction each — editable assumption in formula
TBD — verify the paint label
TBD — enter your local paint price
Show the math
Paintable area = gross surface − openings = 333.0 sq ft
Gallons = 333.0 ÷ 350.0 sq ft/gal × 2 coats = 1.90; buy 2 gallons
Primer = 333.0 ÷ 400.0 = 0 gallons when enabled
Total = paint $0 + labor $0 = $0; cost per sq ft = $0
Interior, exterior, or ceiling
Interior mode adds up to five rooms and deducts standard opening allowances. Exterior mode starts from total wall square footage and subtracts openings. Ceiling mode uses floor area and a flat or textured coverage input. Choose the mode that matches the surface instead of mixing wall and ceiling areas into one rate.
How much paint per room
The calculation is paintable square feet divided by the coverage rate, multiplied by coats, then rounded up to whole gallons. A 12×12 room with 8-ft walls, one door, and two windows has 333 paintable wall square feet; two coats at 350 sq ft per gallon is 1.9 gallons, so two gallons is the purchase quantity.
Larger rooms, tall walls, porosity, and texture all change the result. Use the room repeater for a multi-room project so each opening deduction is visible.
Coverage rates are a starting point
A paint label’s coverage assumes a particular surface and application thickness. Smooth previously painted walls may approach the label; fresh drywall, rough siding, textured ceilings, and dark color changes may require more. Enter the product coverage when it differs from the planning default.
Interior paint is not an exterior coating. Exterior products need to tolerate UV, moisture, and temperature cycling, while ceiling products are formulated for a different sheen and application.
Primer and finish
Primer is useful on bare drywall, repairs, stains, porous surfaces, and dark-to-light color changes. Enable it as a separate one-coat quantity and enter the primer label’s coverage. For finishes, flat hides ceiling imperfections, eggshell or satin is common on many walls, and semi-gloss is useful on trim and high-wear areas.
A single coat can be enough for a same-color refresh on a sound surface. Two coats are the safer plan for a new color or uneven substrate.
DIY versus pro cost
DIY cost depends on paint grade, primer, tools, masking, and prep. Pro cost depends on access, repairs, number of colors, trim, and regional labor. Enter a local labor rate when comparing a quote; this page leaves cost inputs TBD instead of presenting unsupported ranges.
A good quote identifies prep, number of coats, products, protection, cleanup, and whether ceilings and trim are included.
Methodology and assumptions
Interior walls use 2 × (length + width) × height minus 21 sq ft per door and 15 sq ft per window. Exterior mode accepts total wall area minus openings. Ceiling mode uses length × width. Primer is one coat at the entered coverage; paint and labor prices remain editable local inputs.
What the numbers mean
The calculator converts your measurements into a practical purchase quantity using a documented coverage or spacing assumption.
Any whole-unit quantity is rounded up so the result does not leave you short during the project.
Use the cost range as a planning baseline and confirm the final quantity, availability, and delivery charge with your local supplier.
2026 material prices
Ready-mix, bulk material, and labor prices are TBD; enter local prices for a planning estimate. Verify current supplier pricing. 2026 U.S. national averages. Prices vary by region and retailer — confirm locally before buying.
Pro tips
- Two coats are the safe planning number for a color change; one coat can work for a same-color refresh.
- Primer matters on bare drywall, repairs, stains, and strong dark-to-light color changes.
- Interior and exterior coatings are not interchangeable; exterior paint needs resins for UV and moisture exposure.
- Eggshell or satin suits many living-area walls, semi-gloss suits trim, and flat hides ceiling imperfections.
- Buy all gallons together and box them for large jobs so small tint differences do not create visible bands.
- Coverage is a starting point, not a promise; rough, porous, or textured surfaces use more paint.
Common questions
Frequently asked questions
How much paint do I need?
Use paintable square feet ÷ coverage × coats. With the default 350 sq ft/gal interior coverage, a 12×12 room is about 2 gallons for two coats after opening deductions.
How many gallons for a 12×12 room?
The default room has 384 sq ft of gross wall area, less 21 sq ft for one door and 30 sq ft for two windows = 333 sq ft. Two coats at 350 sq ft/gal is 1.9 gallons, so buy 2 gallons.
Do I need primer?
Primer is commonly needed on bare drywall, repairs, stains, and dark-to-light changes. Enable the primer option and enter local coverage and price.
How much paint for the exterior of a house?
Use exterior wall area minus openings, then apply the coverage rate for the siding. The exterior paint calculator child page goes deeper on perimeter and wall-height measurements.
How much paint for a ceiling?
Flat ceilings use a higher planning coverage than textured ceilings. Choose the ceiling mode and enter the label coverage for your product.
Can I use interior paint outside?
No. Interior coatings are not designed for the UV, moisture, and temperature cycling that exterior paint must withstand.
What is the best paint finish for walls?
Eggshell or satin is common for living areas, semi-gloss is useful for trim and high-wear surfaces, and flat is best at hiding ceiling imperfections.
How much does it cost to paint a room?
DIY and professional costs vary by region, paint grade, prep, and labor. Enter local paint, primer, and labor rates above; defaults are intentionally TBD.
One coat or two?
Use two coats for color depth and durability. One can be enough for a same-color refresh on a sound, previously painted surface.
How do I calculate paint for multiple rooms?
Enter additional room dimensions in the repeater fields. The calculator adds each room’s paintable wall area before applying coverage and coats.
Related calculators
Planning estimates only. Prices are 2026 U.S. national averages and vary by region — confirm with your local supplier before buying.