Buy too little paint and you’re back at the store mid-project, praying the new can matches the old batch. Figuring out how much paint do I need with a simple calculator prevents that headache, and the math is easier than most people expect: measure the wall area, subtract the doors and windows, and divide by the coverage on the can. One gallon covers roughly 350 to 400 square feet with one coat, which is the number everything else hinges on.
Part of our Paint & Walls guide — explore the full guide →
I’ve painted enough rooms to know the estimate people botch most is coats, not square footage. Let’s walk through the whole calculation the way a pro does it.
The Basic Formula
Here’s the paint calculator in four steps:
- Measure wall area. Add up the length of every wall, then multiply by the ceiling height. (Perimeter x height = total wall square footage.)
- Subtract openings. Deduct about 20 sq ft per door and 15 sq ft per average window.
- Divide by coverage. Take your net square footage and divide by 350 for the number of gallons per coat.
- Multiply by coats. Most jobs need two coats, so double it.
Example: a 12×14 room with 8-foot ceilings has a 52-foot perimeter, giving 416 sq ft of wall. Subtract one door and two windows (about 50 sq ft) for 366 sq ft. Divide by 350 to get about 1 gallon per coat, so buy 2 gallons for two coats.
Coverage Rates You Can Count On
Not all paint covers the same. Use these real-world numbers:
- Standard wall paint: 350 to 400 sq ft per gallon, one coat
- Primer: 200 to 300 sq ft per gallon (it soaks in more)
- Textured or porous surfaces: as low as 250 sq ft per gallon
- Smooth, previously painted drywall: up to 400 sq ft per gallon
When in doubt, use 350 as your working number. It builds in a small cushion and keeps you from coming up short.
When You Need Two Coats (Usually)
Plan on two coats in almost every real scenario. You’ll definitely need two when:
- Going from a dark color to a lighter one
- Painting over bold or saturated colors (reds, deep blues)
- Covering new drywall or fresh primer
- Using deep-base tints, which are naturally more translucent
A single coat only works when you’re refreshing the same color over a clean, previously painted wall. Most homeowners underestimate here and end up with patchy coverage, so budget for two coats unless you’re certain.
Ceilings and Trim
Ceilings and trim get calculated separately. For a ceiling, multiply length by width for the square footage, then divide by 350. A 12×14 ceiling is 168 sq ft, well under a gallon per coat. For trim, baseboards, casings, and crown, estimating by linear footage is tricky, so a good rule of thumb is one quart of trim paint per average room, or one gallon for a whole floor of a house. Trim paint stretches further because you’re covering narrow surfaces.
Don’t Forget These Factors
A few variables push your paint needs up:
- Primer: New drywall, bare wood, and stains need a separate primer coat. Calculate it like a paint coat but at 250 sq ft per gallon.
- Rough textures: Popcorn, knockdown, and stucco drink paint. Add 10% to 20%.
- Color change: Big shifts mean extra coats, which means extra gallons.
- Waste and touch-ups: Always buy a little extra; keep a labeled can for future scuffs.
Quick Reference by Room Size
For standard 8-foot ceilings and two coats on walls:
- Small room (10×10): about 2 gallons
- Average bedroom (12×14): about 2 gallons
- Large room / living room (15×20): 3 to 4 gallons
- Whole 1,500 sq ft house interior: roughly 12 to 15 gallons plus primer
Buying Smart
Paint costs $25 to $60 a gallon for quality interior products from brands like Sherwin-Williams and Benjamin Moore, so accuracy saves real money both ways. Buying gallons instead of quarts is cheaper per square foot once you need more than a couple quarts. When your calculation lands just over a gallon, round up rather than gambling on a short can. And always have the store note the exact tint formula so a future touch-up matches, because even the same color can differ slightly between batches.
Run the numbers before you shop, and how much paint you need stops being a guess. Measure the walls, subtract the openings, divide by 350, and multiply by two coats. Add primer and a texture cushion where they apply, round up when you’re on the fence, and you’ll finish the job with one trip to the store and a small can left over for touch-ups.
Exterior Paint Calculations
Outside surfaces follow the same math but with lower coverage rates. Exterior siding, especially rough or textured surfaces like stucco and cedar shakes, soaks up paint, so plan for 250 to 300 square feet per gallon instead of 350. Measure the perimeter of the house times the height for the wall area, then subtract windows and doors. Add extra for trim, fascia, soffits, and any decorative details. Because exterior jobs almost always need two coats for weather protection and often a primer coat on bare or previously stained wood, exterior projects typically require noticeably more paint per square foot than interior ones.
Choosing the Right Sheen and Quality
Coverage isn’t just about square footage; paint quality matters too. Cheap paint with low solids content covers less and often needs an extra coat, which can erase the savings. Premium paints from Sherwin-Williams, Benjamin Moore, and Behr hide better in fewer coats, so paying more per gallon sometimes means buying fewer gallons overall. Sheen affects durability more than coverage: flat and matte hide wall imperfections but scuff easily, while eggshell and satin clean up better in kitchens, bathrooms, and hallways. Higher-sheen paints can also require more careful application to avoid lap marks, so factor a little extra for touch-ups.
