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Paint Calculator

Estimate paint volume, primer required, number of paint cans, and total project costs for interior and exterior walls, ceilings, and custom surfaces.

CalculatorHow to CalculateFormulas & ExamplesFAQs

Select surface type, shape, dimensions, coats, primer, and container specifications.

Interior Wall: Estimates paint required for indoor wall surfaces. Excludes openings like doors and windows.

Surface Dimensions

Subtract Openings (Optional)

Subtracts doors & windows from painting area

Paint Specifications

ft²/gal
10%

Paint Can Sizing

Optional Material Cost

Calculation Results

Awaiting Calculations

Fill in the surface type, room/wall shape, dimensions, paint coverage, and optional pricing, then click Calculate.

Formula Details

Gross Surface Area Formula
Depends on selected Surface Type and Shape:

Rectangle Surface: Area = Width × Height Square Surface: Area = Side × Side Rectangle Room: Area = 2 × (Length + Width) × Height [walls] + (Length × Width) [ceiling] Square Room: Area = 4 × Side × Height [walls] + (Side × Side) [ceiling]

Variables Explained
  • Width / Height
    Surface Dimensions:Width and height of walls or ceiling surfaces.
  • Length / Side / Height
    Room Dimensions:Room lengths, widths, and wall heights.
Opening Exclusion Formula
Opening Area = (Doors × Door Area) + (Windows × Window Area)

Calculates the combined surface area of doors and windows to subtract from the paintable area. Standard sizes: Door is 21 sq ft (2.0 m²) and Window is 15 sq ft (1.4 m²).

Variables Explained
  • Door Area
    Standard Door Size:Typical door covers 21 sq ft or 2.0 sq m.
  • Window Area
    Standard Window Size:Typical window covers 15 sq ft or 1.4 sq m.
Net Paintable Area Formula
Net Area = Gross Surface Area - Opening Area

Represents the actual wall or ceiling area that will physically receive paint. Opening area is subtracted because you paint around windows and doors.

Variables Explained
  • Gross Area
    Total Raw Area:Calculated surface area before subtracting openings.
  • Opening Area
    Exclusion Area:The surface area covered by doors and windows.
Paint Quantity Formula
Paint Required = (Net Area × Coats × Waste Factor) / Paint Coverage Rate

Determines the total paint volume required in Liters or Gallons. We multiply the net paintable area by the number of paint coats and apply the waste factor (e.g., 1.10 for 10% waste), then divide by the coverage rate.

Variables Explained
  • Coats
    Number of Paint Layers:Usually 2 coats are recommended for deep, durable color.
  • Paint Coverage Rate
    Spreading Rate:Coverage rate of the paint (std. 350 sq ft/gal or 10 m²/L).
  • Waste Factor
    Waste Buffer Multiplier:1 + (Waste Percentage / 100). Default is 1.10 (10% safety margin).
Paint Can Quantity Formula
Paint Cans Required = ⌈Paint Required / Can Package Size⌉

Calculates the whole number of cans needed to purchase. Since paint is sold in full cans, we divide total paint required by the chosen package size (e.g. 1 Gal, 5 Gal, 4L, 10L) and round up to the next integer.

Variables Explained
  • Can Package Size
    Can Volume:Selected paint container size (metric or imperial presets, or custom).
Material Cost Formula
Estimated Paint Cost = Paint Cans Required × Price Per Can

Estimates the total paint container material expense if a single can price is provided. Excludes primer cost, paint rollers, brushes, tape, and installation labor.

Variables Explained
  • Price Per Can
    Container Cost:Retail price of one paint container.

Calculation Example

Worked Example
Scenario

Bedroom: Calculate interior wall and ceiling paint requirements for a rectangular room measuring 15 feet long by 12 feet wide with an 8-foot ceiling height. You want to apply 2 coats of paint with a 10% waste factor. The room has 1 door and 2 windows. You will use standard 1 Gallon cans costing $45.00 each. Coverage rate is 350 sq ft/gal.

Calculation Steps
  1. 1.Calculate Gross Surface Area (walls + ceiling).
    Wall Area = 2 × (15 + 12) × 8 = 432 sq ft; Ceiling Area = 15 × 12 = 180 sq ft; Gross Area = 432 + 180 = 612 sq ft
  2. 2.Calculate Opening Exclusion Area.
    Opening Area = (1 Door × 21 sq ft) + (2 Windows × 15 sq ft) = 21 + 30 = 51 sq ft
  3. 3.Calculate Net Paintable Area.
    Net Area = 612 sq ft - 51 sq ft = 561 sq ft
  4. 4.Calculate Paint Volume Required (2 coats + 10% waste).
    Paint Required = (561 sq ft × 2 coats × 1.10) / 350 sq ft/gal = 1234.2 / 350 = 3.53 Gallons
  5. 5.Calculate paint cans to purchase (round up).
    Cans Required = ⌈3.53 / 1.0 Gallon⌉ = 4 Cans
  6. 6.Calculate Estimated Paint Cost.
    Cost = 4 Cans × $45.00 = $180.00
Final Quantity Needed4 Gallon cans required, with an estimated paint cost of $180.00.
Worked Example
Scenario

Living Room Walls: Calculate paint requirements for a square room with walls measuring 12 feet on each side and an 8-foot ceiling height (ceiling is not painted). You want to apply 2 coats of paint. There is 1 door, 1 window, and you apply a 10% waste buffer. You select 1 Quart cans priced at $15.00 each.

