How Much Paint Do I Need

Figuring out how much paint do I need is one of the most common questions before any painting project. This calculator gives you a fast answer. Enter your room dimensions or known square footage below, adjust coats and coverage, and get an accurate gallon estimate with waste and primer factored in.

1. Choose your project type. Select "Entire room" for walls (with optional ceiling), "Single wall" for an accent wall, "Ceiling only" for overhead surfaces, or "Known square footage" if you've already measured.
2. Enter dimensions carefully. For a standard bedroom, you might enter 12 ft × 10 ft × 8 ft. The calculator multiplies the perimeter by wall height to get total wall area.
3. Set door and window counts. Each standard door subtracts 20 sq ft (a 3 ft × 6 ft 8 in opening). Each window subtracts 15 sq ft (a typical 3 ft × 5 ft window). The calculator removes these unpainted areas automatically.
4. Toggle ceiling on if you're painting the ceiling too. The calculator adds length × width to your paintable total. Ceiling paint often has different sheen, so buy it separately from wall paint.
5. Adjust coats and coverage to match your paint. Most interior paints need 2 coats at 350 sq ft per gallon. Check the label on your specific paint can. Sherwin-Williams Duration covers 350 to 400 sq ft, while Behr Premium Plus covers 250 to 400 sq ft.
6. Turn on primer if you're painting new drywall, covering stains, or switching from dark to light. The calculator uses separate primer coverage (default 200 sq ft per gallon) since primer is thicker than finish paint.
7. Add trim if you're painting baseboards, door casings, or crown molding. Enter the total linear feet of all trim pieces.
8. Click "Calculate paint" to see how much paint you need in gallons, with waste and purchase suggestions.

Pro tip: Measure at three spots along each wall and average the numbers. Older homes have walls that bow or lean, so a single measurement can be off by an inch or more. That small difference adds up across four walls.

When to use primer

When figuring out how much paint you need, primer changes the math. Skip primer on walls that are already painted in a similar color with no stains or damage. Use primer when painting over new drywall, joint compound patches, water stains, smoke damage, or when switching from dark to light colors. One coat of quality primer (like Kilz 2 or Zinsser Bulls Eye 1-2-3) often saves you from needing a third coat of expensive finish paint.

How much paint do I need by surface type

Touch-up reserve

Part of calculating how much paint you need is planning for the future. Always keep leftover paint from the same batch. Later batches from the store can look slightly different, especially with custom-mixed colors. Store the can upside down (with lid sealed tight) to create an airtight seal. Leftover latex paint stored properly lasts 2 to 10 years.

Four numbers determine how much paint you need: paintable area, openings to subtract, coats, and coverage rate. This calculator measures your surface area, removes doors and windows, multiplies by coats, and divides by your paint's coverage per gallon. Every step uses numbers you can verify against your room measurements and the label on your paint can.

Wall area

For a full room, the calculator finds wall area from the perimeter. Add room length and width, double it for the full perimeter, then multiply by wall height. A 10 × 10 ft room with 8 ft walls: perimeter = 2 × (10 + 10) = 40 ft, wall area = 40 × 8 = 320 sq ft.

Door and window deductions

Doors and windows don't need paint, so the calculator subtracts them. Each standard interior door (3 ft × 6 ft 8 in) removes about 20 sq ft. Each typical window (3 ft × 5 ft) removes 15 sq ft. A room with 1 door and 2 windows loses 50 sq ft total. For oversized openings like sliding glass doors, reduce the door count and enter the known area mode instead.

Coats and coverage

Multiply your paintable area by the number of coats. Most paint jobs need 2 coats for even color. Divide the total coat area by coverage per gallon (from the paint can label) to get exact gallons, then round up. Paint stores sell whole gallons and quarts, not fractions.

Waste factor

Understanding how much paint you need means accounting for waste. The default 10% factor covers paint left on rollers, in the tray, on cut-in edges, and absorbed into texture. Bump this to 15% for heavily textured walls or when using a paint sprayer (overspray wastes 20% to 30% of product).

Worked example: 10 × 10 ft bedroom

Given:

• Room = 10 ft × 10 ft, wall height = 8 ft
• 1 door, 2 windows, no ceiling, 2 coats
• Coverage = 350 sq ft/gal, waste = 10%

Calculations:

• Perimeter = 2 × (10 + 10) = 40 ft
• Wall area = 40 × 8 = 320 sq ft
• Deductions = (1 × 20) + (2 × 15) = 50 sq ft
• Paintable area = 320 − 50 = 270 sq ft
• Total coat area = 270 × 2 = 540 sq ft
• With 10% waste = 540 × 1.10 = 594 sq ft
• Gallons = 594 / 350 = 1.70, rounded up = 2 gallons

Buy 2 gallons. That's how much paint you need for a standard 10 × 10 bedroom with 2 coats. You'll have enough for both coats plus some left over for touch-ups after furniture goes back in.

Primer calculation

When determining how much paint you need, keep primer separate. Primer goes on thicker than finish paint. Most primers cover 200 sq ft per gallon compared to 350 for finish paint. For the same 270 sq ft bedroom, one coat of primer needs 270 × 1.10 / 200 = 1.49, rounded up to 2 gallons. New drywall absorbs primer heavily, so the lower coverage rate accounts for that.

Vaulted and cathedral ceilings

Standard wall height calculations assume flat ceilings at 8 or 9 ft. Vaulted ceilings add triangular wall area above the plate line that this calculator does not automatically measure. For vaulted rooms, measure the wall height at the peak and at the lowest point, average them, and use that as your wall height. Or switch to "Known square footage" mode and enter the total measured area yourself.

Estimate accuracy

These results are approximations based on standard coverage rates and door/window sizes. Actual paint needs vary by surface condition, paint brand, application method (brush, roller, or sprayer), and how evenly you apply each coat. Always round up when buying and keep receipts for returns.