How do you calculate how many gallons of paint you need?

When getting ready to paint a room or house, one of the most important things to determine is how much paint you will need to buy. Buying too much paint is wasteful and expensive, while not buying enough can result in having to make multiple trips to the store to get more. So it’s crucial to accurately calculate the amount of paint needed for your particular painting project.

What factors determine how much paint is needed?

There are several key factors that determine how many gallons of paint will be required:

  • Size of the area – The total wall space that will be painted (length x height of each wall)
  • Type of paint – Light-colored paint may require 2 coats, dark colors could cover in 1
  • Desired coverage – Touching up existing paint vs. painting bare drywall require different amounts
  • Surface type – Rough surfaces like stucco require more paint than smooth drywall
  • Waste/spills – Adding 5-10% extra helps account for unavoidable mishaps while painting

By carefully measuring the space and accounting for these variables, you can use a simple mathematical formula to calculate the appropriate amount of paint for your needs.

Calculating the Square Footage

The starting point for estimating paint needs is calculating the total square footage of the surfaces to be painted. This means adding together the areas of the walls, ceilings, doors, windows, trim and any other surfaces that will get a fresh coat of paint.

For a basic rectangular room, you can simply multiply the length by the height for each wall, then add those numbers together. So for a 10 x 12 bedroom with an 8 foot ceiling:

  • Wall 1 (length 10 ft x height 8 ft) = 80 sq ft
  • Wall 2 (length 12 ft x height 8 ft) = 96 sq ft
  • Wall 3 (length 10 ft x height 8 ft) = 80 sq ft
  • Wall 4 (length 12 ft x height 8 ft) = 96 sq ft
  • Total = 80 + 96 + 80 + 96 = 352 sq ft

For more complex spaces, sketching a simple floor plan and breaking the walls into sections makes it easier to get accurate measurements.

Don’t forget to also factor in the square footage of ceilings, doors, windows and any trim or molding. Measure these separately then add to your wall square footage total.

Estimate Your Paint Coverage

Once you know the total square footage you’ll be painting, you need to estimate how much paint is needed to cover that space. Paint coverage is measured in square feet per gallon (sq ft/gal).

The two main factors that determine coverage are:

  1. Paint Type – Light matte interior paints require two coats and cover around 400 sq ft per gallon. Darker or glossier paints may only cover around 200-300 sq ft per gallon.
  2. Painting Situation – Touching up existing paint requires less paint, around 300-400 sq ft/gal. Painting bare drywall or a radically different color requires more, upwards of 350-400 sq ft per gallon.

Check the manufacturer’s recommendation, but for most do-it-yourself interior painting projects, you can estimate needing one gallon of light paint for every 350-400 square feet. Darker paints may need a gallon for every 200-300 sq ft. Use the lower end of the range for painting bare surfaces.

Do the Math

Now for the fun part – the actual math to compute the gallons of paint needed. Take your total square footage amount and divide by the paint coverage estimate:

Total square feet / Paint coverage (sq ft per gallon) = Gallons needed

Using the example room above with 352 square feet of walls and ceiling to be painted in a light color:

352 sq ft / 400 sq ft per gallon = 0.88 gallons

Round up to account for waste or spills. So for this scenario, you would want to purchase 1 gallon of paint.

Add Extra for Primer, Ceilings and Touch-Ups

When purchasing paint for an entire room or house, it’s smart to add a little extra beyond your measurements and math. This provides some wiggle room for mistakes in your calculations or uneven application.

Some additional paint to account for includes:

  • Primer/Sealer – Often takes more paint than a wall would normally. Add 10-25% more for primed surfaces.
  • Ceilings – Require 20-30% more paint than walls due to height and spraying.
  • Touch-ups – Having leftover paint makes future touch-ups easier. Get up to 1 extra gallon.

For a 350 square foot room, you might buy 2 gallons instead of 1 to have plenty of extra paint. But avoid purchasing too much extra – only get what you expect to reasonably use.

