How to Calculate How Much Paint You Need

Calculating how much paint you need for a painting project can seem daunting, but it’s actually quite straightforward with some simple math. Having an accurate estimate of the quantity of paint needed is crucial for budgeting and ensuring you purchase enough paint to complete the job. In this comprehensive guide, we’ll walk you through the complete process of calculating paint quantities step-by-step.

Estimate the Total Wall Surface Area to Be Painted

The starting point for calculating paint needs is measuring the total wall surface area to be painted. Here are the key steps:

Measure Wall Length and Height

Use a tape measure to measure the length and height of each wall section to be painted. Measure to the nearest inch or centimeter. Walls are rarely perfectly rectangular, so take multiple length and height measurements if the wall has alcoves, angles or other irregularities.

For example, if a wall is 20 feet long but has a 5 foot long alcove, you would measure the main wall length as 20 feet and the alcove as 5 feet.

Write down the length and height measurements for each wall section.

Calculate the Surface Area of Each Wall Section

For regular rectangular wall sections, calculate surface area by multiplying the length by the height.

For example:

  • Wall length: 15 ft
  • Wall height: 8 ft
  • Surface area: 15 x 8 = 120 sq ft

For more complex wall shapes, sketch the wall shape on grid paper and calculate the surface area by counting squares.

Total the Surface Areas

Add together the surface areas of each wall section to get the total wall surface area.

For example:

  • Wall 1: 120 sq ft
  • Wall 2: 280 sq ft
  • Wall 3: 160 sq ft
  • Total area: 120 + 280 + 160 = 560 sq ft

This 560 sq ft is the total wall area that needs to be painted.

Determine the Number of Paint Coats Needed

The number of coats of paint required depends on the type of paint, color, and condition of the surface:

  • For new drywall or if painting over a dramatically different color, plan on 2-3 coats.
  • For painting over a similar color in good condition, 1-2 coats is usually sufficient.
  • For specialty paint finishes like gloss, enamel or textured paints, 2-3 coats are best.

Consider the manufacturer’s recommendations on the paint can or technical specifications for accurate coat guidelines.

Generally, it’s better to calculate paint needing extra coats to ensure having adequate coverage.

Choose the Type of Paint Product

The type of paint product determines the spreading rate or coverage offered per gallon:

  • Latex or acrylic paint – Covers approximately 400 sq ft per gallon for new surfaces.
  • Oil-based paint – Offers 350 sq ft coverage per gallon.
  • Primers – Tend to cover 200-400 sq ft per gallon.
  • Textured paint – Covers around 100 sq ft per gallon.

Check manufacturer specifications since the spreading rate can vary by brand. Choosing a quality paint product with good coverage will save money by minimizing the quantity of paint needed.

Determine the Paint Coats Required

Using the square footage, paint type and coats needed, you can now calculate the quantity of paint required:

  • Take the total wall area and divide it by the paint product spreading rate per coat.
  • Then multiply this figure by the number of coats needed.

Using our example:

  • Total area: 560 sq ft
  • Latex paint spreading rate: 400 sq ft/gallon
  • Coats needed: 2
  • So, 560 sq ft divided by 400 sq ft/gallon = 1.4 gallons per coat
  • 1.4 gallons x 2 coats = 2.8 gallons of paint needed

Round up the result to ensure having adequate paint – so for this example, round up to 3 gallons.

Factor in Additional Paint Needs

Some additional factors can increase paint requirements, including:

Priming Walls

If the walls need priming before painting, calculate the primer quantity needed using the steps above based on the manufacturer spreading rate.

For example, an oil-based primer may cover 300 sq ft per gallon. For a 560 sq ft room, you would need 560/300 = 1.9 gallons, so round up to 2 gallons of primer.

Ceilings and Trim

Don’t forget to measure ceiling area and trim if they will be painted and factor in this additional square footage.

Waste from Rollers and Brushes

Experts recommend buying 10-15% extra paint to allow for unavoidable roller and brush loss when painting. For a 3 gallon paint total, that would mean buying an extra 1/3 to 1/2 gallon.

Touch-Ups and Multiple Colors

Get extra paint if using multiple colors or buying the same colors for future touch-ups. Buy 1 extra gallon for each core color.

By carefully measuring surface area, choosing quality paint, and accounting for all of these factors, you can determine an accurate total paint quantity needed for your project.

