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Resin Calculator

Use this free resin calculator to find out exactly how much epoxy resin you need for a mold. Pick the mold shape — rectangle, circle, or oval — enter the size and the pour depth, choose your mix ratio, and the calculator gives the total volume in milliliters and fluid ounces, split into resin and hardener. Measuring before you mix saves resin, saves money, and stops the two classic mistakes: running out halfway through a pour, and mixing far more than the mold can hold.

How it works

Resin volume is surface area times depth. First the calculator finds the area of the mold from its shape, then it multiplies by the pour depth to get cubic inches, then it converts to milliliters and fluid ounces. For a rectangle, the area is length times width. For a circle, it is pi times the radius squared, where the radius is half the diameter. For an oval, it is pi times half the length times half the width. Once the area is known, the rest is the same for every shape, and one cubic inch converts to 16.387 milliliters or about 0.554 fluid ounces.

The mix ratio then splits the total into its two parts. Epoxy resin is a two-part system: the resin itself and the hardener that cures it, and the ratio on the label is by volume unless the label says otherwise. A 1:1 ratio means half resin and half hardener. A 2:1 ratio means two parts resin to one part hardener, so the resin is two-thirds of the total and the hardener is one-third. A 3:1 ratio gives three-quarters resin and one-quarter hardener. The custom option covers any other ratio on a label — enter the two part numbers and the calculator splits the total in the same proportions. Getting the ratio right matters more than getting the total right: the wrong ratio can leave a piece soft, sticky, or never fully cured, while a small error in total volume just leaves a little extra or a slightly shallow pour.

Measure the mold, not the finished piece you imagine. For a rectangular mold, measure the inside length and inside width at the top — molds taper slightly, so the inside top is the widest point and the honest measure of what the pour will fill. For a round mold, measure straight across the center for the diameter. For an oval, measure the longest run and the widest point. Measure depth from the mold floor to the level you plan to fill, not to the rim, unless you plan to fill it to the brim. If the mold holds objects — dried flowers, stones, a photo, or wood — those objects take up space, so the resin needed is less than the empty-mold volume. As a rough guide, subtract about the volume of the embedded objects, or fill the mold with water first, pour the water into a measuring jug, and use that as your total instead.

Pour depth deserves its own check. Resin cures by a chemical reaction that gives off heat, and a deep pour traps that heat. Most standard casting resins are made for layers of about 1/4 to 1/2 inch; pouring one deep layer can yellow the resin, crack it, or in extreme cases overheat enough to damage the mold. Deep-pour resins are formulated for layers of 2 inches or more and cure more slowly to stay cool. If your depth is more than your product allows in one pour, divide the total into layers, let each layer gel before pouring the next, and use this calculator once for the whole mold — the total resin is the same, it is just mixed and poured in stages.

A small waste allowance covers the resin that clings to cups and stir sticks and the drops that miss the mold. Five percent is enough for a careful pour with a single mix; ten percent suits a first project or a mold with awkward corners. The allowance is added to the total before the ratio split, so both parts scale up together. Remember that mixed resin has a working time — often 20 to 45 minutes — so mix only what you can pour and spread before it starts to thicken, and keep a heat gun or torch nearby to pop surface bubbles in the first few minutes.

Worked example

A rectangular tray mold measures 10 inches long and 6 inches wide, and the pour depth is 1 inch, using a 1:1 epoxy.

Step 1: Surface area — 10 × 6 = 60 square inches.
Step 2: Volume — 60 × 1 = 60 cubic inches.
Step 3: Convert to milliliters — 60 × 16.387 = 983.2 mL, about 983 mL.
Step 4: Convert to fluid ounces — 60 × 0.554 = 33.2 fl oz.
Step 5: Split 1:1 — 983 ÷ 2 = 491.6 mL of resin and 491.6 mL of hardener.

You need about 983 mL of mixed resin in total: roughly 492 mL of resin and 492 mL of hardener. With a 5 percent waste allowance, the total rises to about 1,032 mL — about 516 mL of each part.

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