Paint Calculator

The short answer

A gallon of latex paint covers roughly 350 to 400 square feet in one coat on a previously painted surface in good condition. A 12 by 14 foot room with an 8 foot ceiling has about 416 square feet of wall and needs about 1.2 gallons for two coats. Add ten percent for cutting in and waste, and most people end up buying two gallons.

What it depends on

Surface condition changes the figure more than area does. Previously painted walls in good condition are close to the rated coverage; new plasterboard, bare wood and patched areas absorb noticeably more. Textured or popcorn ceilings reduce coverage further. A dark-to-light colour change needs three coats because first-coat coverage on a colour change is poor.

Where this stops being useful

Coverage rates on the tin assume ideal application with good preparation, and real projects fall short, especially on first-time paint. Measuring walls gross and forgetting to deduct doors, windows and cupboards is the most common cause of buying too little. Paint quality and age both affect spread rate — verify with a wet film gauge for larger jobs. General information for planning, not professional advice.

Coverage rates on the tin assume ideal application with good preparation, and real projects fall short, especially on first-time paint. Measuring walls gross and forgetting to deduct doors, windows and cupboards is the most common cause of buying too little. Paint quality and age both affect spread rate — verify with a wet film gauge for larger jobs. General information for planning, not professional advice.

he coverage figure on the tin is measured on a smooth, sealed surface. Bare plaster, fresh drywall, unpainted masonry and textured walls all absorb appreciably more on the first coat, which is why priming those surfaces is not optional. The stated coverage also assumes the paint is applied evenly — a heavy-handed roller or a poorly thinned paint reduces it. If you are painting a dark colour over a light one, or the reverse, budget an extra coat. Trying to stretch a tin to save money costs more in time and materials than buying the correct quantity.

Paint is one of the few materials where the cost of getting it wrong is not the paint. Reordering mid-project means drying the walls properly before the second application, and a rushed coat is visible for years.

Paint quantity depends on the number of coats, so decide that before you buy rather than discovering you are short. Cutting in around windows and doors takes longer than rolling the wall and uses more paint because the brush is less efficient, so allow more time than the wall area suggests. If the room will later be redecorated, choose a quality finish rather than the cheapest — repainting over a poor surface costs more in preparation than the paint itself ever cost.

Paint is sold by volume but its job is to cover area, so the whole calculation is about turning a room into square feet of paintable surface. The starting point is the wall area: twice the sum of the room's length and width, multiplied by the ceiling height. Add the ceiling if you are painting it, and subtract openings, because a door or a window is about twenty square feet of wall you do not have to cover. Coverage per gallon is the number that decides the answer, and it is printed on the tin. Most interior paints cover between 350 and 400 square feet per gallon on a smooth, sealed wall for a single coat, but that figure falls sharply on rough, porous or unpainted surfaces, which drink the first coat. If you are painting bare plaster or fresh drywall, assume less coverage and expect to need a primer. Coats are the other variable people underestimate. One coat rarely gives an even finish unless you are refreshing the same colour; a colour change usually needs two, and a dramatic change, or covering a dark colour with a light one, sometimes three. Multiply the wall area by the number of coats before dividing by coverage, or you will buy half the paint you need. This calculator gives the paintable area, the total area including coats and the number of gallons, rounded up because paint is sold in whole containers. Keep a little left over for touch-ups. It assumes standard openings and a fixed coverage figure — measure your own walls rather than trusting a nominal room size, and check the coverage claim on the tin you actually buy.

This calculator returns estimates for planning and general information only. It is not engineering, architectural, structural, electrical or code advice, and it cannot replace a qualified professional who has seen the site. Material quantities depend on the actual dimensions, the product you buy and local building codes, so confirm the figures before ordering materials or starting work, and always follow local regulations.