Paint Calculator: How Much Paint Do I Need?

Calculating how much paint you need for a painting project can seem daunting, but with some simple math and planning, you can determine the exact amount of paint required. An accurate paint calculation helps minimize waste, saves money on unnecessary paint purchases, and ensures you buy enough to complete the job.


Determining the right amount of paint for any painting project is critical. Buying too much paint leads to waste and unnecessary expense. Not purchasing enough can lead to having mismatched colors as you try to finish the job with another batch of paint. Using a paint calculator is the best way to get an accurate estimate and ensure you have the precise quantity needed.

Some key factors go into determining paint needs:

  • Total wall area – Measure the total square footage of surfaces requiring paint. This is the starting point for your calculations.
  • Paint type – The type of paint, such as flat, eggshell, etc. impacts coverage rates.
  • Paint coat coverage – Most paints require two coats for full coverage and protection. Primer and extra coats add more paint.
  • Material type – Are you painting sheetrock, brick, wood, or other surfaces? Each has different porosities requiring more or less paint.
  • Material condition – Paint adheres differently on new vs previously painted surfaces. Chipped or damaged areas may need extra paint.

Once you know these specifics, you can accurately calculate paint quantities and get the right amount of paint the first time.

How to Calculate How Much Paint You Need

Follow these steps to precisely calculate paint needs for any painting project:

Step 1: Measure Room Dimensions

The starting point is measuring the length and width of each wall to find the square footage area:

  • Use a tape measure to get the length and width in feet of each wall section.
  • Multiply length x width for the square footage of each wall section.
  • Add up the square footage of each wall section for the total area.

Be sure to include doors, windows, and any specialty areas like trim or crown molding in your measurements.

Step 2: Subtract Door and Window Areas

To get an accurate wall area total, subtract the square footage of doors, windows, or any areas you won’t paint:

  • Measure height x width of each door and window for square footage.
  • Subtract each door and window square footage from the total wall area square footage.

This gives you the net wall area requiring paint.

Step 3: Calculate Paint Quantity Needed

Using the net wall area total, calculate gallons of paint needed:

  • Check the paint can label for paint coverage in square feet per gallon.
  • Divide net wall area by the paint coverage rate per gallon.

This gives you the estimated minimum gallons of paint needed, assuming one coat.

Step 4: Factor in Extra Coats or Primer

Most painting projects require two topcoats of paint, plus primer on new surfaces:

  • Multiply paint gallons needed by two if applying two finish coats.
  • Add one gallon or more if priming new drywall or wood.
  • Add more paint for heavily chipped or damaged existing surfaces.

This ensures you purchase enough paint to complete all required coats.

Step 5: Factor in Material Type and Texture

Some surfaces require more paint for full coverage:

  • Porous new wood may take two primer coats and two finish coats.
  • Textured surfaces like stucco require more paint than flat drywall.
  • Previously painted glossy surfaces may need extra primer and paint.

Add more paint for multiple coats on highly porous, textured or slick surfaces.

Step 6: Factor in a 10% Repaint Margin

It’s smart to add 10% more paint than your total estimate:

  • This provides extra for touch ups, edging and cut ins.
  • You can return unopened cans but not custom mixed colors.
  • Repainting walls years later is nearly impossible with custom colors.

Adding a 10% overage provides a buffer for repainting rooms later on.

Follow these steps to determine exactly how much paint is required using room dimensions, surface porosity, necessary coats, and repaint margins.

Paint Calculator Formulas

There are two basic formulas for determining gallons of paint needed:

Total square feet method:

  • Total wall square footage
  • Minus door/window square footage
  • Divided by paint coverage rate per gallon
  • Multiplied by number of coats needed

Individual wall section method:

  • Length x Width of Wall Section 1 = Square Feet
  • Repeat for each wall section
  • Add all square footages
  • Minus door/window square footage
  • Divided by paint coverage rate
  • Multiplied by number of coats needed

Use whichever formula is easiest based on your individual room and project measurements.

Both formulas account for total wall area, windows/doors, coverage rate, and coats needed to give you an accurate total gallons required.

Paint Coverage Rates Per Gallon

Paint coverage rates per gallon vary by paint type and brand. Check the label or spec sheet for exact coverage of the paint you are using.

