Coaster resin calculator

Many silicone coaster molds are about 4 in (10 cm) across and around 3/8 in (1 cm) deep. The grams-per-coaster figure lets you pour each mold to weight on a scale.

The coaster formula

A coaster is a very short cylinder (or prism): face area × depth. For a round coaster that’s π × radius² × depth; square is side²; a regular hexagon measured across the flats is (√3 ÷ 2) × width², about 0.866 × width². Multiply by the number in the set and add a little for what stays in the cup.

round: π × (diameter ÷ 2)² × depth
square: side² × depth
hexagon: 0.866 × (flat-to-flat)² × depth
1 fl oz = 1.805 cu in = 29.6 ml

A 4 in round coaster poured 3/8 in deep is π × 2² × 0.375 ≈ 4.7 cubic inches, or about 2.6 fl oz. A set of four is about 10.4 fl oz, or 11.5 fl oz with 10% extra — the full breakdown is in how much resin for 4 coasters. Square molds of the same width hold more because they keep the corners; see how much resin for 6 square coasters.

Measuring a hexagon mold

Hexagon molds are sized two ways, and they differ by about 15%. The calculator wants the distance across the flats — from one straight edge to the opposite one. If you only have the corner-to-corner measurement, multiply it by 0.866 to get the flats. Entering the corner-to-corner figure as if it were the flats overestimates the resin by about a third. How much resin for 6 hexagon coasters works through both.

Pour to weight, not to the brim

Filling four molds by eye leaves one thin and one overflowing. Put each mold on a scale and pour to the per-coaster weight the calculator gives. It weighs Part A and Part B at typical densities (about 1.05 g/ml mixed for a 1:1 kit); enter your data sheet’s figures under More options if you want it exact.

Pick the right resin for the depth

Coasters sit in an awkward middle ground. ArtResin, for example, is designed to go on in layers of about 1/8 in (3 mm), so a 3/8 in coaster is three layers — each poured once the previous layer has thickened to a gel, soft but no longer runny (ArtResin suggests roughly 3–5 hours), or after a full cure. Deep-pour resins are designed for far thicker castings. Many crafters use a casting resin rated for thin-to-medium pours instead, which is why the calculator suggests one. The label’s maximum pour depth is the number that matters: as Epoxyworks explains, a thick mass of epoxy traps its own heat, and a pour deeper than the resin is designed for can overheat and crack.

Tips

• Level the molds. A tilted mold gives a coaster that’s thick on one side.
• Fill the molds from one batch if you want the colors to match.
• Coaster sets often include wood, dried flowers or glitter — anything you embed displaces resin, so you’ll need a bit less.
• Epoxy coasters can mark from hot mugs. ArtResin, for example, lists heat resistance up to about 120°F once cured — cooler than a fresh cup of coffee — so check the heat resistance on your label.