Paint Calculator
Calculate gallons of paint needed for your room dimensions, with coverage and cost estimates.
Room Dimensions
Smooth (previously painted)
375 sq ft/gal
Standard coverage on previously painted walls.
Ready to calculate
Enter your room dimensions
Last updated June 11, 2026 by our expert review team
Choose Paint by Surface
Pick the product by where it will live first, then let the calculator adjust gallons for surface condition, coats, primer, and cutouts.
Interior walls
Latex eggshell or satin
Best for bedrooms, halls, and living spaces. Textured walls usually need more paint.
350-400 sq ft/gal
Trim + doors
Acrylic enamel or alkyd hybrid
Harder finish, better cleaning, lower spread rate than wall paint.
300-350 sq ft/gal
New drywall + patches
PVA or stain-blocking primer
Primer evens out porous paper, joint compound, and repair spots before paint.
300-350 sq ft/gal
Cabinets + tough surfaces
Bonding primer + enamel
Plan extra sanding, dry time, and thin coats so doors do not stay tacky.
300-400 sq ft/gal
Expert Contributors
Cherry Jane Limbago
Licensed Civil Engineer & Master Plumber
Licensed civil engineer and master plumber with 10+ years in structural design, estimation, and project management. Currently Project Engineer at Universal Robina Corporation.
See full profileHawkin
Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileUpdated June 2026
How we verify our calculators
Coverage by Finish Type
ft²/galPer gallon on smooth, previously painted walls. Textured or bare surfaces reduce coverage 15-25%.
Which Finish for Which Room?
Hides flaws, easy touch-up
Subtle sheen, washable
Wipes clean from grease
Resists moisture
Durable, cleans easily
No glare, hides marks
Pro Tips
Buy all your gallons at once and have the store shake them together. Same color code can shift slightly between batches.
Cut in edges with a 2.5" angled brush before rolling. Rolling first leaves visible lap marks at corners.
Use a bonding primer (Zinsser or Kilz) on glossy surfaces, dark colors, or fresh drywall. Without it, your topcoat peels within a year.
Painting new construction or fresh walls? Use our drywall calculator to estimate sheets before painting new walls.
Considering wallpaper instead? Our wallpaper calculator estimates rolls, strips, and paste for any room.
Roll in a W pattern across 3-4 foot sections, then fill in. Straight parallel strokes leave streaky lines.
Aim for 50% relative humidity when you can. Many paint specs use 77°F and 50% RH for dry times; above 85% RH, wait for drier air because cure time and adhesion get unpredictable.
Finishing a deck or fence too? Use our stain calculator to figure out how much stain you need for exterior wood.
How the Calculator Works
Wall height times room perimeter gives wall area for each matching room. We subtract 20 sq ft per door and 15 per window, then divide by the coverage rate for your surface condition (smooth: 375, textured: 300, new drywall: 325, bare wood: 300 sq ft/gallon).
The 10% buffer covers touch-ups, cut-in overlap, and paint left in the tray and roller. Ceiling paint uses flat coverage (400 sq ft/gal), trim uses semi-gloss (300 sq ft/gal), and primer gets one coat at 300 sq ft/gal.
Formula
((Wall Height × Perimeter - Doors × 20 - Windows × 15) × Rooms × Coats × 1.10) ÷ Coverage Rate = Gallons
Quick Reference
- Standard door
- 20 sq ft
- Standard window
- 15 sq ft
- 1 gallon covers
- 350-400 sq ft
- 1 quart covers
- ~100 sq ft
Frequently Asked Questions About Paint Coverage
How much paint do I need?
Multiply wall perimeter by ceiling height, subtract 20 sq ft per door and 15 per window, then divide by 350-400. A 12x12 room needs about 2 gallons for two coats.
How many gallons for a room?
10x12 bedroom: 2 gallons. 15x20 living room: 3-4 gallons. Bathroom: 1-2 gallons. All assume two coats at 350-400 sq ft per gallon.
How many sq ft does a gallon cover?
350-400 on smooth walls. Drops to 250-300 on textured, new drywall, or bare wood. Semi-gloss covers slightly less (300-350).
One coat or two?
Almost always two. One coat leaves thin spots visible in side lighting. Only skip on exact same-color recoats.
When do I need primer?
New drywall, raw wood, patched spots, dark-to-light changes, and glossy surfaces. Zinsser and Kilz are the go-to brands.
How does the room multiplier work?
Enter one room's perimeter, doors, and windows, then set Same-size Rooms to the count. Three identical 12x12 bedrooms with one door and two windows each need about 6 gallons for two coats.
Should I use this or the cost calculator?
Use this page when the main question is gallons. Use the paint cost calculator when labor, supplies, primer, and paint tier are part of the budget.
Is premium paint worth it?
For high-traffic areas, often yes. Benjamin Moore Regal Select is best budgeted around $55-65/gal and usually covers better than economy paint, so you may buy fewer gallons on strong color changes.
Paint Coverage Estimates for Common Projects
Standard Bedroom
12x12, 8 ft ceiling1 roomAbout 334 sq ft after subtracting one door and two windows. Two gallons usually covers two coats with a little touch-up paint left over.
Three Matching Bedrooms
12x12 each, 8 ft ceiling3 same-size roomsUse the room multiplier when bedrooms share the same dimensions. The calculator multiplies doors and windows per room, then rounds gallons once for the full order.
Open Living/Dining
20x25, 9 ft ceiling1 roomTall open rooms show roller marks more easily. Buy one batch and box the gallons together before you start cutting in.
Painting Mistakes That Cost You Money
Skipping primer on new drywall
Mudded joints and paper-faced areas absorb paint differently, creating light and dark patches ('flashing'). One coat of PVA primer fixes this.
Painting in bad weather
Dry-time specs are usually based on about 77°F and 50% RH. Above 85% RH or below 50°F, wait for better conditions or expect slow curing and adhesion trouble.
Rolling before cutting in
Cut in one wall with a brush, then immediately roll it while the edges are still wet. If you roll first, the dried roller texture shows through your brush work.
Cheap roller covers
A $2 roller sheds fibers and leaves stipple marks. Spend $8 on a Purdy or Wooster microfiber. Use 3/8" nap for smooth walls, 1/2" for textured.
Leaving tape on too long
Pull tape within 1-2 hours of the final coat while it's still slightly tacky. After 24 hours, the adhesive bonds and tears the paint off in strips.
Buying the exact amount
Always grab one extra gallon. You can't always color-match mid-project, and the extra covers future touch-ups.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.