We have all heard the stories about the professional painter who comes to a site, eyeballs it, and knows exactly how much paint to buy.

Professional painter and author Jack Luts (The Complete Guide to Painting Your Home) claims to always know how much paint will cover the exterior of a home by counting the rooms and hallways inside a home. His single-coat exterior formula is to allow one gallon of paint for each room in the house and one-gallon for each hallway. For interiors, he estimates that applying one coat on four walls of a typical 12’ x14’ room requires one gallon of paint. But, the 200,000 gallons of leftover household latex paint that Metro receives annually indicates that calculating how much paint to buy can be difficult for do-it-yourselfers.Even a Google search using the key words “paint calculator” or “paint estimator” will come up with numerous sites, all of which have disclaimers that at best the estimates are “rough” calculations.

Generally, paint can labels indicate that a gallon of paint will cover 300 to 400 sq ft. This calculation applies to ideal conditions on a primed or smooth surface.

The real spread rate may differ depending on surface irregularities, wall porosity, application method, air and surface temperature, and any loss in mixing (boxing) multiple cans of paint together. The quality of the paint, especially its hiding capacity, also will influence its spread rate.Taking these variables into account can help you to approximate how much paint is required for a project when measuring the square footage of the surfaces to be covered. Professional painters do not deduct for the windows and doors, since there always is some paint wasted from drips and spills, and some paint is needed for touch-ups.

**Interior**

On an interior surface total the width of all the walls to determine the perimeter or distance around the room. Because most rooms are not perfect boxes and may have alcoves there may be more than four wall areas to add together to calculate the perimeter.

Multiply the perimeter by the height of the room to find the square footage to be painted. To estimate how much paint is required for the ceiling simply multiply the room width by the room length for the ceiling area.

Read the paint can label to determine the typical spread rate. Divide the wall area or ceiling area by this number to determine how many gallons of paint to buy. Always round up when estimating how much paint to buy. Rough or textured walls, flocked or popcorn ceilings, walls that have cracks or nail holes can double the amount of paint needed to complete the project. Always double the amount of paint required for a project if two coats are required.

**Exterior**

To estimate the paint required for an exterior project consider the walls and gables as separate areas to be added up.

Measure the width or length of each wall. Measure the height from the top of the foundation to the roofline. This can be done easily by measuring the width of one clapboard or siding row. Multiply this number by the number of siding rows.

Multiply the height by the width of each wall to determine the square footage of each side of the structure. To determine the square footage of a gable (the triangular area of the wall under a pitched roof) measure the height from the gable’s base to roof peak. Multiply the height by the gable base width (the same width as at the ground). Divide this number by 2 because a gable is a triangle.

Add the square footage of the gables, sides, front, and rear of the structure together to get the total square footage. Divide the total by the estimated coverage for a gallon of paint to determine how many gallons of paint to buy.

Exterior painting is at best a guesstimate because the painted area will usually be greater than the measurement due to the rough nature of siding and the added paint required for the edges and underside of lap or bevel siding.

**Priming**

Do-it-yourselfers can come closer to the spread rate on the can by priming and sealing bare surfaces, painting when temperatures are between 50 and 85 degrees Fahrenheit, and using a good quality nylon or polyester brush.

If it looks like there will not be enough paint to finish a project do not use up all the paint that is left before buying more paint. Even custom mixed colors vary between batches. Mix the paint that is left with the paint that is purchased to complete the project in uniform color.

Since no one wants the frustration of running out of paint most people buy more paint than they need. After keeping some paint for touch-up, leftover paint can be taken to Metro for safe disposal. Call 503-234-3000 for locations and times.

*Paulette Rossi is a Certified Master Recycler in Portland, OR.*