Free Paint Calculator – How Much Paint Do I Need?

Quick Answer: How much paint do you need? One gallon of paint covers 350 to 400 square feet per coat on a smooth, primed wall. For an average 12×12 bedroom with 8-foot ceilings, that’s about 2 gallons for two coats after subtracting doors and windows. Enter your own measurements into the calculator below for an exact number.

Figuring out how much paint you need comes down to one formula: total wall area, minus doors and windows, multiplied by your number of coats, divided by your paint’s coverage rate. The calculator above does this math instantly, but the sections below walk through where each number comes from — coverage rates by paint type, gallons by room size, and the details on the label that change your final count.

How Many Square Feet Does a Gallon of Paint Cover?

A gallon of paint covers 350 to 400 square feet per coat under normal conditions — smooth drywall, one even coat, standard latex paint. That number drops with every variable that makes a wall harder to cover: texture, a dark-to-light color change, or a porous, unprimed surface. Coverage also depends on what you’re painting, not just the paint itself.

Brands like Benjamin Moore and Sherwin-Williams print their exact coverage rate on the can label — always check it before buying, since premium formulas like Benjamin Moore Aura or Sherwin-Williams Emerald can cover closer to 400-450 sq ft per gallon, while budget paints often fall closer to 250-300 sq ft.

How to Use This Paint Calculator

The calculator is built around the same measurements a painter would take on-site:

1. Wall Width and Height — Measure each wall in feet and add it as a row. Use “+ Add Wall” for every wall in the room; the calculator totals them automatically.
2. Number of Doors and Windows — Enter how many of each you have. The calculator subtracts 21 sq ft per door and 15 sq ft per window by default, which lines up with industry averages for standard-sized openings. Adjust the “Average Door Area” or “Average Window Area” fields if yours run larger or smaller.
3. Number of Coats — Most rooms need two coats for solid, even color. Stick with one coat only if you’re touching up the same color with premium paint.
4. Paint Coverage per Gallon — Defaults to 350 sq ft, the realistic real-world rate. Bump this to 400 if you’re using a premium paint and rolling on smooth, primed drywall.
5. Waste / Extra Paint — The 10% default accounts for spills, roller absorption, and touch-ups. Don’t skip this; running short mid-coat almost always means a mismatched batch on the second trip to the store.
6. Include Ceiling Paint — Check this box to add ceiling square footage (length x width) to your total.

Once you hit “Calculate Paint Needed,” you’ll get gallons required, rounded up to the nearest practical purchase size.

Paint Needed by Room Size: Quick Reference Chart

If you want a ballpark before measuring, here’s what typical rooms need for two coats, based on 8-9 ft ceilings and standard door/window counts:

Kitchens are deceptive on paper — cabinets often cover 30-40% of the wall area, so the actual paintable surface runs smaller than the room’s square footage suggests.

What Changes Your Paint Coverage

The 350-400 sq ft figure assumes ideal conditions. Real walls rarely match that exactly:

• Finish: Flat and matte finishes hide imperfections and spread further. Semi-gloss and high-gloss finishes are thinner and often need an extra coat.
• Color change: Going from a dark wall to a light color (or the reverse) usually adds a coat, even with primer.
• Surface texture: Stucco, brick, and popcorn ceilings can eat up 20-30% more paint than smooth drywall.
• Surface porosity: Bare drywall, unfinished wood, and old plaster soak up the first coat fast. Priming with a product like Zinsser first prevents this from inflating your final paint count.
• Application method: Spraying uses more paint than rolling; rolling uses more than brushing. A quality roller from a brand like Purdy gives more consistent coverage than a budget one.

Common Paint Calculator Mistakes to Avoid

• Measuring floor area instead of wall area. Floor square footage has nothing to do with how much paint you need — always measure the walls.
• Skipping door and window deductions. A standard door and two windows save roughly 51 sq ft, which is 15% of a small room’s wall area.
• Forgetting the ceiling. If you’re painting it, add it separately — it doesn’t show up in your wall total.
• Assuming all paint covers the same. Budget paint can run as low as 250 sq ft per gallon; premium paint can hit 400-450. Check the can.
• Skipping primer on bare drywall. New drywall is porous and will absorb your first coat almost entirely if it isn’t primed first.

How Much Will Your Paint Project Cost?

Once you know your gallon count, cost is straightforward math: gallons x price per gallon. Budget interior paint runs roughly $25-$35 a gallon, mid-range options like Behr or Valspar run $35-$55, and premium lines like Benjamin Moore Regal Select or Sherwin-Williams Emerald run $65-$80. For a standard 12×12 bedroom needing 2 gallons, that’s roughly $50-$70 in budget paint versus $130-$160 in premium — though premium paint’s better hiding power often means you need fewer total coats, narrowing the real-world cost gap.

Knowing how much paint you need is half the job — getting clean lines and full coverage once you start applying it is the other half, and a few prep habits make the difference between a smooth finish and a streaky one.

FAQs

How many square feet does a gallon of paint cover? A gallon of paint covers 350 to 400 square feet per coat under normal conditions on a smooth, primed wall. Textured or porous surfaces can drop that to 250-300 square feet.

How much paint do I need for a 12×12 room? A 12×12 room with 8-foot ceilings needs about 2 gallons of paint for two coats on the walls, after subtracting a standard door and two windows.

Does a gallon of paint cover 2 coats? Not usually for a full room. One gallon covers 350-400 sq ft for a single coat, so two coats on that same wall area typically requires close to double the paint — most rooms need 2 gallons minimum for two coats.

How much primer do I need before painting? Primer covers less than paint — about 200-300 sq ft per gallon — because it soaks into the surface rather than sitting on top. Bare drywall or major color changes usually need a full coat of primer first.

Do I need more paint for textured walls? Yes. Textured surfaces like stucco or popcorn ceilings can need 20-30% more paint than smooth drywall because of the extra surface area.

How much paint do I need for an average bedroom? Most bedrooms between 10×10 and 15×15 feet need 2-3 gallons of paint for two coats, depending on ceiling height and the number of doors and windows.

Is it cheaper to buy a 5-gallon bucket instead of individual gallons? Yes, once a project needs 4 or more gallons. A 5-gallon bucket usually saves $20-$50 and guarantees one consistent batch code, so the color doesn’t vary slightly between cans.

How much extra paint should I buy for touch-ups? Add about 10% to your calculated total. This covers spills, roller absorption, and future touch-ups without leaving you short mid-project.