Woodworking calculator
Finish Coverage Calculator
Buying finish is easy to get wrong: too little stops a job halfway, too much wastes money and shelf life. Enter the surface area you are coating, how many coats you plan, and the spread rate printed on the can, and this calculator returns how much finish to buy, including a sensible allowance for what soaks in and gets lost on the brush.
Spread rates are typical defaults; always use the figure printed on your product for the most accurate result.
How it works
Coverage is just area divided by spread rate. The raw amount of finish for one coat equals the surface area divided by the square feet one unit covers, and the total scales with the number of coats. The calculator then adds a waste factor, ten percent by default, for the finish that soaks into open grain, clings to the applicator, and is lost to overspray and cleanup.
Spread rates vary widely by product and by how porous the wood is, so the figure on the can is always the most accurate input. As a rough guide, a coat of oil-based polyurethane covers around 400 square feet per gallon and thinner finishes like shellac and lacquer cover less per coat because more coats are expected. Bare and open-grained woods drink the first coat, so the real coverage on coat one is often lower than the label rate.
Remember to count every face you are finishing. A tabletop finished on both sides has roughly double the area of the top alone, and edges and aprons add more, so measure the true coated surface rather than just the footprint of the piece.
Worked example
A small table with 60 sq ft of surface, 3 coats of polyurethane at 400 sq ft/gal, plus a 10% waste allowance: (60 / 400) × 3 × 1.1 = 0.49 gallons.
Frequently asked questions
How much finish do I need per coat?
Divide the surface area in square feet by the spread rate on the can, the square feet one gallon covers. That gives the gallons for a single coat before adding any allowance for waste and soak-in.
How much extra should I buy for waste?
A ten percent allowance is reasonable for most jobs to cover soak-in, brush loss, and cleanup. Add more for very open-grained woods or spraying, where losses to absorption and overspray run higher.
What spread rate should I use?
Always use the rate printed on the product, since it varies by finish type and formulation. As a rough default, oil-based polyurethane covers about 400 square feet per gallon for one coat on sealed wood.
Do I count both sides of a panel?
Yes. Finish the back of a tabletop or panel to balance moisture exchange, and count that area too. A board finished both faces has nearly double the area of the visible surface alone, plus edges.
Why does the first coat use more?
Bare and open-grained wood absorbs finish, so the first coat soaks in and covers less area than later coats over a sealed surface. Build in extra for that first coat on raw or porous species.
Related calculators
Sources
These calculators are for planning and estimation. Engineering results (shelf sag, wood movement) use published average material properties; real boards vary by grade, grain, moisture and defects. Verify load-bearing designs with a professional.