Weight ratio and volume ratio are different numbers
This is the part worth reading twice. A two-part resin is sold with a mix ratio, and that ratio is stated on one of two bases: by weight or by volume. They only coincide when part A and part B have the same density, and they usually do not. A typical casting epoxy has a resin side near 1.1 g/cm3 and a hardener nearer 0.95, and that difference is enough to move a 1:1 by volume product to about 1.16:1 by weight.
Mix a 1:1 by volume product as 1:1 by weight and you have put in roughly 14 percent too much hardener by volume. Depending on the chemistry, the result is a piece that stays soft in the middle, or one that goes brittle and yellows early, or one that never reaches full hardness no matter how long you leave it. There is no correction after the fact. The page shows both bases every time for this reason: enter the ratio as the label states it, tell the page which basis that is, and read off the other one.
Where the data sheet is genuinely unclear, the tell is often in the printed figure itself. A ratio quoted with decimals, such as 100:44 or 100:31, is usually a weight ratio, because volume ratios are normally set up as round numbers for dispensing pumps. That is a hint and not a rule, and a small test batch settles it in an afternoon.
Getting the cavity volume right
For a simple box or disc, the dimensions give you the answer directly. For anything else, the fastest accurate method is to fill the mold with water, pour that into a measuring jug, and read it. Do this before the mold has ever seen release agent, and dry the mold thoroughly afterwards, because trapped water in a silicone mold will cause surface defects on the first casting.
The volume this page wants is the cavity, not the mold. If you came here straight from the silicone mold calculator, the number you want is the part volume you entered there, not the silicone volume. Those are the two halves of the same subtraction and it is easy to grab the wrong one.
Pigment, dye and filler
Colorants come in two families that behave differently in the arithmetic. Transparent dyes are used at fractions of a percent and are essentially invisible to the mix. Opaque pigment pastes are used at a few percent and are not: past the manufacturer's stated maximum, the excess pigment sits in the cured resin as unreacted material and the piece is measurably softer. The common figure is a few percent of the mixed weight, but the only number that applies to your product is the one on your product's data sheet.
Fillers are a separate calculation and this page treats them separately. Powders, aggregates and flakes displace resin, which means a heavily filled casting takes less resin than the cavity volume suggests. That saving is real but it is not linear, because a packed powder has air between the grains that the resin then has to fill. If you are filling heavily, cast one test piece and weigh what you actually used rather than trusting a percentage.
What the cost figure does and does not include
The price side converts your kit price into a price per gram and multiplies. It assumes the kit price covers both parts, which is how most kits are sold. It does not include pigment, mold release, mixing cups, the mold itself, or the pieces you throw away while learning the product. On a small run those non-resin costs are frequently larger than the resin, which is why a per-piece figure from resin alone tends to look better than the shelf does. If you are pricing work for sale rather than costing a hobby pour, take the resin number here and put it into the shop labor estimate, where the hours are the part that usually decides whether the job was worth doing.
Questions people ask
How do I convert a mix ratio from volume to weight?
Multiply the volume ratio by the ratio of the densities. If the product is r parts A to 1 part B by volume, then by weight it is r multiplied by the density of A divided by the density of B, still to 1 part B. Worked through: a 1:1 by volume product with resin at 1.10 g/cm3 and hardener at 0.95 gives 1 x 1.10 / 0.95 = 1.158, so 100 g of A takes about 86 g of B. Going the other way, divide instead of multiplying. Both densities come off the data sheet, and if only one is listed, do not guess the other — mix by the basis the label states and use the correct measuring tools for that basis.
Why does my resin weigh less than the volume suggested?
Because resin is not water. Volume in millilitres equals weight in grams only for a fluid with a density of exactly 1.0, and casting resins run above that, typically 1.05 to 1.15 mixed. A 500 ml cavity therefore takes something like 550 g rather than 500. The direction people get caught by is the opposite one: they weigh out a batch that matches the cavity in grams, pour it, and come up short by roughly the density difference. Enter the density from your data sheet rather than accepting the default here, since the difference between 1.05 and 1.15 is nearly 10 percent of the batch.
How much extra should I mix?
Around 5 to 10 percent for most work, which is the default range here. What it covers is the film that stays in the mixing cup, what clings to the stir stick and any transfer container, and the last of the pour that simply will not leave. Deep narrow cups waste proportionally less than wide shallow ones. Push it higher if you are pouring through a funnel, degassing in a vacuum chamber, or splitting one batch across several small molds where each transfer leaves a residue. The asymmetry is heavily in favour of mixing extra: leftover resin costs a few dollars, while a short pour on a piece that has already started to gel is a remake.
Can I scale a batch down for a small pour?
Yes, and the ratio holds at any size, but accuracy does not. A scale that reads to one gram is fine when the batch is 500 g and useless when part B is 4 g, because a one gram error there is a 25 percent ratio error. For batches under about 50 g use a scale reading to 0.1 g, and consider whether the product tolerates small batches at all — some fast-curing systems need a certain mass to reach temperature and cure properly, which is the opposite of the exotherm problem large pours have. Very small batches also mix badly, since the residue that stays on the stick is a larger share of the total.
Does adding pigment change how much resin I need?
Barely, at normal loadings. Pigment at 2 or 3 percent of the mixed weight adds a little volume and the effect is well inside the waste allowance, so it is not worth adjusting for. What it does change is the cure. Pigment does not react; it is carried in the cured resin, and above the manufacturer's stated maximum the excess prevents the resin reaching full hardness. Heavy fillers are a different story and do change the resin quantity meaningfully, which is why they are entered separately here. If you are casting a filled piece for the first time, weigh what you actually consume on the test piece and use that number for the run.