How to Measure Resin Mold Volume with the Water Method

Updated 2026-10-16

Most silicone molds don't tell you how much resin they hold, and odd shapes — hearts, geodes, letters, trays with wells — make measuring with a ruler impossible. The water method solves that with nothing more than a kitchen scale and a cup of water. It's quick, accurate enough for resin work, and works on any shape. Here's how to do it properly and how to turn the result into the amount of epoxy to mix.

Why the water method works

At room temperature, water has a density very close to 1 gram per milliliter (about 0.997–0.998 g/ml). That means the weight of the water in grams is, for practical purposes, its volume in milliliters. Weigh the water that fills your mold and you know the mold's volume.

What you need

• A digital kitchen scale with a tare function (grams give finer readings than ounces)
• Water
• Your mold, clean
• A towel and time to dry the mold completely

Step-by-step

1. Place the empty mold on the scale. If the mold is floppy, set it on a rigid tray or board first so it doesn't distort when full — then put the tray on the scale too.
2. Tare the scale so it reads zero with the mold (and tray) on it.
3. Fill with water to your intended resin level. This matters: if you plan to fill the mold only partway, fill the water only that far. Pour slowly to avoid spills; spilled water on the scale adds to the reading.
4. Read the weight. That number in grams is approximately the volume in milliliters.
5. Empty and dry the mold completely. Water and epoxy don't mix — moisture can cause cloudiness, bubbles or a poor cure. Pat dry, then let the mold air-dry fully, paying attention to deep details.

Enter the reading into the mold volume calculator to convert it to fluid ounces, cubic inches and Part A/B amounts.

Converting the reading

• Grams of water ≈ milliliters
• Milliliters ÷ 29.574 = US fluid ounces
• Milliliters ÷ 16.387 = cubic inches
• If your scale reads in ounces (weight), multiply by 28.35 to get grams — and therefore ml

Watch out for "ounces". A scale displays weight ounces (oz), while resin kits often use fluid ounces (fl oz), a volume. One weight ounce of water is about 0.96 fluid ounces, so they're close but not identical. The calculator lets you pick which you measured.

Example

A coaster mold holds 85 g of water. That's about 85 ml, or 2.87 fl oz. With a 10% buffer for the cup and stir stick, mix about 94 ml (3.2 fl oz). For a 1:1 by volume epoxy, that's about 47 ml of Part A and 47 ml of Part B. For a set of four identical coasters, multiply by four — the calculator has a "number of cavities" field for exactly this.

Mixing by weight instead of volume

If your epoxy is mixed by weight, you need the weight of epoxy, not water. Epoxy is usually denser than water — many products are roughly around 1.1 g/cm³ — so the same volume weighs more. Multiply the milliliters by your epoxy's density (from the technical data sheet; it may be listed as specific gravity) to get grams. In the example: 85 ml × 1.1 ≈ 93.5 g before the buffer. Then split that by the product's weight ratio.

Measuring simple shapes without water

For geometric molds you can calculate volume from dimensions instead — handy if you don't want to get the mold wet:

• Rectangle / square: length × width × depth
• Cylinder (round coaster, tumbler): π × (diameter ÷ 2)² × depth
• Sphere: 4/3 × π × (diameter ÷ 2)³
• Cone: π × (diameter ÷ 2)² × height ÷ 3

Measure the inside of the cavity, not the outside of the mold. A round coaster mold 4 inches across inside and 0.25 inch deep holds π × 2² × 0.25 ≈ 3.14 in³ ≈ 51.5 ml ≈ 1.74 fl oz. The mold volume calculator includes these shapes.

Alternatives to water

Some people fill molds with rice, sand or dry beans and then measure that in a measuring cup, to avoid moisture. It's convenient, but granular materials pack unevenly and leave gaps, so the result is less precise, and grains can get stuck in fine details. If you do use a dry filler, measure it by volume in a measuring cup — not by weight, since its density isn't 1 g/ml. For accuracy, water plus thorough drying is hard to beat.

Tips and common mistakes

• Forgetting to tare. If you don't zero the scale with the mold on it, you'll include the mold's weight.
• Floppy molds bulging. A flexible mold can stretch when full, holding more water than it does when supported. Support it the same way you will when pouring resin.
• Not drying fully. Hidden droplets in details cause cloudy spots. Let it air-dry, or use a soft cloth and patience.
• Measuring once for a whole set. Similar-looking cavities in a multi-cavity mold may differ. Measure each, or fill all at once and weigh the total.
• Not adding a buffer. Epoxy clings to cups and sticks. For small amounts, that loss is a bigger share of the total — 10–20% extra is reasonable for small molds.
• Embedding objects. Items you embed displace resin, so you'll need a little less. You can account for this by placing the objects in the mold before filling it with water — just make sure they're waterproof and dry them afterward.

Record your molds

Once you've measured a mold, write its volume on a label or keep a list. Next time you'll skip the water step entirely and just enter the number into the epoxy calculator using the custom volume option.

Summary

Tare the scale with the empty mold, fill it with water to the level you want, read the grams — that's your volume in milliliters — then dry the mold completely. Convert to fluid ounces if your kit uses them, multiply by your epoxy's density if you mix by weight, and add a small buffer. The mold volume calculator does all of the conversions for you.