How much paint do I need? The complete method

The answer in one line

Paint needed is the area you are painting, divided by what one gallon covers in one coat, multiplied by the number of coats, plus a small allowance, rounded up to cans you can buy. Everything else in this guide is about getting each of those four inputs right, because each one has a common way to go wrong.

The wall paint calculator does this for a room in one step. The rest of this guide explains what it is doing, so you can trust the answer and adjust it when your room is not typical.

Step 1: measure the walls

Measure the length of each wall along the floor and the height from floor to ceiling in a corner. For a rectangular room, the wall area is the perimeter times the height: a 12 × 12 ft room with 8 ft walls has a perimeter of 48 ft and 384 sq ft of wall.

Rooms with alcoves, chimney breasts, bay windows or a sloping ceiling are easier wall by wall. The square footage calculator takes up to six walls separately and adds them up. Measure to the nearest half foot; more precision than that is lost in the rounding to cans anyway. The guide on measuring a room for paint covers the awkward cases.

Step 2: subtract the openings

Doors and windows are not painted with wall paint, so take them out. Estimators use standard sizes: 20 sq ft for a door and 15 sq ft for a window. Our example room has one of each, so 35 sq ft comes off and 349 sq ft is left to paint.

Do not subtract small things like outlets and switches, and do not subtract openings smaller than a window at all; the paint you save is less than the allowance. Large openings matter more: a patio door or a wide archway is close to two standard doors. The full list of sizes is in the standard sizes reference.

Step 3: find the coverage

The can says how much area a gallon covers in one coat, usually 350 to 400 sq ft for interior wall paint. That figure is measured on a smooth, sealed surface, and real walls rarely match it. We use 350 sq ft per gallon as the default for walls, the low end of what the data sheets print, because running short costs more than a little leftover.

Texture and porosity lower coverage further. On the same 349 sq ft room:

The paint coverage calculator applies these surface factors, and paint coverage explained goes into why the label figure rarely survives a real wall.

Step 4: count the coats

Two coats is the standard for any change of colour, and the one most people should plan for. One coat is enough only for a refresh in the same colour over sound paint. Three coats, or two over a tinted primer, are needed for big jumps in colour, especially to dark, saturated colours that cover poorly.

The coats calculator applies these rules to your colours and surface, and how many coats of paint explains each one.

Step 5: add an allowance and round up to cans

Some paint stays in the roller, the tray and the bottom of the can, and you want a little for touch-ups later. A 5% allowance covers both on walls; trim, which is brushed in narrow strips, needs about 10%.

Then round up to the cans that are actually sold: quarts, gallons and 5-gallon pails. Always round up. A can short means a trip to the store in the middle of a coat, and a second batch of a custom colour may not match the first. Our rule is gallons first, then quarts for the remainder, with four quarts becoming a gallon and anything over four gallons becoming a pail, because a pail costs less than four gallons and a quart. The can size optimizer finds the cheapest mix at your store’s prices.

A whole room, worked

Beyond walls: every interior surface

Each surface has its own coverage and its own way to measure it, which is why there is a calculator for each:

• Ceilings take flat ceiling paint and lose coverage to texture; a popcorn ceiling can need two thirds more paint than a smooth one. The ceiling paint calculator applies the texture.
• Trim is measured in linear feet and painted in quarts; the trim paint calculator turns lengths and casings into area.
• Doors are counted, not measured, with a factor for panels; the door paint calculator does both faces and the edges.
• Windows with their sashes are fiddly and small; the window frame and sash calculator counts them by size.
• An accent wall in a dark colour needs extra coats or a tinted primer; the accent wall calculator applies the colour-depth rules.
• Primer has its own coverage by surface; the primer calculator covers drywall, patches, stains and glossy surfaces.
• A whole house is quicker by room counts; the whole-house estimator gives wall, ceiling and trim paint for every room at once.
• Cabinets are counted by doors and drawer fronts; the cabinet paint calculator gives primer and enamel.
• Stairwells have tall walls and awkward shapes; the stairwell calculator takes each wall separately and warns about ladders.

Common ways the estimate goes wrong

• Forgetting the second coat. The single most common mistake. Divide by coverage, then multiply by coats.
• Trusting the 400 on the can for a textured wall. Use 350 on smooth walls and lower on texture.
• Subtracting too much. Only doors, windows and large built-ins come out; leave small openings in.
• Buying exactly the calculated amount. Round up to cans and keep the leftover for touch-ups.
• Mixing the ceiling and wall paint in one figure. They are different products with different coverage.

What to do with the number

Take the result to the store as a can list rather than a gallon figure: “2 gallons and a quart of wall colour, a gallon of ceiling white, a quart of trim enamel”. Ask for all the wall colour to be mixed at once, and box the cans (pour them together and stir) before starting, so the colour is identical from wall to wall. Keep a labelled jar of the leftover for touch-ups; the leftover and touch-up planner says how much is worth keeping.