How Much Paint Do I Need for a Room?

A painter measuring a room for paint coverage in dublin

How Much Paint Do I Need for a Room?

By Paul McGovern, Master Craftsman at

If you’re getting ready to paint a room in your home, one of the most important calculations you need to make is how much paint to buy. Buying too little can lead to mismatched colors or having to make multiple trips to the store. Getting too much paint means wasted money and dealing with disposal. 

So how do you estimate the right amount? As a professional painter with over 10 years of experience, I’ve painted hundreds of rooms. In this article, I’ll share my insider tips on determining paint quantities for any room in your home. We have also included a handy Paint Quantity Calculator further down the page to help you work out how much paint you will need.

The Main Factors That Determine Paint Quantity

The amount of paint needed depends on a few key factors:

  • Size of the room – The room’s square footage and wall space
  • Type of paint – Water-based or oil-based paint; finishes like flat, eggshell, etc.   
  • Application method – Roller, brush, sprayer; and coat thickness
  • Primer/Finish coats – Whether you need separate primer and finish paint
  • Brand of paint – Quality paint covers better with less coats
  • Wall condition – Paint soaks into new drywall more than existing painted walls 

Take measurements of the room to calculate total wall space, including doors, windows, and trim. This gives you the surface area to be painted. The general rule of thumb is that one gallon of paint covers 400 sq ft of wall space with one coat. But this varies based on the other factors above.

Now let’s look at how each factor impacts the quantity of paint you’ll need.

a photo of a professional decorator, getting ready to paint a small sitting room in an Irish house

How Room Size Impacts Paint Quantity

The bigger the room, the more paint you’ll need to buy. To measure room size:

  1. Measure the length and width to get the square footage of each wall
  2. Measure height to estimate wall space 
  3. Add up the square footage of all walls

This gives you the room’s total wall space. An average 10×12 ft room with 8 ft high walls is approximately 480 sq ft of wall space. For such a room, one gallon of paint covers with one coat. For two coats, you need 2 gallons.

For high-ceiling rooms, measure wall height in feet and multiply by width and length to get wall square footage.

How Paint Type and Finish Impacts Quantity

Water-based or latex paint is thicker than oil-based alkyd paint. Latex paint requires less quantity – about 350-400 sq ft per gallon per coat. Oil-based paint covers around 300-350 sq ft per gallon.

Flat or matte finishes are thinner and require more paint. Eggshell, satin, and semi-gloss finishes are thicker. 

High gloss paints have the most solids and require the least amount of paint per coat.

How Primer vs. Paint Impacts Quantity 

Primer seals surfaces and allows finish paint to adhere better. Primers are thinner, so you may need more than one coat to cover surfaces evenly. 

New drywall or heavily stained/damaged walls need primer. For new drywall, one coat of high-quality primer followed by two finish paint coats is best. For existing painted walls in good shape, you may only need one coat of finish paint.

How Application Method Impacts Quantity

Paint application method also affects quantity needed. Thicker applications naturally require more paint.

  • Roller – Standard 3/8″ nap covers 400 sq ft per gallon per coat  
  • Brush – Can cover 300-350 sq ft per gallon per coat
  • Sprayer – Highly efficient but causes paint overspray/waste

Proper roller and brush techniques also help optimize paint usage. Apply an even coat without excessive dripping or thin patches.

How Brand Quality Impacts Quantity

Higher quality paints made with more solids cover better. With premium brands like Benjamin Moore, Farrow & Ball, you may be able to get away with one finish coat. 

Lesser quality economy brands may require multiple coats to get full coverage. Read the manufacturer labels for expected coverage per coat.

How Wall Condition Impacts Quantity  

Wall condition is a very important factor. Paint soaks into unfinished new drywall more than existing painted walls. Previously painted surfaces in good condition will need the least amount of paint.

Consider the following wall conditions:

  • New drywall/plaster: will absorb more paint; needs primer 
  • Lightly stained walls: may require spot priming only
  • Heavily stained/damaged walls: requires full coat of primer
  • Glossy walls: sanding improves absorption; primer recommended
  • Freshly painted walls in good condition: least paint needed

Now let’s look at a paint quantity example for a standard room.

Paint Quantity Example for a 10×12 Bedroom

Here is how much paint you’ll need for a typical 10×12 ft bedroom with 8 ft high walls and 400 sq ft of standard drywall in good condition. 

For one coat of quality latex wall paint applied with a roller, you need 1 gallon. For two finish coats, you’ll need 2 gallons. 

Adding a full coat of primer on top of the 2 finish coats, you’ll need around 3 gallons total:

  • 1 coat primer: 1 gallon 
  • 2 coats paint: 2 gallons

Therefore, to be on the safer side, buy 4 gallons total for this room. Having some extra is better than running short!

For oil-based paint, you may need up to 5 gallons for 3 full coats on new drywall. For spray painting, add up to 10% more paint.

Use the same calculations based on square footage when estimating paint for ceilings and trim.

Paint Quantity Calculator

This calculator will take the room dimensions, number of coats, and paint coverage as inputs, and output the amount of paint needed.

Paint Calculator

This is a simple calculator and doesn’t account for every possible scenario. For example, it assumes that the room is a perfect rectangle and that the doors and windows are all the same size. You may want to adjust the code to better fit your specific needs.

Frequently Asked Questions

Here are some common questions people have about estimating paint quantity for rooms:

How much paint is needed for interior vs. exterior painting?

Interior walls require less paint – about 400 sq ft per gallon per coat. Exterior surfaces require 100-150 sq ft per gallon per coat.

Does ceiling height affect paint needed?

Yes, tall walls and high ceilings require painting more surface area, meaning more paint.

Should I buy paint in 1-gallon or 5-gallon buckets?

For most rooms, 1-gallon buckets are easier to manage. Get 5-gallon buckets only if painting very large rooms. 

Should I get more paint than calculated for my rooms?

Get at least 10% extra paint than your estimate to allow for mistakes, spills, or touch-ups.

How do I calculate trim, doors, and windows paint quantity? 

Measure linear feet of trim and multiply by 0.333 to get square feet. Get about 10-15% extra for trim and doors.

Does primer and finish paint come in the same quantities?

Yes, both primer and paint come in standard sizes like quarts, gallons, and 5-gallons.


Estimating the right quantity of paint for a room is not difficult if you account for all the key factors. Measure room size, calculate total wall space, factor in paint type/finish, wall conditions, and application method. 

Be sure to get 10-15% extra paint than your estimate to have some spare. Apply primer and finish coats as needed. Use high quality paint and proper application techniques to maximize coverage.

I hope these tips give you a good understanding of how to determine how much paint you need for any room in your home. Let me know if you have any other paint or painting questions!

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