Resin mold volume calculator

Three ways to measure a mold

Rectangular molds are boxes: inside length × inside width × the height you will pour to. Measure inside the walls, and measure to the pour line rather than to the top of the mold.

Round molds are cylinders: π × (diameter ÷ 2)² × pour height. For a tapered cup, measure the diameter halfway up the pour height. This slightly underestimates a true cone frustum: by about 0.3 % when the wide end is 20 % wider than the narrow end, 1.3 % at 50 % wider and 3.6 % at twice as wide, so add a little more allowance for steep tapers.

Irregular molds (silicone shapes, bowls, carved forms) are easier to measure than to calculate. Fill the mold with water to the pour line, then pour the water into a measuring cup, or weigh it. The calculator converts the water’s weight to volume using a density of 0.998 g/mL. The USGS lists water at 0.99802 g/cm³ at 70 °F (21 °C), 0.99975 at 50 °F and 0.99510 at 90 °F (USGS), so between 50 °F and 90 °F the temperature error stays under 0.3 %.

TotalBoat’s casting steps call for a clean, dry mold (TotalBoat), so dry the mold completely after measuring with water and before any resin goes in.

Formula and assumptions

V_resin = V_mold − V_inclusions    V_total = V_resin × (1 + loss %)

• V_mold comes from one of the three methods above.
• V_inclusions is the volume of wood, stones, flowers or other objects you will embed. Measure it by displacement: put water in a measuring cup, read the level, submerge the object, read again. The difference is its volume. Seal porous objects such as wood before embedding them: TotalBoat explains that air trapped in an unsealed porous object escapes into the resin as a stream of small bubbles (TotalBoat). An unsealed object can also soak up some of the water while you measure it.
• Mixing loss defaults to 10 %, an assumption rather than a manufacturer figure. Small castings lose a larger share to the cup than big ones.

The calculator assumes the mold does not flex or bulge when filled. A flexible mold that bulges under the weight of the resin holds more than its measured volume.

Worked example

A wooden box mold for a resin-and-wood tray: 10 in long, 4 in wide inside, poured 1 in deep. A piece of burl that displaces 6 fl oz of water goes in the middle.

1. Mold volume: 10 × 4 × 1 = 40 in³. One US gallon is 231 in³ and holds 128 fl oz (NIST), so 40 in³ = 22.2 fl oz (655 mL).
2. Minus the burl: 22.2 − 6 = 16.2 fl oz (478 mL).
3. With 10 % loss: 17.8 fl oz (526 mL) of mixed resin.

The same mold measured with water: 655 mL of water weighs about 654 g, which the calculator turns back into 655 mL.

Before you pour

• Check the depth limit. A casting that is thick in one place can exceed a product’s maximum pour depth even if the total volume is small. See the layer planner.
• Seal and release the mold. Tape, release agents and sealed seams are covered in the mold release guide.
• Price it. Carry the result to the cost estimator.