Paint Calculator

How to Use This Paint Calculator

This paint calculator helps you estimate the exact amount of paint needed for your room in just a few steps. It accounts for room shape, doors, windows, the number of coats you plan to apply, and even estimates the total cost.

Steps:

1. Select your room shape — Rectangular for standard rooms, or L-Shaped for rooms with an alcove or extension.
2. Enter the room dimensions in feet. For rectangular rooms, you need length, width, and wall height. For L-shaped rooms, enter the two sections' dimensions plus the wall height.
3. Enter the number of doors and windows. Each standard door is estimated at 20 sq ft and each window at 15 sq ft.
4. Choose the number of coats. Two coats is standard for most interior paint jobs.
5. Adjust the coverage per gallon if your paint specifies a different rate (the default is 350 sq ft/gallon).
6. Optionally enter the price per gallon to see the estimated total cost.

How to Measure a Room for Painting

Accurate measurements are the foundation of a good paint estimate. Here is how to measure your room properly:

For a rectangular room:

• Measure the length and width of the room at floor level.
• Measure the height of the walls from floor to ceiling (or to the top edge if you are not painting the ceiling).
• Count the number of doors and windows.

For an L-shaped room:

• Think of the L-shape as two connected rectangles.
• Measure the length and width of each section separately.
• Measure the wall height (which should be the same throughout).

Pro tip:Use a laser measure for faster, more accurate readings. If you don't have one, a steel tape measure works fine. Measure at multiple points along each wall and use the average if the walls are not perfectly straight.

The Paint Calculator Formula

The calculator uses a straightforward formula to determine how much paint you need:

Rectangular room:

L-shaped room:

Paintable area:

Gallons needed:

The result is always rounded up because you cannot buy a fraction of a gallon at most stores. This ensures you have enough paint to finish the job.

How Much Paint Do You Need?

Most interior latex paint covers approximately 350 to 400 square feet per gallon for a single coat on smooth surfaces. However, several factors can reduce coverage:

• Texture: Rough surfaces like stucco or heavily textured drywall can reduce coverage by 20–40%.
• Color change: Painting over a dark color with a light one may require an extra coat or primer.
• Paint quality: Higher-quality paints with more pigments often cover better in fewer coats.
• Application method: Rollers tend to be more efficient than brushes, and sprayers can use more paint due to overspray.

As a general rule, buy 10–15% more paint than you calculate to account for touch-ups and minor variations in surface area. It is better to have a little left over than to run out midway through a project.

Choosing the Right Paint

Not all paint is created equal. The type of paint you choose affects both coverage and the quality of the finished result:

• Flat/Matte: Best for ceilings and low-traffic areas. Hides imperfections well but is harder to clean.
• Eggshell/Satin: A good balance for living rooms and bedrooms. Has a slight sheen and is easier to clean than flat.
• Semi-gloss: Ideal for trim, doors, and kitchens or bathrooms. Durable and easy to wipe clean.
• Gloss: Most durable and easiest to clean, but highlights surface imperfections. Best for cabinets and trim.

Primer:If you are painting over bare drywall, patched areas, or making a drastic color change (e.g., red to white), use a primer first. Primer typically covers 200–300 sq ft per gallon. Using primer can actually save you money because you will need fewer coats of the more expensive finish paint.

Tips for a Better Paint Job

• Prep the surfaces: Fill holes, sand rough spots, and clean the walls before painting. Good prep work makes a bigger difference than expensive paint.
• Use painter's tape: Tape off trim, ceilings, and edges for clean lines. Remove the tape while the paint is still slightly wet to avoid peeling.
• Buy quality tools: A good roller cover and quality brushes hold more paint and leave fewer streaks. Cheap brushes shed bristles into your finish.
• Paint from top to bottom: Start with the ceiling, then the walls, then the trim. This way, any drips from above get covered.
• Maintain a wet edge: Work in small sections and keep a wet edge to avoid lap marks. Don't let one section dry before blending into the next.
• Store leftover paint properly: Seal the can tightly and store it upside down in a temperature-controlled area. It will last for touch-ups for up to 2 years.

Frequently Asked Questions

How many gallons of paint do I need for a 12×12 room?

A standard 12×12 room with 8-foot ceilings has a wall area of about 384 sq ft. After subtracting one door and one window, the paintable area is roughly 349 sq ft. With two coats and standard coverage (350 sq ft/gallon), you need about 2 gallons of paint.

Does this calculator include the ceiling?

No. This calculator estimates paint for walls only. To calculate ceiling paint, measure the length and width of the floor and use that as the ceiling area. Most ceilings need 1–2 gallons depending on size and the number of coats.

How much does a gallon of paint cost?

Interior latex paint typically costs between $20 and $60 per gallon. Budget paints start around $15–$20, mid-range paints run $25–$40, and premium paints can cost $45–$70+. Primer is usually a bit cheaper at $15–$30 per gallon.

Do I need primer before painting?

Not always. If you are painting over a similar color in good condition, you can skip primer. However, you should use primer when painting bare drywall, patched areas, making a dramatic color change, or switching from oil-based to water-based paint. Some paints are marketed as “paint and primer in one,” which can work for moderate color changes but may still need a separate primer for extreme cases.

How long does it take paint to dry between coats?

Most interior latex paint is dry to the touch in 1–2 hours and can be recoated in 4 hours. However, always check the manufacturer's recommendation on the can. Oil-based paints take longer — typically 6–8 hours before recoating. Waiting the full recommended time between coats ensures the best adhesion and finish.

How do I calculate paint for textured walls?

Textured walls absorb more paint than smooth surfaces. As a rule of thumb, reduce the coverage rate by 20–40% depending on the texture depth. For heavily textured walls like knockdown or stucco, use a coverage rate of 200–250 sq ft per gallon instead of the standard 350. Enter this lower value in the coverage field above.