Quick answer: A paint calculator estimates paint from the surface area you plan to coat, the coverage rate of the exact paint, and the number of coats you expect to apply. Measure the walls you are actually painting, subtract large openings when it makes sense, enter the product’s listed coverage, and keep ceilings, trim, doors, and primer separate when they use different products.
Use Cardinal Decor’s free Paint Calculator to estimate paint quantity, save or print the result, and build a project shopping list.
| Calculator Input | What to Use |
|---|---|
| Wall size | Actual width × height for each wall or room |
| Doors/windows | Use actual opening sizes when they materially affect the result |
| Coverage | Use the label or technical data for the exact paint |
| Coats | Enter the number your project and product require |
| Ceiling | Calculate separately when it uses a different paint |
| Trim/doors | Estimate separately when the product or sheen differs |
What a Paint Calculator Actually Calculates
The basic math is:
Paintable area × coats ÷ coverage per gallon = estimated gallons.
Sherwin-Williams describes the same basic approach: calculate the square footage to be coated, determine the coverage of the intended product, then divide the project area by that coverage. It also notes that theoretical coverage can differ from real use because of surface texture, application method, and paint left in tools. See Sherwin-Williams’ current calculation guidance.
That makes a calculator a planning tool, not a guarantee.
Step 1: Measure the Walls You Will Paint
For each wall, multiply width by height. Add the wall areas together.
Example:
- Wall A: 12 ft × 8 ft = 96 sq ft
- Wall B: 10 ft × 8 ft = 80 sq ft
- Wall C: 12 ft × 8 ft = 96 sq ft
- Wall D: 10 ft × 8 ft = 80 sq ft
- Total wall area: 352 sq ft
For vaulted ceilings, stairwells, sloped walls, alcoves, or irregular rooms, break the surface into smaller rectangles or triangles and add the areas.
Step 2: Subtract Openings Only When It Helps
Do not automatically subtract “20 square feet per door” and “15 square feet per window.” Those are rough shortcuts, not measurements.
If a room has one ordinary window and one door, leaving them in the calculation may provide a useful cushion. If a wall is mostly glass, cabinetry, built-ins, or large openings, measure those areas and subtract them.
The goal is a realistic paintable area, not false precision.
Step 3: Use the Coverage Rate for the Exact Paint
Sherwin-Williams says a gallon of paint typically covers about 350–400 square feet, while also warning that actual requirements vary with surface condition and application method. See its paint-calculator FAQ.
BEHR currently lists different coverage ranges across its products, including ranges such as 250–400 square feet per gallon for some paints and up to 400 square feet per gallon for others. See BEHR’s current calculator and product coverage.
Use the label or product data sheet for the paint you are buying. Do not choose a generic “budget,” “standard,” or “premium” coverage number.
For a deeper explanation, see Paint Coverage Per Gallon.
Step 4: Enter the Number of Coats Your Project Needs
Two coats are common, but they are not a universal rule.
The number of coats can change with:
- the paint product
- the old and new colors
- surface porosity
- repairs and patches
- primer use
- application method
- manufacturer instructions
Some products make one-coat-hide claims only for selected colors and conditions. Others recommend two coats for durability or appearance. Follow the exact product instructions.
Step 5: Keep Primer Separate
Primer is not simply “another coat of wall paint.” It may have a different coverage rate and is used for different reasons.
Primer may be appropriate for:
- new or repaired drywall
- porous patches
- stain blocking
- major color changes
- special substrates
- adhesion problems
Sherwin-Williams notes that primer coverage can be lower than finish-paint coverage. Check the exact primer data instead of assuming one gallon covers the same area as your wall paint.
Step 6: Calculate Ceilings Separately
A ceiling is usually width × length.
If the ceiling uses a separate product or sheen, keep it as its own calculation. That matters for projects such as cloud capping and color drenching, where the ceiling may or may not use the same paint as the walls.
