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The short version

The arithmetic is the same shape as any other print: material, plus machine, plus electricity, plus your time, divided by the prints that actually worked. What changes is which consumables exist and how much of the total they are.

On an FDM machine the wear items are cheap and slow — a nozzle, a belt, a build plate. On a resin machine, one of the parts that wears out is a screen, which costs a fair share of the printer's price and lasts a set number of hours of exposure. Leave it out and your cost per print will be too low, and the more you print, the further out it gets.

Weight or volume

Resin is sold by weight — 500g and 1kg bottles — and sliced by volume. Your slicer says 42mL, but you bought the resin by the kilogram, so you need to convert one to the other.

Standard resins are typically around 1.10 g/mL, with most sitting somewhere between 1.05 and 1.20 depending on the formulation. At 1.10, a 1kg bottle is roughly 909 mL, and a 50mL print is about 55g.

Check the figure on your own bottle. Tough, ABS-like and water-washable resins differ from standard, and the number is usually on the label or the datasheet. As with filament, trust the manufacturer's figure over a table on someone else's website.

The consumables FDM does not have

These are the costs that make a resin print more expensive than the resin in it:

• The release film. FEP or nFEP across the bottom of the vat. It clouds, stretches and eventually punctures, and its life is measured in prints rather than years — heavily dependent on how large your prints are and how aggressive the peel is. Each one is cheap, but you keep having to buy them.
• Isopropyl alcohol, or water. Washing is not optional, and the alcohol becomes saturated with uncured resin and has to be replaced. Water-washable resin doesn't get rid of this cost. The water still has to be dealt with, and you can't just pour it down the drain.
• Gloves and paper. Next to nothing per print, but they add up over a month. Like electricity, they are small, regular, and usually left out.
• Curing. A UV station draws power and takes time, and it is part of making a sellable part rather than an optional extra.

The screen is the expensive one

The LCD masking screen wears out and has to be replaced, so treat it as a consumable. UV degrades it, contrast falls off, exposure gets less predictable, and eventually it has to be replaced — at a cost that is a real fraction of what the machine cost in the first place.

Manufacturers quote a life in hours of exposure, and the figures vary too much between machines for us to give one here. Find yours, divide the replacement cost by it, and you have a per-hour number that belongs in every print you make. It is the resin equivalent of printer depreciation, and it is usually larger than people expect — see what it costs to run a printer for the same argument on the FDM side.

Failures and waste

Resin prints fail in a different way. An FDM failure usually costs you one object. A resin failure often takes the whole plate and everything on it, and a failed layer early on can leave a cured sheet stuck to the film that also costs you the film.

The waste that never becomes an object is also higher, and more of it is structural: supports and rafts are not optional the way they often are in FDM, and that resin is paid for and thrown away. Then there is what is left in the vat when you change colour, and what clings to the part and ends up in the wash.

None of that means you are doing something wrong. It is part of the cost. The mistake is dividing by every print you started instead of only the ones you could sell. It is one of the most common errors in print pricing, and what a print actually costs goes through it.

Doing the sum

All of this works the same way as filament. Only the units and the wear parts are different.

Log the resin in mL, and let the density do the conversion. Record each bottle by the weight it was sold in, along with the density from the label or datasheet. When you log a print, enter the mL your slicer reports and it is turned into grams at that bottle's density. Grams are what get costed, because that is how the resin was sold to you.

Put the film and the screen into the machine's wear cost, the same place a nozzle would go. That is a replacement price divided by its expected life, applied per hour.

Count the resin you throw away. When you retire a bottle, the amount left over includes anything you poured out of the vat and did not reuse. It is recorded as waste against what you bought, so it shows in your cost rather than disappearing.

MakerProof does all of this for resin. Tell it you print with resin, or with both, and it talks about bottles rather than spools. If you just want the number for one print, the resin print cost calculator works it out without an account.