How to Calculate Paint for Any Room: A Complete Guide

The Basic Formula

At its core, calculating paint comes down to three numbers: total wall area, coverage rate per gallon, and number of coats. The formula is:

A standard gallon of interior latex paint covers approximately 350–400 square feet on a smooth surface. Most paint jobs require two coats for even color and proper coverage, especially when changing from a dark to a light shade.

Step 1: Measure Your Walls

Measure the length and height of each wall in the room. For rectangular rooms, you can measure the room's perimeter (total length of all walls) and multiply by the ceiling height. Most rooms have 8-foot (2.4 m) ceilings, but always measure — older homes may have 9 or 10-foot ceilings, and basements may be shorter.

Example: A 12 × 14-foot room has a perimeter of 52 feet. With 8-foot ceilings, total wall area is 52 × 8 = 416 sq ft.

Step 2: Deduct Doors and Windows

You don't paint over doors and windows, so subtract their area from the total. Standard deductions used by professionals:

• Standard door: 21 sq ft (3 ft × 7 ft)
• Average window: 15 sq ft (3 ft × 5 ft)
• Large window / sliding door: 35–60 sq ft

Continuing our example: the room has 1 door and 2 windows. Deductions = 21 + (2 × 15) = 51 sq ft. Paintable area = 416 − 51 = 365 sq ft.

Step 3: Account for Coats

Two coats is the standard for most paint jobs. You may need only one coat if you're repainting with the same color and using a premium paint with built-in primer. Plan for three coats if you're painting over a very dark color with a light shade, or if you're using a bold red or yellow which are notoriously hard to cover.

With our 365 sq ft room and 2 coats: 365 × 2 = 730 sq ft of paint coverage needed.

Step 4: Divide by Coverage Rate

Check your paint can for the manufacturer's stated coverage rate. Most interior latex paints cover 350–400 sq ft per gallon. Using the conservative end (350 sq ft):

730 ÷ 350 = 2.09 gallons. Round up to 3 gallons (paint is typically sold in gallons, not fractions). Alternatively, buy 2 gallons and 2 quarts.

Surface Texture Matters

The 350–400 sq ft coverage rate assumes a smooth, previously painted surface. Rough or textured surfaces absorb more paint and reduce coverage significantly:

• Smooth drywall: 350–400 sq ft/gallon
• Light texture (orange peel): 300–350 sq ft/gallon
• Heavy texture (knockdown, popcorn): 200–300 sq ft/gallon
• Raw/unprimed drywall: 200–250 sq ft/gallon
• Brick or stucco: 150–200 sq ft/gallon

If you're painting textured walls, adjust your coverage rate downward. This is one of the most common reasons people run short.

Common Mistakes to Avoid

1. Forgetting the waste factor. Even careful painters lose 5–10% to roller trays, brush loading, drips, and touch-ups. Add 10% to your calculated amount.

2. Mixing up wall area and floor area. Your room's floor might be 168 sq ft (12 × 14), but the wall area is 416 sq ft. Always calculate walls, not floor.

3. Assuming one coat is enough. Even "one-coat" paints rarely deliver perfect coverage in a single application, especially over repairs or when changing colors.

4. Not buying enough of the same batch. Paint colors can vary slightly between batches. Buy all the paint you need at once, from the same batch if possible, and have the store shake all cans together.

5. Ignoring the ceiling. If you're painting the ceiling too, calculate its area separately. Ceiling paint typically has similar coverage but is formulated to reduce dripping.

Quick Reference Table

Trim, Doors, and Accent Walls

Trim paint (for baseboards, crown molding, and door frames) is sold in quarts. A typical room requires about 1 quart of trim paint. For doors, plan 1 quart per two standard doors.

If you're painting an accent wall in a different color, calculate it separately. Measure that single wall and apply the same formula with the accent paint's coverage rate.

Try Our Free Paint Calculator

Don't want to do the math by hand? Our Paint Calculator handles all of this automatically. Enter your room dimensions, specify doors and windows, choose your number of coats, and get an instant estimate — including the 10% waste factor built in.

Key Takeaways

• Measure walls (perimeter × height), not floor area
• Subtract doors (21 sq ft each) and windows (15 sq ft each)
• Plan for 2 coats minimum; 3 for dramatic color changes
• Use 350 sq ft/gallon as a conservative coverage estimate
• Add 10% for waste and touch-ups
• Buy all your paint from the same batch for color consistency
• Adjust coverage rate downward for textured surfaces