How to Calculate Concrete for Craft Molds
Whether you are casting geometric planters, terrazzo-style coasters, or minimalist candle holders, knowing the exact amount of concrete saves money and prevents mid-pour shortages that ruin your finish.
The Math Behind the Mix
For cylinders, we use π × r² × h to get volume in cubic inches, then convert to milliliters (1 cu in = 16.387 ml). For rectangles, it is simply L × W × H. Craft concrete has a density of roughly 2.3 g/ml, so we multiply volume by density to get the weight in grams, then convert to pounds. A 10% waste factor is added to cover mixing losses and spillage.
Choosing the Right Mix
Standard concrete from the hardware store contains large gravel aggregate that does not work well in small molds. Look for countertop mixes, craft-specific cement, or pure Portland cement mixed with fine sand for the smoothest results. Adding a concrete plasticizer helps the mix flow into detailed molds without air bubbles.
Pro Tip: Spray the inside of your silicone mold with cooking spray or use a mold release agent before pouring. This makes demolding easier and gives you a smoother surface finish with fewer pinholes.
Ready to price your finished concrete pieces? Use our Etsy Profit Calculator to make sure every planter and coaster covers your materials, fees, and profit margin.
Frequently Asked Questions
What type of concrete mix is best for small craft projects?▼
For small craft items like planters, coasters, and candle holders, use a fine-grain craft concrete or a cement-based mix without large aggregate. Brands like Quikrete Countertop Mix or Smooth-On's Forton MG work well because they produce a smooth finish and flow easily into silicone molds.
How long does craft concrete take to cure?▼
Craft concrete can be demolded after 24 to 48 hours, but it takes about 28 days to reach full strength. For best results, keep pieces in a cool, humid environment and mist them with water during the first few days to prevent cracking.
Why does the calculator add 10% for waste?▼
Concrete sticks to mixing containers, spills during pouring, and some gets trapped in texture details of the mold. The 10% waste factor accounts for these losses so you do not run short mid-pour, which would create visible seam lines in your finished piece.