What a pound cut actually means
One pound of dry shellac flake dissolved in one gallon of alcohol is a 1 pound cut. Two pounds in a gallon is a 2 pound cut. The definition is per gallon of solvent, not per gallon of finished mix, which is why the numbers behave the way they do when you dilute.
Suppose you have a quart of 3 pound cut. That quart contains three quarters of a pound of flake, because a quart is a quarter of a gallon. To make it a 2 pound cut, that same three quarters of a pound has to end up in three eighths of a gallon, which is 48 fluid ounces. You have 32, so you add 16. That is half again on top of what you started with, not a third, and the shortcut is simply the ratio of the two cuts: 3 divided by 2 is 1.5, so the volume grows by half.
Why the finished volume is not quite the alcohol volume
The flakes are a solid with a volume of their own, and once dissolved that volume is still in the jar. Mix 8 ounces of flake into a gallon of alcohol and you get slightly more than a gallon of shellac. For ordinary work nobody cares, and the traditional definition ignores it, which is why this page leaves the correction off unless you fill in a specific gravity. When it matters is when you want the solids by volume so you can put shellac into a film build calculation alongside other finishes, because that comparison needs the solids as a share of the finished liquid rather than a weight per gallon of solvent.
The correction itself is simple. Weight divided by the specific gravity times 8.345 pounds per gallon gives the volume the resin occupies, and adding it to the alcohol gives the finished volume. Your supplier is the source for the specific gravity, and if you do not have one, leave the field blank rather than guessing.
Cut, coats and film
The film a coat of shellac leaves scales with the cut, near enough, because the alcohol all leaves. So two coats of a 3 pound cut and three coats of a 2 pound cut land in roughly the same place, and the three lighter coats are usually easier to apply evenly. That is the whole argument for reducing a stock cut rather than brushing it heavy, and it is the same trade the film build calculator makes explicit for varnishes.
Where this sits
For the film side of the same decision, run the cut through the finish film build calculator once you have solids by volume. Quantities for a whole job by product type are in the finish coverage calculator, and the elapsed time a multi-coat schedule takes is the finishing schedule calculator. If shellac is going on as a sealer before a stain or a filler, the grain filler calculator covers what the pores still need.
Questions people ask
Why is diluting a 3 pound cut to 2 not just adding a third more alcohol?
Because the cut is defined against the solvent volume, so halving the concentration means the same flake has to end up in a larger total volume, and that total is set by the ratio of the cuts. Three divided by two is 1.5, so a quart of 3 pound cut becomes a pint and a half, or 48 fluid ounces: you add 16 to the 32 you had. Adding a third, which is about 10.7 ounces, would leave you at roughly 2.25 pounds. The mistake is small enough that nothing obvious goes wrong, and large enough that the film per coat is out by a tenth.
Does this page work in grams and millilitres?
The inputs are pounds per gallon and US fluid measure, because that is how the cut convention is written, but the flake weight is also given in grams so you can weigh it on a kitchen scale. For a metric batch, work out the batch in fluid ounces here and convert the volume afterwards: a US fluid ounce is 29.5735 millilitres. Weighing in grams and measuring in millilitres is genuinely easier than the traditional units, and the cut number stays the same because it is only a label for the ratio.
How accurate does the flake weight have to be?
Less accurate than people assume for the finish itself and more accurate than they assume for repeatability. A few percent either way changes the film per coat by a few percent, which you will never see on a single piece. What it does change is your ability to repeat a result: if you mix by eye and the second batch is thicker, the same number of coats gives a different film and you will blame the technique. A cheap scale removes that variable for good.
Can I mix shellac with something other than denatured alcohol?
Solvent choice is a product question and this page takes no position on it. What the arithmetic assumes is only that the flake ends up dissolved in a known volume of solvent, whatever that solvent is, so the cut is meaningful either way. What varies between solvents is evaporation rate and how the mix behaves under a brush or a pad, along with the hazards, and both belong to the supplier data and the safety data sheet rather than to a calculator.
How long does mixed shellac last?
Shorter than dry flake, and the number depends on the flake, the storage temperature and how the batch was made, so no page can give you a figure. What is worth knowing is the failure mode: old shellac stays liquid and looks fine, and gives itself away by staying soft rather than by looking wrong in the jar. The usual test is to put a few drops on glass and see whether it hardens, and mixing in the volume you will actually use avoids the question entirely.