If your epoxy keeps curing soft, tacky, or with bendy spots, the problem usually is not the resin brand. It is how you measured it. The single most consequential decision in a resin pour happens before you ever open the bottle: do you measure your two parts by weight on a digital scale, or by volume in graduated cups? Get that wrong and even a flawless mix-and-pour technique cannot save the cure.
Here is the bottom line up front. For epoxy resin, weigh it — do not pour it by eye. A 0.1g precision scale meters your two components far tighter than any cup can, and because epoxy cure depends on a precise chemical balance between resin and hardener, that precision is the difference between glass-hard and gummy. The one real exception: if your resin’s label publishes a by-volume ratio and only a by-volume ratio, graduated cups are the correct tool and a scale would actually mislead you. We will cover both paths below, and the comparison table lays the five tools side by side.
The core difference: by-weight vs by-volume
Every two-part epoxy mixes at a fixed ratio of resin (Part A) to hardener (Part B). The catch nobody mentions on the first pour is that the weight ratio and the volume ratio are not the same number. Resin and hardener have different densities — different specific gravities — so a kit that is 1:1 by volume is often roughly 100:83 by weight, and a 2:1-by-volume kit lands somewhere else again depending on the chemistry.
This is the trap that wrecks more first castings than any other: someone reads “1:1” on the bottle, sets a scale to grams, and weighs out 50g and 50g. But that 1:1 was a volume ratio. Weighed 1:1, that batch is off-ratio and may never fully harden. The rule is simple and absolute:
- If the label says “by weight,” use a scale.
- If the label says “by volume,” use graduated cups (or convert — see below).
- Never feed a by-volume number into a scale, or a by-weight number into cups.
Most modern art and casting epoxies publish a by-weight ratio precisely because weight is more repeatable. That is why, for the majority of makers, a scale is the default tool.
Why measuring cups lose accuracy
Cups are not “wrong” — they are just less precise, and the imprecision is structural, not a skill issue. Three errors stack up before your resin even hits the mold:
- Parallax. A graduation line is printed on a curved wall. Read it from above and you see a different level than reading it dead-on at eye level. That viewing-angle error alone is commonly 1-2% off the intended volume.
- The meniscus. Liquid resin curves where it meets the cup wall. Do you read the top of the curve or the bottom? Either way it is a judgment call, and judgment calls are not repeatable to a fraction of a percent.
- Wall film. This one is sneaky. Resin clings to the cup wall as a film. The volume you read at the line is not the volume you pour out — some stays behind. So poured volume is always a little less than read volume, a built-in bias that no careful reading can correct.
Add those up and a volume measurement carries roughly 1-2% inherent error in the best case, and more when batches are small or the maker is rushing. A scale’s equivalent error on a 0.1g step over a 100g batch is about 0.1% — an order of magnitude tighter.
Why off-ratio is so costly: the stoichiometric cure
Epoxy is not like cement, where “close enough” still sets. The resin and hardener react in a fixed molecular proportion — a stoichiometric relationship. When the ratio is right, nearly every molecule finds its partner and you get a fully cross-linked, hard, durable casting. When it is off, you have leftover unreacted material that stays soft.
The two classic failure modes:
- Too much hardener: the excess has nothing to react with, can react with itself and with moisture in the air, and leaves a surface that stays tacky and never fully hardens.
- Too little hardener: not enough cross-linking, so the piece cures soft, rubbery, or with bendy spots.
On a 30g jewelry batch, a 1g slip from a coarse kitchen scale is about a 3% ratio error — enough to risk a tacky surface. A 0.1g scale cuts that same batch’s error to about 0.3%. This is the whole argument in one sentence: small batches punish imprecision hardest, and a scale is where you find precision. Our deeper breakdown of resolution trade-offs lives in the best digital scale for resin guide.
The comparison table: five tools, head to head
The specs below put all five tools on the same axes — method (weight vs volume), resolution, capacity, inherent error, the batch range each suits, and price band. Read it as three scales (escalating precision and capacity) plus two cup styles (generic graduated vs pre-printed ratio lines). The pattern is clean: the scales win on accuracy, the cups win on price and no-battery simplicity, and the right answer for most makers is to own one of each and let the scale do the actual metering.
Pick by batch size, not by price
Resolution should follow your batch, not your budget. The smaller the pour, the finer the scale you need so that the rounding step stays a small percentage of the total:
- Sub-10g jewelry, bezels, domes: a 0.01g scale. A 0.1g step is a meaningful chunk of a 6g batch, so step down to milligram class.
- General work, ~10g to 2000g (coasters, tumblers, small castings, most everyday pours): 0.1g. This is the sweet spot and where most makers should live.
