How Much Paint Do I Need for a Room? (Exact Formula + Calculator)

Buying the right amount of paint saves money, reduces waste, and avoids the frustration of running out mid-wall. One gallon of paint covers approximately 350 square feet, and the average bedroom takes 1.5 to 2 gallons for a single coat. Here is the exact formula to calculate what you need for any room.

The Paint Coverage Formula

The calculation is straightforward: measure the total wall area, subtract doors and windows, then divide by the paint's coverage rate.

Step 1: Calculate the room's perimeter by adding the length of all walls. For a rectangular room, that is (length + width) x 2.
Step 2: Multiply the perimeter by the ceiling height to get the total wall area.
Step 3: Subtract the area of doors and windows.
Step 4: Divide by the coverage rate (typically 350 square feet per gallon).
Step 5: Multiply by the number of coats needed.

Worked example: A 12x14 foot room with 8-foot ceilings, two windows, and one door. Perimeter = (12 + 14) x 2 = 52 feet. Wall area = 52 x 8 = 416 square feet. Subtract one door (21 sq ft) and two windows (15 sq ft each) = 416 - 51 = 365 square feet. At 350 sq ft per gallon, you need 365 / 350 = 1.04 gallons for one coat, or 2.08 gallons for two coats. Buy 2.5 gallons (two gallons plus a quart) to have a small amount for touch-ups.

How to Measure Your Room

Use a tape measure to measure the length of each wall at the base. Do not assume opposite walls are equal, especially in older homes where rooms may not be perfectly rectangular. Measure the ceiling height from floor to ceiling at any point along the wall.

For rooms with angled or vaulted ceilings, measure the height at the tallest point and shortest point and use the average. For rooms with chair rail or wainscoting where you are painting only above or below a line, measure from that line to the ceiling or floor rather than the full wall height.

Subtracting Doors and Windows

Standard measurements for common openings save time:

Standard interior door: 21 square feet (3 feet wide by 7 feet tall)
Standard window: 15 square feet (3 feet wide by 5 feet tall)
Sliding glass door: 42 square feet (6 feet wide by 7 feet tall)
French doors (pair): 42 square feet

If your doors or windows are non-standard sizes, measure the actual width and height and multiply them together. Deducting these areas prevents you from buying too much paint.

One Coat or Two?

Two coats is the standard for most paint jobs. A single coat is sufficient only when you are refreshing the same color or going from a lighter shade to a very similar lighter shade. Two coats are necessary when changing colors, when painting over a darker color with a lighter one, when using a sheen change (switching from flat to semi-gloss), or when painting over a patched or repaired wall surface.

If you are making a dramatic color change, such as painting over a dark red or navy blue, consider using a tinted primer first. A primer coat covers dark colors far more effectively than an extra coat of finish paint, and primer is cheaper. Without primer, you may need three or even four coats of finish paint to fully hide a dark base color.

Paint Coverage by Type

Different paint finishes cover at different rates because of their varying viscosity and solids content:

Flat/matte finish: 350-400 square feet per gallon. Best for ceilings and low-traffic walls. Hides imperfections well but is harder to clean.
Eggshell finish: 300-350 square feet per gallon. The most popular choice for living rooms and bedrooms. Easy to clean with a slightly soft sheen.
Satin finish: 300-350 square feet per gallon. Good for kitchens, bathrooms, and kids' rooms. More washable than eggshell.
Semi-gloss finish: 250-300 square feet per gallon. Ideal for trim, doors, cabinets, and moisture-prone areas. Very durable and easy to clean.
Primer: 200-300 square feet per gallon. Coverage varies widely by brand and type. Primer designed for dark colors or stain blocking covers less than general-purpose primer.

Quick Reference for Common Room Sizes

Here are approximate paint amounts for standard rooms with 8-foot ceilings, one door, and two windows, assuming two coats:

Small bedroom (10x10): 2 gallons
Average bedroom (12x12): 2.5 gallons
Large bedroom (14x16): 3.5 gallons
Living room (16x20): 4.5 gallons
Bathroom (5x8): 1 gallon

These are estimates. Always measure your specific room, because ceiling height, number of windows, and door count can significantly change the calculation.

Use Our Free Paint Calculator

Our paint calculator does all the math instantly. Enter your room dimensions, ceiling height, number of doors and windows, paint type, and number of coats. It shows the exact gallons needed plus a recommended purchase amount rounded to standard can sizes. The average gallon of interior paint costs $30-60, so accurate calculation can easily save you $30-$60 by avoiding an unnecessary extra gallon. No signup, works on your phone, runs entirely in your browser.