Interior paint calculator

How this calculator works

It measures the four walls as one long strip, subtracts openings, multiplies by the number of coats and divides by the paint’s coverage. Then it adds a waste allowance and rounds up to cans you can actually buy.

Worked example

A 14′ × 12′ 6½″ bedroom with 8′ walls has 2 × (14 + 12.54) × 8 = 424.7 sq ft of wall. Take off one door (21) and two windows (30) and 373.7 sq ft is left to paint. Two coats at 350 sq ft per gallon is 2.14 gallons; with 10% waste that is 2.35 gallons, which the list buys as 2 gallons plus 2 quarts.

Tips before you buy

• Measure each wall at the floor, to the nearest inch. Rooms are rarely square.
• Buy all the paint for a room from one batch, or mix the cans in a bucket before you start.
• Keep one quart back for touch-ups and label it with the room and date.
• Painting the ceiling too? It is measured separately; ceiling paint is a different product.

Questions

How many square feet does a gallon of paint cover?

Most interior wall paints cover 350 to 400 square feet per gallon for one coat on a smooth, primed wall. Textured walls, fresh drywall and deep colors drink more, so this calculator defaults to the conservative end: 350 sq ft.

Do I need two coats?

Plan on two coats whenever you change color, paint over patches, or use a lighter color over a darker one. A single coat can be enough when you refresh the same color on a wall in good condition.

When do I need primer?

Prime new drywall, bare patches, stains and glossy surfaces, and when going from a very dark color to a light one. Paint-and-primer products help on sound walls but do not replace a primer on raw drywall.

Should I buy quarts or another gallon?

Quarts top up a small shortfall. Three quarts usually cost about as much as a gallon, so the list switches to a full gallon at that point. Buying slightly over also leaves paint for touch-ups from the same batch.