HomecalcTool

Paint Calculator

Enter your room dimensions to get the exact number of gallons needed, including 10% waste.

Your Price (optional)

Enter your paint's per-gallon price for an exact cost estimate.

How to Use This Calculator

A paint calculator tells you exactly how many gallons of paint to buy for any room, based on wall area, number of coats, and deductions for doors and windows. Enter your room's length and width in feet. Set the wall height — 8 feet for standard rooms, 9 feet for newer construction, and up to 12 feet for great rooms or open plans. Choose the number of coats: 1 coat for touch-ups over the same color, 2 coats for most jobs, 3 coats when going from dark to light or painting new drywall. Enter the number of interior doors (each subtracts 21 sq ft) and windows (each subtracts 15 sq ft) so you don't overbuy. Results include a 10% waste factor for roller nap absorption and tray waste. Use the project presets to pre-fill typical room sizes and get an instant result.

How to Calculate How Much Paint You Need

The calculation starts with total wall area: 2 × (length + width) × wall height. From that, subtract painted surfaces you won't paint: each door is 3 × 7 = 21 sq ft, each window is 3 × 5 = 15 sq ft. That gives paintable area.

Formula: gallons = ⌈(paintable area × coats × 1.1) ÷ 350⌉

Standard interior latex covers 350 sq ft per gallon on smooth surfaces. The 1.1 factor adds 10% for roller nap absorption, tray waste, and touch-ups. Math.ceil ensures you always round up — never run out mid-wall.

Example: a 12 × 12 room with 8-ft walls, 2 coats, 1 door, 2 windows. Wall area = 2 × (12 + 12) × 8 = 384 sq ft. Subtract 1 door (21) + 2 windows (30) = 333 sq ft paintable. Gallons = ⌈333 × 2 × 1.1 ÷ 350⌉ = ⌈2.09⌉ = 3 gallons.

Paint Tips

Buy the same lot number when buying multiple cans. Paint varies slightly between production batches. Check the lot number printed on the lid or side label — if you're buying 4 gallons, all 4 should share a lot. If you run out mid-room and buy a new lot, the difference shows up as a faint line on the wall.

Prime new drywall or major repairs before finish painting. New drywall is highly porous and will absorb the first coat unevenly, leaving a patchy appearance called "flashing" under raking light. A single coat of drywall primer seals the surface and cuts your finish coat count from 3 to 2. This saves more in finish paint than the primer costs.

Dark to light always takes 3 coats minimum. Going from a deep color — navy, forest green, burgundy — to a lighter shade almost always requires a gray or white tinted primer plus two finish coats. Budget 3 coats in the calculator whenever you're lightening more than two shades.

What to Buy

For most interior walls: Benjamin Moore Regal Select or Sherwin-Williams SuperPaint in eggshell finish. Both cover 350–400 sq ft per gallon, have excellent washability, and go on in two coats over primed surfaces. Expect to pay $65–80 per gallon at full retail.

For bathrooms and kitchens: specify a paint labeled "moisture-resistant" or "kitchen and bath" — same coverage rate but formulated to resist mildew and withstand humidity. Sherwin-Williams Emerald Interior and Benjamin Moore Aura Bath & Spa are the benchmarks.

Budget option: any major brand's "value" line at $30–40 per gallon covers adequately in 2 coats on previously painted surfaces. Avoid it for new construction or dark-to-light color changes — the coverage is too thin and you'll end up using more cans than you saved.

Frequently Asked Questions

How many gallons of paint do I need for a 12×12 room? +
A 12 × 12 room with 8-foot walls, 2 coats, 1 door, and 2 windows needs 3 gallons of paint. That covers 333 sq ft of paintable wall area at 350 sq ft per gallon with 10% waste included. If the room has higher ceilings or more windows, re-run the calculator with your actual dimensions.
How much does a gallon of paint cover? +
A gallon of standard interior latex covers 350–400 sq ft on smooth, previously painted surfaces. On new drywall or rough textures, expect 250–300 sq ft per gallon. Our calculator uses 350 sq ft per gallon, the conservative end of the range, to ensure you don't run short.
How many coats of paint do I need? +
Two coats is the standard for most paint jobs: one coat to establish color, a second for even coverage and durability. Use one coat only for touch-ups over the exact same color. Use three coats when going from a dark color to a light one, painting over a heavily stained surface, or applying paint over bare drywall that wasn't primed.
Should I subtract doors and windows from my paint calculation? +
Yes, but only if they're substantial. Our calculator subtracts 21 sq ft for each standard interior door and 15 sq ft for each standard window. For a small room with 1 door and 1 window, this saves about half a gallon. For larger rooms with many openings, the savings are significant — always enter your actual count.
What finish should I use for walls? +
Eggshell is the best all-purpose finish for most interior walls. It has just enough sheen to be wipeable without looking glossy. Flat or matte is better for ceilings and low-traffic bedrooms since it hides imperfections. Semi-gloss is the standard for trim, doors, and cabinets. Satin sits between eggshell and semi-gloss — use it in hallways and high-traffic areas.
Why do I need to add 10% extra when buying paint? +
The 10% covers roller nap absorption (a 3/8-inch nap absorbs about 1/4 gallon on first use), paint tray waste, brush loading for cutting in, and touch-ups after the job is done. Without the 10% buffer, a job that theoretically needs exactly 4 gallons often runs short in the final room or leaves you without paint for inevitable dings and scuffs.
Can I use this calculator for exterior paint? +
Yes, with one adjustment: exterior paint typically covers 250–300 sq ft per gallon on rough or weathered siding, versus 350 sq ft on smooth interior walls. If using this calculator for exterior, add 15–20% more gallons to your result to account for the lower coverage. Rough stucco and wood siding absorb more paint than smooth surfaces.
Is it cheaper to buy one gallon or a five-gallon bucket? +
A five-gallon bucket costs 20–30% less per gallon than buying five individual cans. If your result is 4 gallons or more, buy a five-gallon bucket. For 2–3 gallons, individual cans make sense unless you plan to repaint the same color soon. The bucket also ensures all the paint is from the same batch, which prevents visible color differences between walls.

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