How much silicone for a 4 inch coaster mold?
You need about 12.9 ounces (365 grams) of silicone rubber to mold a 4 inch round coaster that is 3/8 inch thick, with 1/2 inch walls — 6.44 ounces each of Part A and Part B, poured in a 5 × 5 × 0.88 in box. That assumes a rubber with a specific gravity of 1.18, like Mold Star 15 SLOW.
How to work it out
- Box 5 × 5 × 0.88 in = 21.9 cu in − model 4.7 cu in = 17.2 cu in (9.51 fl oz)
- × SG 1.18 + 10% = 12.9 oz → A 6.44 oz + B 6.44 oz, 1 × 2 lb kit (19.1 oz left over)
A single coaster is the classic first mold: flat, small, and forgiving. The master here is a 4 in round coaster 3/8 in thick — the same size the coaster resin calculator starts from — sitting in a square mold box with 1/2 in of rubber around the sides and over the top.
Adjust for your project
More options: waste
- Part A by weight
- 6.44 oz (183 g)
- Part B by weight
- 6.44 oz (183 g)
- Silicone volume (no waste)
- 9.51 fl oz (281 ml)
- Mold box inside (L × W × H)
- 5 × 5 × 0.88 in
- Kits to buy
- 1 kit
- Left over from the last kit
- 19.1 oz (1.2 lb)
- Mix by the ratio on your data sheet. Many silicones must be weighed; some, like Smooth-On’s Mold Star line, are 1A:1B by volume as well as by weight. Parts A and B here are by weight.
Silicone for a 4 in coaster mold, by coaster thickness
| Coaster thickness | Silicone to mix | Silicone volume (no waste) | Kits to buy |
|---|---|---|---|
| 1/4 in | 11.7 oz | 8.65 fl oz | 1 kit |
| 3/8 in | 12.9 oz | 9.51 fl oz | 1 kit |
| 1/2 in | 14 oz | 10.4 fl oz | 1 kit |
| 3/4 in | 16.4 oz | 12.1 fl oz | 1 kit |
What to know about making a 4 inch coaster mold
Glue the master face up. The side stuck to the base becomes the open back of the mold, so the face you want to copy — a texture, a bevel, a logo — points up into the silicone. A dab of hot glue or double-sided tape keeps the coaster from floating when the rubber rises around it; silicone that creeps underneath leaves a thin skin you have to trim off.
Watch what the master is made of. If it is a resin coaster you poured yourself, Smooth-On lists newly cast epoxy among the materials that can stop platinum silicone curing, leaving a sticky layer against the model. Let the master cure fully, seal it with a clear acrylic lacquer, and let that dry before the pour.
One 2 lb trial kit covers this with about 19 oz spare — enough for a second coaster mold from the same batch, which is handy because coasters are usually made in sets. Mold Star 15 SLOW gives you about 50 minutes to work and cures in about 4 hours at room temperature. A round container that hugs the coaster, such as a length of pipe or a straight-sided plastic cup, uses roughly a quarter less rubber than the square box, at the cost of a round mold that is harder to line up in a row.
For the bigger job of molding a serving tray, compare how much silicone for a 12×8 tray mold — the tray’s hollow changes the math completely.
Different size? Use the full Silicone Mold Calculator.
What you’ll need
- Silicone mold-making rubberMid-range price
Two-part liquid silicone for pouring block molds. Check the mix ratio (often 1:1 or 10:1 by weight) and specific gravity on the TDS.
See options on Amazon (opens in a new tab) - Digital scale (0.1 g)Budget price
For mixing by weight. Far more accurate than cup lines for small batches — and it doubles for the water method on molds.
See options on Amazon (opens in a new tab) - Graduated mixing cups & stir sticksBudget price
Small cups marked in ounces and ml for art, coasters and tumblers. Use a fresh cup when you transfer to a second container.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much silicone for a square 4 inch coaster mold?
About 11.9 ounces in the same 5 × 5 in box. A square coaster fills more of the box than a round one, so it needs slightly less rubber.
How much silicone for a 4 inch coaster mold with 3/8 inch walls?
About 9.16 ounces in a 4.75 × 4.75 × 0.75 in box. Thinner walls save rubber but flex more when you demold.
How much silicone to mold a 1/4 inch thick coaster?
About 11.7 ounces for a 4 in round coaster 1/4 in thick with 1/2 in walls.
Other common sizes
Sources
- Smooth-On — How much mold rubber do I need? (opens in a new tab) — Container volume minus model volume; the water method; convert volume to weight with the specific volume (cu in per lb) on the technical bulletin
- Smooth-On — Mold Star 15 SLOW product page (opens in a new tab) — 1A:1B by volume or weight; specific gravity 1.18 (23.5 cu in/lb); 50 min pot life, 4 h cure; sulfur clays, latex and fresh epoxy can inhibit the cure
- Smooth-On — How to make a silicone mold using Mold Star 15 (opens in a new tab) — Glue the model and mold box to a base; pour about 1/2 in (1.25 cm) over the model’s high point