Here are some typical coverage rates by paint type:

  • Flat paint – 400-450 sq ft per gallon
  • Eggshell paint – 400-450 sq ft per gallon
  • Satin paint – 350-400 sq ft per gallon
  • Semi-Gloss paint – 300-350 sq ft per gallon
  • Gloss paint – 250-300 sq ft per gallon
  • Primer – 300-400 sq ft per gallon
  • Stain blocking primer – 250-350 sq ft per gallon

Higher sheens provide good durability but offer lower coverage rates. Primer also covers less than paint. Verify exact rates for the specific products used on your project.

Factors Impacting Paint Quantity Needed

Several factors affect how much paint your particular project requires:

  • Wall height – Tall walls require more paint than shorter walls of the same length and width.
  • Surface porosity – New drywall is very porous requiring extra primer and paint.
  • Surface texture – Popcorn ceilings, knocked down textures, or stucco need more paint to fully coat peaks and valleys.
  • Previously painted – New unpainted surfaces absorb more paint than existing painted walls.
  • Paint color – Darker colors may require an extra coat for full coverage over light wall colors.
  • Repainting – Areas with chipped or damaged existing paint need extra coats of new paint.

Consider each of these factors when estimating your total paint needs to ensure complete coverage.

Ordering Tips for Paint

Follow these tips when purchasing paint for your project:

  • Buy all paint at the same time for each room. Paint can vary between batches.
  • Opt for quarts rather than gallons when possible to keep paint fresh if not using it all quickly.
  • Purchase the same brand and sheen for all coats including primer and finish coats.
  • Buy extra for future touch ups. Custom mixed colors cannot be exactly matched later.
  • Save leftover paint for future color matches or repairs. Store where it won’t freeze.
  • Order extra primer as well as finish paint to have on hand for touch ups or changes.

Planning ahead helps ensure you have the right quantity of all necessary paints for a smooth from start to finish.

Common Questions About Calculating Paint Needs

Here are answers to some frequently asked questions about determining paint requirements:

How do I calculate paint needed for interior walls?

For interior walls, measure the length and width of each wall to get square feet of surface area. Subtract door and window areas, multiply by two coats, and divide by the paint coverage rate per gallon for total gallons needed.

How do I calculate paint needed for trim and molding?

Measure the linear feet of all trim pieces including baseboards, crown molding, chair rails, window and door trim. Multiply linear feet by width of trim, multiply by two coats needed, divide by coverage rate to get gallons of trim paint needed.

How do I calculate paint for ceilings?

Measure length x width of ceiling to get total square feet. Subtract fixture square feet. Multiply total square feet by two coats and divide by coverage rate per gallon for ceiling paint needed. Consider ceiling height and texture which may require more paint.

Can I return unused paint?

Most retailers allow unopened cans of paint to be returned. However, custom mixed colors cannot be returned. Save extra for future touch ups and repairs.

Should I use a paint calculator or estimate by rooms/walls?

For accuracy, use a calculator and exact measurements vs rough room estimates. Estimating by rooms can result in too little or wasted paint. Precise calculations eliminate guesswork and minimize errors.

How much extra should I buy to allow for touch ups?

Add approximately 10% more paint than your total calculation to allow for future touch ups, edging, and cut ins needed after the initial painting is complete.

What tools do I need to calculate how much paint I need?

Gather a tape measure for room dimensions, pencil/pad for notes, access to wall square footage formulas, and paint can labels for coverage rates per gallon. These basic tools allow you to precisely calculate paint needs.

Carefully calculating your exact paint needs using room dimensions, surface types, necessary coats, and proper tools can help guarantee you purchase the right amount of paint for any project.

Paint Calculator Tips and Resources

Here are some additional tips and helpful resources for determining paint requirements:

  • Use a paint calculator tool to do the math for you if preferred over manual calculations.
  • Check out online guides from Home Depot and Lowe’s for estimating paint quantities.
  • For DIY guides and project calculators, visit GoPuffer and House Painting Guide.
  • Buy quality brushes, rollers, trays, and painters tape to apply paint smoothly and neatly.
  • Move furniture to the center of rooms and cover floors with drop cloths when painting walls and trim.
  • Follow paint can directions for proper drying times between coats for the best finish.

Precisely calculating your paint ensures you get the right amount the first time for any project. Follow the tips above to make your painting job easy, efficient, and long-lasting.


Determining the exact quantity of paint for your particular project takes a bit of pre-planning and simple math. Following the step-by-step process outlined above will help you calculate how much paint is needed based on room dimensions, surface type, necessary coats, and other job-specific factors. The small time investment to accurately estimate paint requirements can prevent shortages or costly overages, saving you both money and extra trips to the store. Approach your next painting project with confidence by taking the time to precisely calculate how much paint you need and get the results you want.