Get an instant, accurate gallon estimate for any room, free.
Before you buy a single can, answer one question: how much paint does this room actually need? Guess too low and you are back at the store mid-project with a color that will not quite match the old can. Guess too high and you have gallons sitting in the garage for the next five years. The calculator below solves that in under a minute. Enter your wall measurements, tell it about your windows and doors, and get a straight answer in gallons, not a vague range.
Paint Calculator
Total Paint Needed:
0 Gallons
Note: This tool gives a basic estimate using 350 square feet of coverage per gallon per coat. For rectangle and square rooms, enter two adjoining walls and the calculator accounts for the opposite walls automatically. Your actual paint usage may differ depending on surface type, paint quality, and how it’s applied.
How the calculator works
The math behind the tool is simple, and knowing it helps you trust the number it gives you.
Most paint covers about 350 square feet per gallon on a properly prepped wall. The calculator takes your wall dimensions, multiplies width by height for each wall, then subtracts standard allowances for windows (about 15 square feet each) and doors (about 20 square feet each). If you are painting two coats, which is standard for most interior work, it doubles the adjusted square footage before converting to gallons.
Add the ceiling if that is part of the job. Ceilings get calculated the same way: length times width, run through the same per-gallon coverage rate.
What to enter
Room shape. Pick rectangle, square, or custom if your space has an unusual layout or more than four walls.
Wall dimensions. Measure each wall's width and height in feet. If a wall has a bump-out, closet, or angled section, treat it as a separate wall in the custom option.
Windows and doors. Count each one. The calculator subtracts their average area automatically so you are not paying for paint you will never apply.
Ceiling. Toggle this on only if the ceiling is part of the scope. Skip it if you are just refreshing walls and trim.
Number of coats. Two coats is the right default for almost every interior repaint. Drop to one only if you are using a premium one-coat product over a similar existing color, or bump to three if you are covering a dark wall with a much lighter shade.
Why the number can still be off
A calculator gives you a starting estimate, not a purchase order. A few things shift the real number in either direction:
- Surface texture and porosity. Bare drywall, unfinished wood, and older plaster all soak up more paint than a wall that has already been painted and sealed. Budget extra on first-time-primed surfaces.
- Color change. Going from a dark, saturated color to a light one (or the reverse) often needs an extra coat regardless of what the calculator says. A tinted primer can sometimes close that gap in one coat instead of two.
- Application method. Sprayers use noticeably more paint than rollers and brushes because of overspray. If your project is being sprayed, add a buffer.
- Trim, doors, and detail work. This calculator is built for wall and ceiling square footage. Trim, baseboards, doors, and built-ins are typically priced and painted separately, since they use a different sheen and go through the coverage rate much faster per square foot.
A worked example
Here is how the math plays out on a typical bedroom: a 10-by-12-foot room with 8-foot ceilings, two windows, and one door, painted with two coats, walls only.
- Wall area: (10 + 12 + 10 + 12) linear feet × 8 feet high = 352 square feet
- Subtract openings: (2 windows × 15 sq ft) + (1 door × 20 sq ft) = 50 square feet
- Adjusted area: 352 − 50 = 302 square feet
- Two coats: 302 × 2 = 604 square feet of total coverage needed
- At 350 square feet per gallon: just under 2 gallons
That is the kind of number the calculator hands you instantly, without the manual subtraction.
When a calculator estimate is not enough
A DIY estimate works fine for a single room with a quick color refresh. It gets shakier on anything with real complexity: multiple rooms with different ceiling heights, a full-home repaint, an exterior with varied siding materials, or any project where getting the gallon count wrong means a mismatched batch halfway through.
That is where a walkthrough estimate from GREENHAUS Painting earns its keep. We measure every wall, window, and door in person, factor in the actual condition of your surfaces, and give you a number tied to a real proposal, not an average. You also get guidance on sheen, coat count, and whether your walls need a stain-blocking or bonding primer before color goes on at all.
Call 203-429-4424 - Get a Free, In-Person Estimate
What a GREENHAUS estimate includes that a calculator can't
- We measure everything on-site. No guessing on wall dimensions, ceiling height, or trim linear footage. The number in your proposal is the number we paint to.
- We flag prep needs before they become surprises. Patchy old paint, bare drywall, and stained ceilings all change how much product and labor a room actually needs, and we call that out during the walkthrough, not after the crew arrives.
- We help you choose sheen and coat count. The right finish depends on the room, the light, and how the space gets used, not just the math.
- We use premium paint systems from Benjamin Moore and Sherwin-Williams, applied with the prep and technique that make the coverage rate hold up in real rooms, not just on paper.
- Every project closes with a detailed final walkthrough and is backed by our all-inclusive warranty and 100% Satisfaction Guarantee, with no fine print.
Call 203-429-4424 - Schedule Your Free Estimate
Frequently asked questions
How accurate is a paint calculator?
It is a solid starting estimate for straightforward rooms, typically within 10 to 15 percent of what you will actually use. Surface porosity, color change, and application method are the main reasons the real number can shift.
Does one gallon really cover 350 square feet?
That is the standard figure most paint manufacturers use for a smooth, previously painted wall with a roller. Textured surfaces, unprimed drywall, and darker color changes reduce that coverage, sometimes significantly.
Should I round up when buying paint?
Yes. Buying slightly more than the calculator suggests avoids a mid-project trip back to the store, and most paint stores will accept unopened returns.
Do I need to calculate trim and doors separately?
Yes. Trim, doors, and baseboards use a different paint (usually a higher sheen) and a different, more labor-intensive coverage rate than flat wall paint. This calculator is built for walls and ceilings only.
What if my room has an unusual shape?
Use the custom room option and add each wall segment individually. Breaking an irregular room into smaller rectangular sections and adding them together gets you the most accurate total.
Is it cheaper to paint myself or hire a professional?
Paint and supplies for a DIY job cost less upfront, but a professional estimate accounts for prep work, primer, and application quality that extends how long the paint job actually lasts. Many homeowners find that a professional repaint needs to happen less often, which changes the real cost comparison over several years.
Ready to skip the guesswork entirely?
Use the calculator above to get a ballpark, then let us turn it into an exact plan. GREENHAUS Painting serves Westport and the surrounding Fairfield County and New Haven County area with premium interior and exterior painting, free estimates, and a process built around clear scope and clean execution from the first wall to the last.
