Resin Casting Volume Calculator

About Resin Casting Volume Calculator

The Resin Casting Volume Calculator tells you exactly how much epoxy resin and hardener to mix for any mold. Describe your mold as a rectangle, cylinder, half-sphere dome, or sphere, enter a volume you already know, or use the water-weight method — then the calculator adds a waste margin, splits the total by your mix ratio (1:1, 2:1, 3:1, or any custom ratio, by volume or by weight), and gives you pour amounts in milliliters, fluid ounces, and grams. It works for epoxy, UV resin, polyurethane, and polyester casting resin alike, for projects from coasters and jewelry to river tables and paperweights.

How Resin Amounts Are Calculated

Three steps turn a mold into a mixing recipe:

1. Mold volume. For a rectangular mold, volume = length × width × depth. For a cylinder, \( V = \pi r^2 h \). For a half-sphere dome, \( V = \tfrac{2}{3} \pi r^3 \), and for a full sphere, \( V = \tfrac{4}{3} \pi r^3 \). Dimensions in centimeters give cubic centimeters, which equal milliliters directly; cubic inches are multiplied by 16.387 to get ml.

2. Waste margin. Some resin always stays on the cup walls and stir stick, so the mold volume is multiplied by your mold count and increased by a waste percentage — typically 10%.

3. Ratio split. The total is divided by your resin-to-hardener ratio. For a 2:1 ratio, resin gets \( \tfrac{2}{3} \) of the total and hardener \( \tfrac{1}{3} \). If the ratio is by weight, the split is done on grams (volume × density) instead of milliliters.

The Water-Weight Trick for Irregular Molds

Geometry only works for simple shapes. For an irregular silicone mold — a flower, a skull, a geode tray — fill the mold with water, pour that water into a cup on a kitchen scale, and read the grams. Since water weighs almost exactly 1 g per ml, the gram reading is the mold volume in milliliters. Select "Water-weight method" and enter that number. Just remember to dry the mold thoroughly before pouring resin, because moisture causes clouding and surface defects.

By Volume vs. By Weight — Why It Matters

Resin and hardener usually have different densities, so a 1:1 ratio by volume is not 1:1 by weight. Mixing the wrong basis can leave a cast sticky, bendy, or permanently uncured. Always check your bottle label or technical data sheet: brands like ArtResin specify 1:1 by volume, while many casting epoxies specify ratios like 100:45 by weight. This calculator handles both — pick the basis from the dropdown and it converts using the mixed density (about 1.1 g/ml for most epoxies).

Tips for an Accurate Pour

Measure the inside of the mold cavity, not the outside of the silicone. If you only plan to fill a deep mold partway, use the fill depth, not the full mold depth. For embedding objects (flowers, photos, hardware), the displaced volume is usually small enough to be covered by the waste margin. For very deep casts, check your resin's maximum pour depth — thick pours may need to be done in layers to avoid overheating, but the total amount stays the same.

Frequently Asked Questions

How do I calculate how much resin I need for a mold?

Work out the mold's inner volume, add about 10% for waste, then split by the mix ratio. For example, a 10 × 10 × 2.5 cm coaster mold holds 250 ml; with 10% waste you mix 275 ml, which at 1:1 means 137.5 ml resin and 137.5 ml hardener.

What does a 2:1 mix ratio mean?

Two parts resin for every one part hardener — e.g. 200 ml resin + 100 ml hardener. The first number on the bottle is almost always the resin (Part A).

How many grams is 100 ml of epoxy resin?

At a typical mixed density of 1.1 g/ml, 100 ml of epoxy weighs about 110 g. Check the technical data sheet for your exact product — casting resins range from roughly 1.0 to 1.25 g/ml.

How much extra resin should I mix?

5–15% extra is normal. Use ~10% for typical molds, more when filling many small cavities (more cup-wall loss per ml), less for one big pour.

Can I use this calculator for UV resin or polyurethane resin?

Yes — the volume math is identical. UV resin is one-part (no hardener), so just read the "Total to mix" line and ignore the split. For polyurethane, use its density (often ~1.05 g/ml) for accurate gram figures.

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