Paint Calculator Formulas

Here are the basic formulas used:

Total paint needed (gallons) = (Total wall area ÷ Paint coverage per gallon) x Number of coats needed

Primer needed (gallons) = (Total wall area ÷ Primer coverage per gallon)

Total quantity needed = Paint total + Primer total + Extra 10-15% for waste

Let’s look at two more examples:

Example 1:

  • Total wall area: 850 sq ft
  • Latex paint covers 400 sq ft/gallon (2 coats needed)
  • Oil-based primer covers 300 sq ft/gallon
  • Paint: 850 sq ft ÷ 400 sq ft/gal x 2 coats = 4.25 gallons. Round up to 5 gallons.
  • Primer: 850 sq ft ÷ 300 sq ft/gal = 2.8 gallons. Round up to 3 gallons.
  • 5 gallons paint + 3 gallons primer = 8 gallons total
  • Add 10% extra for waste: 0.8 gallons
  • Total to buy: 8.8 gallons, so round up to 9 gallons total

Example 2:

  • Total wall area: 1500 sq ft
  • Specialty textured paint covers 100 sq ft/gallon (2 coats needed)
  • Latex primer covers 400 sq ft/gallon
  • Paint: 1500 sq ft ÷ 100 sq ft/gal x 2 coats = 30 gallons
  • Primer: 1500 sq ft ÷ 400 sq ft/gal = 3.75 gallons. Round up to 4 gallons.
  • 30 gallons paint + 4 gallons primer = 34 gallons
  • Add 15% extra for waste: 5 gallons
  • Total to buy: 34 + 5 = 39 gallons

Tips for Accurate Paint Calculations

Follow these best practices for accurate paint quantity estimates:

  • Use a laser distance measurer tool instead of manually measuring for much faster and more precise surface area calculations.
  • Create a detailed diagram of each wall with measurements when dealing with unusually shaped rooms.
  • Use painters tape to measure irregular walls by running tape along each wall and then measuring the used tape.
  • Measure and calculate wall area in the metric system (square meters) instead of imperial (square feet) to avoid rounding errors.
  • Verify paint coverage rates in both sq ft and m2 since manufacturer specs can vary between units.
  • Read the manufacturer’s application guide for exact spreading rates based on surface material and porosity.
  • Purchase all paint for a single wall or room together to ensure color consistency.
  • Save all paint receipts to reference for future touch-ups.

Paint Coverage Tool

For convenience, you can use paint calculator tools online to estimate the quantity of paint needed for your particular project.

These paint estimation tools simplify the process by allowing you to input the total wall area and product details to auto-generate the recommended quantity to buy.

Be sure to add the extra waste factor after using these calculators.

Here are some popular online paint calculators:

These tools provide a quick and convenient way to estimate the quantity of paints, primers, and coatings needed for your next project.

FAQs About Calculating Paint Quantity

Here are some frequently asked questions about determining how much paint is required:

How much paint is needed per square foot?

On average, 1 gallon of latex paint covers 400 sq ft and oil-based paint covers 350 sq ft. Primer can cover 200-400 sq ft per gallon. Actual coverage depends on paint quality, surface porosity, application method and coat thickness.

How much extra paint should I buy?

Purchase an extra 10-15% beyond the estimated quantity to allow for waste from rollers, brushes and paint trays. Get whole extra gallons for colors being used in multiple rooms.

Should I use a paint coverage calculator?

Online paint calculators are handy for quick estimates, but always round up their results and add extra for waste. Precisely measuring and calculating each wall gives the most accurate quantity estimate.

What measurements are needed?

You need the length, height and total surface area of each wall section being painted. Also note the number of doors, windows, and any complex architectural features.

How do I measure a round room?

Use the formula: Wall area = π x (radius x radius). Measure radius length and plug the number into the formula to get surface area.

Can I return unused paint?

Most retailers allow unopened paint cans to be returned. Opened paint can’t be returned, so properly measuring and calculating quantity is important.

Should I buy paint and primer together?

Primer and paint absorbency can impact results, so use separate primer and paint for best coverage on new or unpainted surfaces. On previously painted surfaces in good condition, quality paint often has sufficient primer built in.

Get the Paint Volume Right

Determining the correct quantity of paint for your particular painting project takes some careful calculations, but the math involved is straightforward. The key steps are:

  1. Precisely measure and calculate the total wall surface area to be painted.
  2. Determine how many paint coats are needed based on surface condition.
  3. Identify the type of paint product and verify spreading rate per gallon.
  4. Plug these variables into the formula to estimate paint quantity.
  5. Add extra paint to allow for unforeseen waste and touch ups.

Taking the time to accurately calculate paint needs using measurements, manufacturer specs and some simple math will save you from both exceeding your paint budget or running out of paint too soon. With these techniques, you’ll buy just the right amount of paint for complete coverage on every project.

How to Calculate How Much Paint You Need for Your Walls

Figuring out the amount of paint needed for painting the walls in a room or entire interior is an important early step of any painting project. Having insufficient paint can lead to mismatched colors or having to abruptly stop painting and purchase more paint. Similarly, purchasing excess paint is wasteful and costly. So learning how to properly measure wall space and translate that into the appropriate paint volume is a handy DIY skill. Here is a step-by-step guide to help calculate how much paint you need to paint the walls in any residential or commercial interior painting project.

Measure the Wall Dimensions

The starting point is measuring the length and height of each wall section that needs to be painted:

  • Use a steel tape measure for accuracy. Make sure to measure to the nearest 1/4 inch.
  • For standard rectangular walls, measure the length and height. Write down both measurements.
  • For more complex wall shapes, make a sketch on graph paper or map out the wall in sections. Measure each section.
  • Use a laser distance measurer for faraway or awkward spaces like stairwells.
  • For irregular spots like soffits and alcoves, carefully measure both the length and height.
  • Note measurements separately for each wall in a room or interior space.