Calculation Steps
  1. 1.Calculate Gross Surface Area (walls only).
    Wall Area = 4 × 12 × 8 = 384 sq ft
  2. 2.Calculate Opening Exclusion Area.
    Opening Area = (1 Door × 21 sq ft) + (1 Window × 15 sq ft) = 36 sq ft
  3. 3.Calculate Net Paintable Area.
    Net Area = 384 sq ft - 36 sq ft = 348 sq ft
  4. 4.Calculate Paint Volume Required (2 coats + 10% waste).
    Paint Required = (348 sq ft × 2 coats × 1.10) / 350 sq ft/gal = 765.6 / 350 = 2.19 Gallons
  5. 5.Calculate paint cans needed (1 Quart = 0.25 Gallons).
    Cans Required = ⌈2.19 / 0.25 Gallons⌉ = ⌈8.76⌉ = 9 Cans
  6. 6.Calculate Estimated Paint Cost.
    Cost = 9 Cans × $15.00 = $135.00
Final Quantity Needed9 Quart cans required, with an estimated paint cost of $135.00.
Worked Example
Scenario

Exterior House Wall: Calculate paint and primer requirements for a rectangular exterior house wall measuring 10 meters wide by 3 meters high. You want to apply 2 coats of paint plus 1 coat of primer. You set a 5% waste buffer. The wall has 1 door and 2 windows. You purchase 4 L cans priced at €35.00 each. Coverage rate is 10 m²/L.

Calculation Steps
  1. 1.Calculate Gross Surface Area.
    Gross Area = 10 m × 3 m = 30.00 sq m
  2. 2.Calculate Opening Area (Door = 2.0 m², Window = 1.4 m²).
    Opening Area = (1 × 2.0) + (2 × 1.4) = 4.80 sq m
  3. 3.Calculate Net Paintable Area.
    Net Area = 30.00 sq m - 4.80 sq m = 25.20 sq m
  4. 4.Calculate Paint Required (2 coats + 5% waste).
    Paint Required = (25.20 sq m × 2 coats × 1.05) / 10 m²/L = 52.92 / 10 = 5.29 Liters
  5. 5.Calculate Primer Required (1 coat + 5% waste).
    Primer Required = (25.20 sq m × 1 coat × 1.05) / 10 m²/L = 26.46 / 10 = 2.65 Liters
  6. 6.Calculate paint cans (4 L size).
    Cans Required = ⌈5.29 / 4 Liters⌉ = 2 Cans
  7. 7.Calculate Estimated Cost.
    Cost = 2 Cans × €35.00 = €70.00
Final Quantity Needed2 cans of 4 L (5.29 L paint and 2.65 L primer required) with an estimated cost of €70.00.

How to Use

Estimate paint and primer volumes, whole cans, and project material budgets with these simple steps:

  1. Select the surface type: Choose between Interior Wall, Exterior Wall, Ceiling, Room (All Walls + Ceiling), or Custom Surface.
  2. Choose Room / Wall Shape: Select Rectangle, Square, or Custom Dimensions (direct area entry).
  3. Enter the required dimensions: Input values using your preferred unit system (Imperial or Metric).
  4. Add doors and windows: Enter the counts of openings to automatically subtract them from the paintable area.
  5. Define paint settings: Enter the coverage rate, select the number of coats, and toggle primer requirements.
  6. Choose paint can size: Select a standard container volume preset or provide a custom size.
  7. Optionally input waste percentage and price: Adjust waste allowance sliders and enter price per can to estimate paint material costs.
  8. Click Calculate: Process all inputs to receive results for gross area, excluded openings, net area, paint/primer volumes, and cans required.

Frequently Asked Questions

How much paint do I need?
Typically, one gallon of paint covers about 350 to 400 square feet of smooth wall with a single coat. In metric systems, one liter covers about 10 square meters. To find your paint requirement, divide the total paintable surface area (multiplied by the number of coats) by the paint coverage rate.
How many coats of paint should I apply?
For most projects, two coats of paint are recommended to achieve a smooth, rich, and uniform color. A single coat may look uneven or allow the old paint color to show through, while three or more coats are generally only necessary when painting light colors over extremely dark walls.
Should I subtract doors and windows from my calculations?
Yes, you should subtract the area of doors and windows since you will not paint over glass or wooden door leaves with wall paint. Our calculator automatically handles this. Standard door sizes are 21 sq ft (2.0 m²) and standard windows are 15 sq ft (1.4 m²).
How do I estimate the amount of primer needed?
Primer is estimated as a single coat applied directly to raw drywall, plaster, or unpainted wood before topcoats. Select the primer option in the calculator, and it will compute the volume in Gallons or Liters using the spreading rate.
What paint coverage rate should I use?
The spreading rate depends on wall texture and paint type. Smooth drywall uses about 350–400 sq ft/gal (10 m²/L). Rough plaster or porous brick walls absorb more paint, reducing coverage to 200–250 sq ft/gal (6–7 m²/L). Check the manufacturer label on your paint container.
Why should I include a waste percentage?
A waste percentage (typically 10%) accounts for spills, roller absorption, drip loss, wall texture absorption, and paint left behind inside the tray or container. It ensures you don't run out of paint in the middle of a coat.

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Disclaimer

Calculations are estimates based on the values you enter and standard engineering formulas. Results are intended for planning and estimation only. Always verify critical measurements, specifications, and local building requirements before purchasing materials or beginning construction.