- Large pours, hundreds of grams to multi-kilo (trays, shallow river tables, coating batches): a 1g kitchen scale is fine, because one gram is a tiny fraction of a 500g batch.
Scale picks
AWS SC-2KG (2000g x 0.1g) — the value default. This is the one to buy if you buy only one. The 0.1g readability meters epoxy’s ratio far tighter than any cup, 2000g of capacity covers the great majority of hobby batches, and two-point linear calibration with 1kg/2kg weights keeps it honest over years. The stainless platform takes a mixing cup directly, and tare lets you zero the cup, weigh resin, then add hardener to the target weight. At roughly $26 with a 10-year warranty, nothing else in the lineup matches the value. Just budget for a calibration weight set — it is not in the box.
Truweigh Enigma (500g x 0.01g) — tiny batches. When you are casting jewelry, bezels, and domes under ~10g, 0.1g rounds too hard and the Enigma’s 0.01g resolution earns its place. Its 4-inch platform is unusually large for a pocket scale. The caveats are real: 500g max rules out coasters and deep pours, and inexpensive 0.01g sensors drift with temperature, so let it acclimate and recheck against a calibration weight before precision work.
Greater Goods kitchen scale (5kg / 11 lb x 1g) — big batches. For large coaster runs, trays, and shallow river-table pours, the 5kg ceiling is the headline feature, with four sensors quoted accurate to within about 1g. Two warnings. First, the 1g resolution is too coarse for small work — do not use it for jewelry. Second, ignore the mL mode entirely: it assumes water density (1 g/mL), which is simply wrong for resin. Always weigh resin in grams.
Cup picks: when each is right
Cups still belong in the studio — for by-volume resins and for eyeballing how much pour a mold needs.
Pixiss 10oz graduated cups (mL/oz) are the cheapest entry: a 20-pack costs less than one scale, the clear PET lets you watch the meniscus, and you toss them after cure. But the mL/oz marks are generic graduations, not ratio lines — you read each part and do the division yourself, on top of the inherent 1-2% volume error and wall-film loss.
WiseBond / GlassCast calibrated ratio cups solve the math: pre-printed 1:1 and 2:1 ratio lines let you fill Part A to a line, then Part B to the next, no division. Measuring both parts in one cup halves cup waste. The trade-offs: the lines only fit the printed ratios, it is still a by-volume tool carrying that ~1-2% error, and single-cup mixing leaves off-ratio wall film in the final pour.
How to mix by weight correctly
The technique is what converts a scale into accurate castings:
- Place your empty mixing cup on the scale and tare to zero.
- Pour in Part A (resin) to your target weight. Note the exact number.
- Compute Part B from the published by-weight ratio. For 100:50 by weight, 80g of resin needs 40g of hardener.
- Re-tare (or watch the running total) and add Part B to the combined target. Add slowly near the end — you cannot un-pour hardener.
- For the gold standard, use the double-cup method: mix thoroughly in cup one, then scrape into cup two and mix again. This leaves the off-ratio wall film behind in the first cup instead of in your pour.
Converting a by-volume ratio to weight
If you only know a by-volume ratio but want to weigh, convert with specific gravity: multiply each component’s volume parts by its specific gravity to get relative weight parts. A 1:1-by-volume kit whose hardener is less dense than the resin lands near 100:83 by weight. But treat this as a fallback — the manufacturer’s published weight ratio always wins over any conversion you calculate, because it folds in the exact chemistry.
Troubleshooting
- Tacky or sticky surface: almost always off-ratio, usually too much hardener. Re-measure your next batch by weight with a tare between components.
- Soft or bendy spots: under-hardened (too little hardener) or under-mixed. Weigh precisely and use the double-cup method.
- mL-mode mistakes: if you ever set a scale to mL for resin, stop — it assumes water density and will throw your ratio off. Weigh in grams.
- Drifting numbers on a 0.01g scale: temperature sensitivity. Let the scale acclimate to the room and recheck against a calibration weight before a precision batch.
A conservative safety note: always mix and cure in a well-ventilated space, follow your resin’s published safety data sheet, and wear nitrile gloves — accurate measuring does not reduce the need for skin and respiratory protection.
Verdict
Buy a 0.1g digital scale (the AWS SC-2KG is the value pick) for the actual metering, and keep a pack of graduated cups on the bench for estimating pour sizes and for the occasional by-volume resin. Mix the real batch by weight, tare between components, and finish with the double-cup method. That single workflow change fixes more cure failures than any resin upgrade. For the full equipment picture, see our best resin equipment buyer’s guide and browse every head-to-head in the comparisons hub.