Accurately measuring before painting is crucial for purchasing the optimum quantity of paint.

Determine the Square Footage Per Wall

Use the length and height measurements for each wall section to find the area:

  • Standard wall: Multiply length x height = area in sq ft
  • Irregular wall: Count the squares on graph paper sketch or add the areas of each section

For example:

  • Wall 1 – Length: 25 ft Height: 8 ft Area: 25 x 8 = 200 sq ft
  • Wall 2: Section A – Length: 10 ft Height: 8 ft Area: 80 sq ft
    Section B – Length: 15 ft Height: 8 ft Area: 120 sq ft
    Total Wall 2 Area: 80 + 120 = 200 sq ft

Carefully calculating the area of each interior wall surface needing paint ensures accurate results.

Add Up the Total Square Footage

To determine the total wall surface area in the room or space, add together the area (sq ft) figures for each wall section.

For a room with:

  • Wall 1: 200 sq ft
  • Wall 2: 200 sq ft
  • Wall 3: 250 sq ft

The total would be 200 + 200 + 250 = 650 sq ft

Tallying the total space needing paint is essential for choosing adequate paint amounts.

Determine Number of Paint Coats

The required number of paint coats depends on the current wall condition:

  • New drywall or concrete: 3 coats typically needed
  • Dramatic color change: 2-3 coats recommended
  • Painting over similar color in good condition: May only need 1 coat

Check paint can directions for manufacturer recommendations on coats needed.

Accounting for multiple coats ensures having enough paint from the start.

Calculate the Paint Quantity

Use a simple formula to calculate the quantity of paint needed:

Total sq ft of walls ÷ paint coverage per coat ÷ number of coats = gallons needed

For example, for 650 sq ft of wall using regular latex paint that covers 400 sq ft per gallon with 2 coats:

650 sq ft ÷ 400 sq ft/coat ÷ 2 coats = 0.8125 gallons

Round up to nearest whole gallon, so 1 gallon needed

Buying the precise quantity of paint for the project saves money and eliminates waste.

Add Additional Paint

It’s smart to purchase an extra 10-15% paint for:

  • Minor spills, drips or overpainting
  • Hard to reach spots needing touch-ups
  • Future paint repairs

For a 1 gallon paint total, buy an extra 1-2 quarts.

Padding the paint quantity guarantees you won’t run out before the job is finished.

Carefully measuring walls, calculating area, and factoring in extras results in purchasing the perfect volume of paint. Accurately estimating paint quantities takes a bit of effort but saves time, money and hassle in completing interior painting projects.

How Much Primer Will You Need?

If the painting project involves priming the walls and ceilings before painting, you’ll need to buy an adequate amount of primer in addition to the topcoat paint. Here are some tips for estimating how much primer to purchase:

Measure and Calculate Total Wall Area

Follow the steps previously outlined to accurately measure the total square footage of the surfaces to be primed.

Determine Primer Type

Common options:

  • Latex or water-based primer
  • Oil-based primer
  • Spray primer
  • Stain-blocking primer for covering stains

Check Manufacturer Coverage Rates

  • Latex primer: 300-400 sq ft/gallon
  • Oil-based primer: 350-400 sq ft/gallon
  • Spray primer: 150-300 sq ft/gallon

Use Primer Quantity Formula

Total square feet ÷ Primer coverage per coat = Gallons needed


  • Total wall area: 500 sq ft
  • Latex primer covers 350 sq ft per gallon

500 sq ft ÷ 350 sq ft/gallon = 1.4 gallons

Round up to 2 gallons

Add Extra Primer

Get 10% extra primer beyond the calculated quantity to allow for mistakes, over-application or missed spots.

Accurately measuring the square footage, verifying the manufacturer spreading rate, and padding the total ensures having sufficient primer on hand for flawless, uniform coverage on the topcoat paint.

How Much Paint Is Needed for Different Room Sizes?

The quantity of paint needed for an interior painting project depends on the room dimensions. Here are estimates for the amount of latex wall paint needed for rooms of various sizes:

Small Room

10′ x 10′ room with 8′ high ceilings

10 x 10 = 100 sq ft per wall
4 walls so 100 sq ft x 4 = 400 sq ft total

Paint Needed:
400 sq ft ÷ 350 sq ft per gallon = 1 gallon

1-2 gallons needed

Medium Room

12′ x 15′ room with 9′ high ceilings

12 x 15 = 180 sq ft per wall
180 x 4 walls = 720 sq ft

Paint Needed:
720 sq ft ÷ 350 sq ft per gallon = 2 gallons

2-3 gallons needed

Large Room

20′ x 20′ room with 10′ high ceilings

20 x 20 = 400 sq ft per wall
400 sq ft x 4 walls = 1600 sq ft

Paint Needed:
1600 sq ft ÷ 350 sq ft per gall