Paint Calculator

Enter your room dimensions to get the exact amount of paint, primer and supplies you need — with doors, windows, ceiling and multiple coats taken into account.

Dimensions Room length (ft) Room width (ft) Wall height (ft) Include the ceiling

Openings & coats Doors (count) Windows (count) Number of coats Include primer

Paint & pricing Coverage per gallon (sq ft) Price per gallon ($)

Coverage figures come from typical manufacturer labels. Always round up — leftover paint is cheaper than a second trip.

Estimates only. Textured walls, high ceilings, sprayers and major colour changes can all increase the amount you need. Always buy slightly more than the calculation suggests.

How this paint calculator works

The calculator measures the actual surface you are going to paint, then divides it by the coverage printed on the paint can.

Wall area. The perimeter of the room (2 × length + 2 × width) multiplied by the wall height.
Openings subtracted. Each door counts as 21 square feet and each window as 15 square feet — the standard allowances used by paint manufacturers.
Ceiling. Length × width, added only if you tick the box.
Coats. The net area is multiplied by the number of coats you select.
Gallons. The result is divided by coverage per gallon and rounded up to the next whole gallon.
Primer. Calculated separately at 250 square feet per gallon, one coat.

How many coats of paint do you actually need?

Two coats is the standard, and it is what the calculator assumes. One coat only works when you are refreshing the same colour on a sound surface. You need three coats — or a primer plus two — when you are painting bare drywall, going from a dark colour to a light one, or covering a strong colour such as red or navy.

The cheapest way to reduce the number of coats is primer. A gallon of primer usually costs less than a gallon of quality topcoat, and it often saves you an entire coat of paint.

Do not forget the supplies

Paint is rarely the whole cost. A typical room also needs a roller and sleeve, a tray, an angled brush for cutting in, painter's tape, a drop cloth and sandpaper. The calculator adds a fixed allowance for these so the total is closer to what you will actually spend at the register.

Paint calculations by project

Frequently asked questions

How much paint do I need for a 12x12 room?

A 12x12 room with 8-foot ceilings has about 384 square feet of wall. After subtracting one door and two windows you are at roughly 333 square feet. At two coats and 350 square feet per gallon, that is about 2 gallons.

How much does a gallon of paint cover?

Most interior wall paint covers 350 to 400 square feet per gallon on a smooth, previously painted surface. Bare drywall, stucco and textured walls drop to 250 to 300 square feet per gallon.

Should I buy extra paint?

Yes. Buy one extra quart if you are close to a whole gallon, and keep the label and the tint code. Touch-ups a year later will never match if you cannot reproduce the colour.

Does this calculator work for exterior painting?

The maths is the same, but exterior surfaces are rougher and absorb more paint. Use the 250 or 300 square feet per gallon option for siding, fences and decks.