Painting & Finishing
Updated Jun 28, 2026

Paint Calculator

Calculate how much paint you need for interior and exterior walls, ceilings, and trim. Get gallon estimates by room size.

How to Calculate Paint

Estimating paint correctly saves money and prevents extra trips to the store. The key is calculating the total wall area, subtracting openings, and accounting for the coverage rate of your chosen paint.

Wall Area Formula

For a rectangular room, calculate the total wall area by finding the perimeter and multiplying by the ceiling height:

Wall Area=Perimeter×Height−Openings

Gallons Needed

Once you know the net paintable area, divide by the coverage rate and multiply by the number of coats:

Gallons=Net Area×CoatsCoverage Rate
Standard Openings
Use these typical sizes when subtracting openings: standard door is 21 sq ft (3' × 7'), standard window is 12 sq ft (3' × 4'), and a sliding glass door is about 40 sq ft.

Coverage Rates by Surface

Different surfaces absorb paint at different rates. Rough or porous surfaces require more paint per square foot:

Surface TypeCoverage (sq ft/gal)Coats Needed
Smooth Drywall350-4002
Textured Drywall250-3002
Bare Wood250-3502 + primer
Brick / Stucco150-2502 + primer
Previously Painted350-4001-2
Color Change Matters
When making a drastic color change (e.g., dark to light or light to dark), always use a tinted primer first. Without primer, you may need 3-4 coats instead of 2, which actually costs more than buying a separate primer.

Paint Finish Types

The sheen of a paint affects how it looks, cleans, and hides flaws. Higher sheen = more durable and washable, but shows every wall imperfection. Match the finish to the room:

FinishSheenBest ForHides Flaws
Flat / MatteNoneCeilings, adult bedroomsExcellent
EggshellLowLiving rooms, diningGood
SatinMediumKids rooms, hallways, kitchensModerate
Semi-GlossHighBathrooms, trim, doorsPoor
High-GlossVery highCabinets, accent doorsNone (shows every bump)

Typical Paint Needs by Room Size

Use these as a sanity check against your calculator result. Assumes 8 ft ceilings, 2 coats, standard coverage (350 sq ft/gal):

Small (10x10)
1 gal
Walls only
Medium (12x14)
1.5-2 gal
Avg bedroom
Large (16x20)
2-3 gal
Living room
Whole house
15-20 gal
Interior 2,000 sq ft

Trim, Doors, and Ceilings

Walls are usually the biggest number, but trim and ceilings matter. Quick rules of thumb:

  • Ceiling: same as floor area. A 12x14 room = 168 sq ft = ~0.5 gal
  • Trim (baseboard + casing): 1 quart per 60-80 linear feet
  • Doors: 1 quart covers ~4 standard doors (both sides, 2 coats)
  • Windows: 1 quart covers ~6-8 window frames (interior)
Drying vs Curing
Most latex paints are dry to touch in 1-2 hours and recoatable in 4 hours. But full cure (maximum hardness and washability) takes 21-30 days. Avoid scrubbing or hanging heavy items in the first 2 weeks - the paint bonds to the wall as it cures.
Reduce Waste
Always buy an extra quart to keep for touch-ups. Paint dye lots vary slightly between batches, and trying to match a color perfectly 6 months later is frustrating. Store the extra can upside-down (briefly, to seal the lid) and label it with room name + color code.

Frequently Asked Questions