Step 7: Calculate Trim and Doors Separately
Trim and doors often use a different sheen or coating than walls.
Keep them separate if you need a useful shopping list. A wall-paint estimate does not tell you how much trim enamel to buy.
Sherwin-Williams’ custom calculator separates walls, baseboards, ceilings, and crown molding for this reason. See Sherwin-Williams’ paint calculator.
What About Textured Walls?
Textured or porous surfaces can use more paint, but there is no universal “add 20%” or “add 30%” rule that works for every surface.
Sherwin-Williams specifically notes that substrate texture and application losses affect real coverage. Use the manufacturer’s guidance, your surface condition, and a reasonable planning cushion rather than one fixed percentage. See Sherwin-Williams’ painting FAQ.
Should You Automatically Buy 10–15% Extra?
No fixed extra percentage fits every paint project.
You may want extra paint for:
- future touch-ups
- porous surfaces
- heavy texture
- deep color changes
- complex rooms
- maintaining one tint batch
BEHR’s current calculator, for example, offers an optional 5% touch-up setting rather than treating one overage percentage as mandatory. See BEHR’s paint calculator.
Choose the cushion based on the project and package sizes available.
Paint Calculator Example
Suppose the paintable wall area is 700 square feet and your paint lists 350 square feet per gallon.
One coat: 700 ÷ 350 = 2 gallons.
Two coats: 700 × 2 ÷ 350 = 4 gallons.
That is a planning estimate. If the wall is rough, highly porous, patched extensively, or the application method changes real coverage, actual use may be higher.
Common Paint Calculator Mistakes
Using Floor Area Instead of Wall Area
A 12 × 12 room has 144 square feet of floor area, but its four walls cover much more surface area. Use wall dimensions for wall paint.
Using a Generic Coverage Number Instead of the Label
Coverage differs by product. Use the technical information for the paint you plan to buy.
Counting Every Project as Two Coats
Coat count depends on the product, color, substrate, primer, and manufacturer instructions.
Mixing Walls, Ceiling, and Trim Into One Number
Separate them when the products or sheens differ.
Subtracting Openings With Fake Standard Sizes
Measure large openings when accuracy matters.
Forgetting Built-Ins and Cabinetry
Large built-ins can remove a meaningful amount of wall area.
Relying on the Calculator Instead of Checking the Can
The label and technical data for the actual product should control the coverage assumption.
How Cardinal Decor’s Paint Calculator Fits the Project
Use the calculator to estimate the material plan, then keep the result with the rest of the project.
- Open the Paint Calculator
- Check Paint Coverage Per Gallon
- Browse Painting Resources
- Prep Walls for Painting
- Choose Painting Tools
- Compare Paint Types and Finishes
- Visit the Cardinal Decor Project Shop
Paint Calculator FAQs
How many square feet does a gallon of paint cover?
Sherwin-Williams says a gallon typically covers about 350–400 square feet, but the exact product, surface, and application can change real coverage. Check the label or technical data for the paint you are buying.
Should I subtract doors and windows?
Subtract large openings when they materially affect the estimate. For a simple room with a small number of normal openings, leaving them in can provide a small planning cushion.
How many coats should I enter?
Use the number of coats expected for the exact product, old/new color, surface, and project. Two coats are common but not universal.
Do textured walls need more paint?
Often, yes. Texture and porosity can lower real coverage, but the amount varies. Do not rely on one fixed percentage.
Can I use the calculator for ceilings?
Yes. Calculate ceiling area separately when it uses a different product or sheen.
Can I use the same estimate for exterior siding?
Use an exterior calculator or product-specific method because siding profile, surface condition, openings, application method, and exterior-product coverage can differ from interior walls.
How accurate is a paint calculator?
It is an estimate based on the inputs. BEHR explicitly states that calculator results are estimates and should be checked against measurements and assumptions. See BEHR’s calculator disclaimer.