Resin mold calculator

For anything irregular — jewelry, figurines, trays with detail — fill the mold with water, weigh it (or pour it into a measuring cup), and let the calculator convert.

Three ways to measure a mold

Simple shapes are geometry: a box is length × width × depth, a cylinder is π × radius² × height, a sphere is 4/3 × π × radius³ and a dome (half sphere) is half that. Measure the inside of the mold and the depth you’ll actually fill to — for example, how much resin for a 12×8 tray mold filled 3/8 in deep.

box = L × W × D
cylinder = π × r² × h
sphere = 4/3 × π × r³   dome = 2/3 × π × r³
water method: 1 g of water = 1 ml (1 oz of water ≈ 0.96 fl oz)

For anything irregular, the water method is more accurate than any measurement — it is the same trick Smooth-On recommends for sizing mold rubber. Put the empty mold on a kitchen scale, zero it, fill it with water to the level you’ll pour resin to, and read the weight. One gram of water is one milliliter, so a mold that holds 250 g of water needs about 250 ml of resin before waste. No scale? Pour the water into a measuring cup instead. Dry the mold completely afterward — water left in silicone can cloud or spoil the cure. Small detailed cavities are where this shines: see how much resin for 12 pendant molds.

Embedded objects

Wood, dried flowers, shells or anything else you cast into the resin takes up space. To find its volume, drop it into a measuring cup of water and read how much the level rises (hold floating items just under the surface with a pin). Enter that per piece and the calculator subtracts it.

Casting tips

• Resin weighs a little more than water — typically about 1.05–1.2 g per ml — so a mold that holds 250 g of water takes roughly 260–300 g of mixed resin. The per-piece weight above uses typical Part A and Part B densities (enter your data sheet’s under More options); pour to that weight on a scale.
• Check the maximum pour depth and batch size on the label. Deep, chunky castings are where coating resins overheat from their own exotherm (the heat resin gives off as it cures); deep-pour resins such as MAS Deep Pour X are made for 2–3 in pours.
• Mix a touch more than you need; topping up a half-cured casting can leave a visible line.
• Warm the molds and resin to room temperature before you pour to help bubbles escape. Epoxyworks notes that cure time roughly halves for every 18°F (10°C) warmer, so a cold shop slows everything down.

Making the mold yourself? The silicone side is worked out the same way, box minus model — see how much silicone for a 4 inch coaster mold or how much silicone for a 12×8 tray mold, where the tray’s hollow changes the answer a lot.