How to Use a Paint Calculator

Paint calculators exist for a reason — nobody wants to measure every wall, subtract every window, and do square-footage math on a scrap of drywall. You just need the right numbers to feed in. Here's what actually matters.

Step 1: Measure Like You Mean It

Grab a 25-foot locking tape measure. Measure length and width along the baseboards, corner to corner. For height, check in at least two spots — floors settle and ceilings aren't always dead level. Write everything down in feet, using decimals for inches.

• L-shaped rooms: Bust them into separate rectangles and measure each one.
• Vaulted ceilings: Measure the highest and lowest points. Use the average.
• Closets: If you're painting inside them, count them.

Step 2: Count Doors and Windows Right

Standard interior door: 21 square feet. Standard window: 15 square feet. The part people forget — you only subtract openings on the walls you're actually painting. A sliding glass door on one wall? That's about 42 sq ft gone. French doors, big picture windows, and fireplaces all tweak the numbers.

Step 3: Nail the Number of Coats

Coat count isn't a preference — it's dictated by what you're working with:

• 1 coat: Only when you're painting over the exact same color with top-shelf paint.
• 2 coats (standard): Covers 90% of jobs. Even color, no thin spots, looks professional.
• 3 coats: Dark-to-light transitions, bright accent walls, fresh drywall, or cheap paint.

Step 4: Know What Your Paint Actually Covers

Every can says 350–400 sq ft per gallon. That's lab conditions — smooth primed walls, pro application. Your walls probably aren't that perfect:

• Textured walls (orange peel, knockdown): Cut expected coverage by 10–15%.
• Stucco or brick: Can drink 2–3x more paint than smooth drywall.
• Bare drywall: First coat soaks in way more, so effective coverage drops hard.

Step 5: Pad the Number

Rule of thumb: buy 10% more than the calculator says. Paint vanishes into roller trays, splatters onto drop cloths, and disappears into uneven spots. Running out mid-wall is the worst — the new gallon's batch might not match perfectly, and now you've got a stripe on your wall you can't unsee.