Finish quantity is where furniture makers over-buy most. A dining table has maybe 60 square feet of surface, and at three coats that is under half a quart — while the smallest can most stores stock is a quart and the impulse is to buy a gallon.
Working out the area
Add up every face. A tabletop is length times width, twice, because the underside counts. Add the edges, the aprons inside and out, and four legs at roughly four times their width times their length.
A 36 by 72 inch table comes to around 36 square feet for the top and underside, plus perhaps 12 for aprons and 8 for legs. Call it 56 square feet.
Enter that as a single rectangle in the area field — the calculator does not care about the shape, only the total.
Coverage is per coat and finishes differ
Wiping varnish and oil finishes go on very thin and wipe back off, so a gallon covers 500 to 600 square feet per coat. Brushed polyurethane lays down thicker at 400 to 500.
Shellac and lacquer sprayed come in lower per coat because of overspray, but they need more coats, so the total is similar.
Three coats is the normal minimum for furniture. A tabletop that takes wet glasses wants four or five, and the difference between three and five coats is the difference between a finish that lasts two years and one that lasts twenty.
Finish both faces or the panel cups
This is the one that costs people a tabletop. Finish slows moisture exchange but does not stop it. A board finished on one face only exchanges moisture faster on the bare side, so the two faces reach different moisture contents and the board cups.
The underside of a top, the inside of a case, the back of a door, the bottom of a drawer — all of it gets finish, even though nobody will see it.
It does not need to be the same number of coats or the same care. It needs to be there.
End grain drinks
End grain absorbs several times what face grain does. On a table with breadboard ends, a butcher block, or any exposed end, the first coat will disappear into it and look starved.
The usual approach is to hit the end grain first with a thinned coat, let it soak and cure, then finish everything together. A sanding sealer or a coat of dewaxed shellac does the same job.
Left unsealed, end grain also pulls in stain much darker than the faces, which is why a stained tabletop often has dark stripes at the ends.
Buy small and buy fresh
Oil-based finishes cure in the can once air gets in. A part-used quart of polyurethane is often unusable a year later, skinned over or full of lumps.
So the right purchase is the smallest container that covers the job, not the best value per gallon. A half-used gallon that has gone off is more expensive than two quarts.
If you do have leftovers, decant into a smaller jar so there is less air above the finish, or use one of the inert gas sprays sold for the purpose.