Calculate volume needed, convert mixing ratios, plan safe pours, and avoid exotherm damage.
Exceeding the recommended load inhibits curing. Your resin will stay sticky and never fully harden in patches. Mix pigment evenly and measure by weight, not guesswork.
The volume of your mould or surface is the starting point, but not the answer. A 1-litre measuring jug seems enough for a 1-litre project, until you realize you have a 5-10% waste factor just from cling to the walls, pouring loss, and the amount that stays in the container. Grab 1.1 litres. If you are mixing by weight, you also need extra to account for the weight that stays in the measuring vessel.
For irregular shapes, split them into rectangles or circles, calculate each section, and add them up. A river table is a simple slab minus a channel; a deep mould with undercuts is multiple pour sections. Do the math for each part.
If your project is thicker than your resin type allows, you will pour in layers. This means you calculate the total volume first, then divide it by the safe pour depth to see how many pours you need. Each layer counts toward your total, so if you need 3 litres total and can pour 1 litre deep at a time, you need three separate pours.
Volume measuring (cups or jugs) is easier and faster. Weight measuring (a gram scale) is more accurate because the resin and hardener have different densities. A 2:1 ratio by volume is not the same as 2:1 by weight; it is typically about 100:47 by weight for casting resin.
If you mix 100 mL resin by volume with 50 mL hardener, but your resin is 1.1 g/mL and hardener is 1.0 g/mL, you are actually mixing 110g with 50g, which is closer to 2.2:1 by weight. This throws off the cure, and your pour ends up soft or tacky in patches. Small projects or very precise work should use weight. The calculator above shows you the correct weight ratio for your specific densities.
The most reliable method is to check your product datasheet. It lists both ratios, and sometimes the densities too. If it only gives one, use it and stick with it; do not try to convert. Different resin systems have different density pairs.
The biggest mistake in resin work is pouring too thick. Standard coating resin should not exceed 1/4 inch (6mm) per layer. Deep pour casting resin is designed for up to 2 inches (5cm) per layer. Specialized products go higher, but check the label first.
When resin and hardener mix, they release heat (exotherm). In thin coats, the heat dissipates quickly into the air and the mould. In thick pours, the surrounding resin acts as insulation. Heat gets trapped inside the mass. Temperatures can spike above 200F (93C). This yellows the resin, causes cracking, shrinkage, surface swirls, and in the worst case, smoke or fire.
If your project is thicker than one safe pour, you layer it. Pour one layer to the maximum for your resin type, then wait for it to start curing (usually a few hours, check the datasheet). Once it is tacky but not fully hard, pour the next layer on top. This spreads the exotherm across time instead of concentrating it in one mass.
The safest approach: use a thermometer inside a test batch, or watch for steam. If the resin heats up faster than expected, you poured too thick. Stop, let it cool, and next time use more, thinner layers.
All figures from manufacturer datasheets. Exceeding these risks exotherm damage. Always check your product label, as formulations vary by brand.
| Resin type | Max per layer | Use case |
|---|---|---|
| Standard coating resin Art resin, doming resin, craft resin standard formula. | 1/4" (6mm) | Tabletop sealing, jewelry, small pours |
| High-quality tabletop Premium brands designed for thick coats. | 1/2" (12mm) | Thick wood slab finishes, river tables |
| Fine detail casting GlassCast 10, for precise mould fills. | 0.4" (10mm) | Jewellery casting, intricate moulds |
| Deep pour casting Craft Resin, TotalBoat Thickset, SIMIRON. | 2" (5cm) | River tables, large castings, thick pours |
| Specialized deep pour Ultra-thick formulations, limited availability. | 4" (10cm) | Large single castings, rare use |
Check your specific product datasheet. Pour limits vary by formulation and hardener type. Exceeding these limits causes exotherm, yellowing, cracking, and fire risk.
| Colorant type | Max loading | Notes |
|---|---|---|
| Pigment paste Most common, good opacity. | 5-6% | Mix evenly, exceeds 6% risks inhibited cure |
| Mica powder Sparkle and shimmer effects. | 1-3% | Above 3% clumps and loses shimmer |
| Liquid resin dye High concentration, spreads easily. | 1-3% | Concentrated, small amount goes far |
| Graphite or powdered tempera Specialty colors, less common. | Up to 10% | Test first, inhibition is unpredictable |
All limits are by weight of total resin. Exceeding safe limits inhibits curing, leaving soft, sticky patches that never fully harden. Measure pigments on the same scale you use for resin and hardener.
A river table, 36 inches long by 20 inches wide, with a channel 2 inches wide and 0.5 inches deep, filled to a total depth of 1.5 inches.
Volume calculation: Main surface (36 x 20 x 1.5) = 1,080 cubic inches. Channel to subtract (36 x 2 x 0.5) = 36 cubic inches. Net volume = 1,044 cubic inches = 17.1 litres. Add 8% waste = 18.5 litres total.
Resin needed: If using deep pour casting resin (2 inches per layer max), one pour suffices. If using standard coating resin (0.25 inches max), you need 6 layers of 0.25 inches, each about 3 litres, for a total of 18-19 litres over 6 sessions.
By weight: Assuming 1.1 g/mL density, 18.5 litres = 20,350 grams. With a 100:47 ratio by weight (typical for 2:1 by volume casting resins), you mix approximately 13.6kg Part A (resin) with 6.4kg Part B (hardener). Use a gram scale reading to 1 gram for accuracy.
Pigment: If adding pigment, limit to 5% of total resin weight. For 13.6kg resin, that is 680 grams max pigment. Most projects use far less, 1-3% for subtle color.
Do a small test batch first. Mix one part (say, 100g resin and 47g hardener), pour it into a small mould at your intended depth, cover it lightly with plastic, and watch it cure over 24 hours. Take its temperature every few hours with a non-contact infrared thermometer. If it gets hotter than 40C above room temperature, you are pouring too thick or your resin is not the type for that depth.
Small batches cure slower and cooler than large ones (surface-area-to-volume ratio matters), so a test is more forgiving than the real thing. But if even the test gets too hot, scale up to thin layers.
Multiply the volume of your mould or surface in cubic inches by 0.016 to get litres. For a rectangular container, multiply length × width × depth. For irregular shapes like river tables, calculate each section separately and add them. Always add 5-10% extra for mixing loss and edge coverage.
Mix by weight if you want maximum accuracy. The resin and hardener have different densities, so a 2:1 ratio by volume is actually about 100:47 by weight for most casting resins. A digital scale reading to 1 gram is more reliable than volume cups, especially for small projects or deep pours where precision matters.
Standard coating resin: 1/4 inch (6mm) per layer. Deep pour resin: 2 inches (5cm) per layer. Specialized deep pour resins: up to 4 inches (10cm) per layer. Exceeding these limits causes exothermic heat buildup, which yellows the resin, causes cracking, and creates a fire risk. Always check your product datasheet.
Excess heat (exotherm) gets trapped inside the resin mass. Temperatures can exceed 200F (93C), causing yellowing, cloudiness, cracking, shrinkage, and in extreme cases, smoke or fire. If you need thickness, pour in layers and allow each layer to partially cure before adding the next. This is the most expensive mistake in resin work.
Maximum 5-6% by weight for most pigments. Mica powders should be limited to 1-3%. Liquid dyes to 1-3%. Exceeding these limits inhibits curing and results in soft, sticky patches that never harden properly. Always measure pigment by weight, not volume.