How this tumbler calculator works
The calculator multiplies the tumbler's circumference by its height to get the surface area, then by the coat thickness to get liquid volume, adding 15% for the resin that stays in the cup and drips off while turning. A typical coat is around 30 mils (0.75 mm); the default two coats means a seal coat plus a flood coat, which is the standard tumbler workflow.
Typical amounts per tumbler size
A 20 oz skinny tumbler (about 3 × 8 inches) takes roughly 1.5–2 fl oz (45–60 ml) of mixed epoxy per coat. A 30 oz tumbler takes about 2–2.5 fl oz, and a small 12 oz kids cup about 1 fl oz. Mixing slightly more than you need beats stretching a thin batch — under-covered spots mean sanding and recoating.
Seal coat, then flood coat
The first thin coat seals glitter, vinyl or paint so it can't bleed or lift. After it tacks up (usually 4–6 hours), the flood coat goes on for the smooth glass finish. Keep the tumbler turning on a slow spinner (5–15 rpm) for the full cure window or the epoxy will sag to one side.
Working cleanly
Measure the two parts precisely — small batches magnify ratio errors, so use marked medicine cups or a scale. Warm the bottles in a water bath for easier mixing, stir 3 minutes, and pop surface bubbles with a brief pass of a heat gun. Wear nitrile gloves; cured epoxy is inert but liquid resin is a skin sensitizer.