How Much Polyurethane Do I Need?

This article defines polyurethane and shares some of its common uses. It tells you how to interpret manufacturer’s coverage estimates and how to plan for multiple coats of polyurethane. By the end, you’ll feel confident in estimating how much polyurethane you need for your next wood finishing project. 

What Is Polyurethane?

Polyurethane, called ‘poly’ for short, is a clear, tintable liquid applied to wood with a sprayer, brush, roller, or rag. It can be oil or water based. It is a type of varnish that hardens through two phases.

• In the first phase, the binders or solvents in the polyurethane evaporate, leaving it dry to the touch. This happens in about six hours for water-based polyurethane or 24 hours for oil-based products. 
• In the second phase, the film formed by polyurethane contracts as the molecules bind more closely together. This polymerization is called ‘crosslinking’, and happens as the finish is exposed to air. For water-based poly, this takes about 20 days. It takes up to 30 days for oil-based poly to cure. 

How Much Polyurethane Do I Need?

Most polyurethane products cover between 300 and 600 square feet per gallon, and require between two and six coats to reach full protection.

The average American living room is 330 square feet. Planning for four coats, you need enough poly to cover 1,320 square feet, which would require two gallons and one quart of polyurethane that covers 600 square feet per gallon, or four gallons and one quart of polyurethane with a lower coverage estimate. 

Polyurethane Uses

Polyurethane is a popular choice for sealing and protecting wood that is exposed to high levels of use or foot traffic. It repels moisture, staves off mold, mildew, and fungus. Of all the film-forming finishes, polyurethane is the most resistant to impact damage such as scuffing, denting, or scratches. 

For this reason, it is popularly used as a top coat for wooden floors, work surfaces, and tabletops. It can also be used to finish interior furniture and trim.

Polyurethane can be used to protect exterior wood, but the inflexible barrier it forms on the surface of woods makes it less able to respond to changes in temperature and moisture content than penetrating oil finishes. 

It has a high-gloss sheen, but satin and semi-gloss versions are also available. It can be mattified through scuffing with abrasive material. Polyurethane can be tinted to act like a stain, or mixed with paint for more opaque coverage. 

How Much Wood Does a Gallon of Polyurethane Cover? 

Polyurethane is a buildable finish that usually requires multiple coats to achieve heavy-duty protection. Thinner coats will build the finish more slowly, resulting in a more natural sheen, while thicker coats tend to look plasticy. 

The coverage of a gallon of polyurethane is dependent on the product. It is listed on the container in square feet per gallon. This measurement is only an estimate, and there are several factors that influence how accurate it is. 

At the low end, some manufacturers estimate that a gallon of their product will cover 300 square feet. This kind of polyurethane is likely to go on very thick, which increases the risk of defects like hazing or bubbles. It is more difficult to apply than thinned polyurethane. Thinning polyurethane will increase the amount of wood you can cover with a single coat, but more coats may be necessary to achieve the same finish. 

At the other end of the scale are manufacturers who claim that one gallon can cover as much as 600 or 700 square feet. Polyurethane with this label is usually thinner and easier to apply, but again, more coats may be necessary, so it might not save you any money on materials. 

Plan on lightly sanding the polyurethane in between coats.

Read more about how many coats of wipe-on poly you need.

How To Calculate Polyurethane Coverage

Before you start your next poly project, you will need to calculate how much polyurethane you need. To make use of the manufacturer’s estimate, measured in square feet per gallon, you need to know how many square feet you have to cover. 

• For flat surfaces, such as wooden floors, decking, fencing, or trim, this requires calculating the area.
• For objects with multiple surfaces, such as furniture, railings, and balusters, you will need to compute the surface area. 

The formula for area is length multiplied by width. For a square room with walls ten feet on each side, the area of the floor is 100 square feet. 

To find the surface area of a three-dimensional object, find the area of each face and add them together, or use a surface area calculator. 

A table might have a top seven feet long and three feet wide, for an area of 21 square feet. If the legs of the table are square shaped posts, four inches to a side and 36 inches tall, the area of each side is one square foot, for a total of four square feet per post, or 16 square feet if you add all four posts together. 

If you were going to apply polyurethane to the entire table, you’d need to double your calculation of the table top, to account for polyurethane on both sides. 

As a rule of thumb, if a surface is less than a few inches wide or tall, such as the edge of a fence, trim, or tabletop, it’s not usually worth calculating the area. 

Instead, play it safe by rounding up your estimate to the nearest quart. This accounts for minor spills, drips, or splashes. 

Water-based polyurethane may take as many as six coats to build up a robust layer of protection. Four coats is usually the minimum. Oil-based polyurethane takes fewer coats; usually two or three. 

In the example of the wooden floor, assuming three coats of oil-based polyurethane, the total area to be covered is 300 square feet. One gallon of polyurethane would likely cover this area, regardless of brand. 

In the wooden table example, let’s say four coats of water-based poly are required. 37 multiplied by four gives us 148 square feet. Depending on the brand of polyurethane you choose, you might be able to get away with a single quart. 

Conclusion

Polyurethane is a clear, tintable liquid that hardens into a durable barrier that protects wood. It is most often used to protect interior furniture and wooden floors. You can estimate how much polyurethane you need by calculating the area or surface area of the wood you want to cover.

Manufacturers estimate the amount of wood their products cover, using square feet per gallon as the measurement of choice.