Wood stain and finish calculator

Coverage comes from the product label, and it depends on the wood as much as the product: sanded hardwood takes far less than rough or weathered boards. No can yet? Pick a typical figure to plan before you buy. New to this? Words explained.

How much finish you need

Divide the area by the product’s coverage to get the amount per coat, multiply by the number of coats, and add a little for what stays in the brush and tray.

finish = area × coats ÷ coverage × (1 + extra)
1 US gallon = 4 quarts ≈ 3.79 L
1 m²/L ≈ 40.7 sq ft per gallon

Example: a 100 sq ft project with two coats of interior stain at a label coverage of 500 sq ft per gallon needs 0.4 gallon, or about 0.44 gallon with 10% extra — 2 quarts. Worked examples with the numbers shown: How much stain for a 12x16 deck and How much polyurethane for a dining table.

Where the coverage figure comes from

Coverage is set by the manufacturer and printed on the can or its technical data sheet, usually as square feet per gallon or square metres per litre, often as a range. Use that figure. The table below only shows the kind of ranges labels tend to give, so you can plan before you have chosen a product — it is not a measurement, and two stains in the same category can differ a lot.

The wood matters as much as the can. The finishing chapter of the USDA Wood Handbook notes that rough-sawn and weathered surfaces take up more penetrating stain than smooth, planed ones, which is why the same deck stain goes much further on new boards than on old grey ones. Where the label gives a range, use the low end for rough or old wood.

Measuring the area

• Tables and shelves: top area, plus edges, plus the underside if you finish it (you should, to balance moisture exchange).
• Decks: floor area plus rails, balusters and stair treads. Balusters add up fast — a 2×2 baluster 36 in tall has about 1 sq ft of surface.
• Fences: length × height × 2 for both sides, plus posts and rails.
• Trim: linear feet × face width (in ft).

Choosing the can size

Pick the size you will actually buy and the calculator counts cans of it. The line below it gives the least finish that covers the job in gallons and quarts (or 2.5 L and 750 ml tins). A part-used can of oil-based finish tends to skin over, so on small jobs the smaller can is often the better buy even when a gallon costs less per quart.