Special Considerations for Exterior Painting

When calculating paint needs for exterior surfaces, you’ll need to account for a few additional factors:

  • Multi-story homes often need more paint for upper surfaces
  • Texture affects coverage, stucco needs more paint than wood or vinyl
  • Darker colors show imperfections – use 100-200 sq ft/gal estimate
  • Add extra for trim, doors, window frames, railings and any other exterior surfaces

A good rule of thumb when estimating for outdoor painting is to figure around 100-150 sq ft of coverage per gallon of paint depending on your specific situation.


Figuring out paint needs for a painting project doesn’t have to be a guessing game. Follow this structured process:

  1. Calculate total wall square footage
  2. Estimate paint coverage needed
  3. Divide square footage by coverage to get gallons required
  4. Round up for waste and add extra for primer, ceilings and touch-ups

Carefully measuring the space and using a paint gallons calculator makes it easy to determine exactly how much paint to buy with minimal waste and extra trips to the store. With the right amount of paint from the start, you can ensure your painting project goes smoothly and efficiently.

Room Square Footage Paint Needed (sq ft/gal) Gallons Needed
Bedroom 352 400 1
Bathroom 150 400 1
Kitchen 250 400 1
Total 752 3

As shown in this example, for a 752 square foot interior painting project using a light paint requiring 400 sq ft per gallon coverage, you would need approximately 3 gallons of paint. Round up to 4 gallons to have plenty of extra paint for priming, ceilings, and touch-ups.

Frequently Asked Questions

Does the brand of paint make a difference in coverage?

Yes, paint brands can have an impact on coverage. Less expensive brands may require an additional coat and therefore more paint. Higher quality paints tend to go further. Always check the manufacturer’s recommendation for square foot coverage.

How much extra should I buy to account for imperfections and touch-ups?

It’s smart to add at least 10-20% more paint than your measurements suggest to account for uneven application, drips, spills and future touch-ups. 1 extra gallon for a small room and 2-3 extra for a whole house.

Should I use the same brand paint for future touch-ups?

Yes, it’s best to stick with the same paint brand and type for any future paint touch-ups. Paints can vary in sheen, quality and color, so your touch-ups may not match with a different brand.

Can I return unused paint to the store?

Possibly. Some paint stores allow returns of unopened cans of paint within 30-90 days. Make sure you check your retailer’s return policy before purchasing.

How do I calculate exterior painting needs?

For exterior surfaces, measure the house perimeter, surfaces, doors, windows, etc. and multiply by the surface height to get square footage. Use 100-150 sq ft per gallon as an estimate for exterior paint coverage.

How often should exterior paint be refreshed?

On average, exterior paints should be reapplied every 3-5 years. Harsher climates or inferior quality paint may need more frequent refreshes. Inspect paint annually and look for fading, cracks or peeling.

Should I use a paint calculator app or online calculator?

Apps and online calculators can provide quick estimates, but are not as accurate as measuring and doing the math yourself. Use them for a ballpark figure then adjust as needed for your specific situation.

How do I calculate paint for oddly shaped rooms or high ceilings?

For irregular room shapes, sketch the room to scale on graph paper and break up the walls into smaller sections for easier measuring. Add 20-30% more paint for ceilings over 9 feet tall which require painting with extension tools.

Should I paint the walls and ceiling the same color?

Painting the walls and ceilings different colors can help visually define the spaces. But use similar sheens to avoid seeing cut-lines between the wall and ceiling. Matte or eggshell paint is ideal for ceilings and walls.

Key Takeaways

  • Carefully measure all wall, ceiling and surface areas to find total square footage
  • Estimate paint coverage based on paint type and painting situation
  • Divide square footage by estimated coverage to get gallons needed
  • Add extra for primer, ceilings, imperfections and touch-ups
  • Use 100-150 sq ft/gal estimate for exterior painting

With some simple math and advance planning, you can determine the precise amount of paint for any painting project. Avoid underbuying or wasting money on excess paint by following these tips and techniques for accurately calculating gallons needed. Investing the time upfront to measure and estimate ensures your paint job will be completed efficiently, affordably and hassle-free.

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