Epoxy Resin Mixing and Curing: Why Ratios and Depth Both Matter
Why a 1:1 ratio by volume is never 1:1 by weight, and how mold depth determines whether your resin cures clear or cracks from heat.
Most epoxy resin failures — cloudy patches, a tacky surface that never fully hardens, or a deep pour that cracks from internal heat — trace back to one of two things: an imprecise mix ratio, or using the wrong resin type for the pour depth. Both are avoidable once you understand what the ratio number on a bottle actually means and why "thin coat" and "deep pour" resins are not interchangeable.
This guide covers the volume-versus-weight ratio distinction in detail, along with the heat-buildup science behind deep pours. It pairs with the epoxy resin calculator, which converts your mold dimensions and mix ratio directly into exact resin and hardener amounts by both volume and weight.
Quick Answer: Ratio Numbers Mean Different Things
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| Ratio | Typical product type | What it usually specifies |
|---|---|---|
| 1:1 | Tabletop / deep pour, casting resins | Usually by volume; check the data sheet |
| 2:1 | General purpose casting, some deep pour | Usually by volume |
| 3:1 or 4:1 | Some laminating and coating resins | Can be volume or weight — verify |
| 100:X (e.g., 100:47) | Industrial / technical adhesives | Always by weight |
Why 1:1 by Volume Is Not 1:1 by Weight
Resin and hardener are different chemical formulations with different densities — resin typically sits around 1.05-1.15 g/mL, while hardener is often lighter, around 0.90-0.98 g/mL. Because a mix ratio by volume assumes equal-sized measuring containers regardless of what's inside them, a 1:1 ratio by volume routinely converts to something like a 100:83 to 100:86 ratio by weight once you account for that density difference — the two liquids simply don't weigh the same amount per milliliter.
Two Worked Examples
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| Component | Volume | Weight |
|---|---|---|
| Resin | 250 mL | 275.0 g |
| Hardener | 250 mL | 237.5 g |
| Weight ratio (normalized) | 100 : 86.4 |
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| Component | Volume | Weight |
|---|---|---|
| Resin | 200 mL | 216.0 g |
| Hardener | 100 mL | 95.0 g |
| Weight ratio (normalized) | 100 : 44.0 |
Volume ratio vs. weight ratio, side by side
The hardener's share of the mix looks smaller by weight than by volume in both examples, since hardener is the lighter component.
1:1 volume → weight
100 : 86.4 by weight
2:1 volume → weight
100 : 44.0 by weight
Both bars show the hardener's normalized share by weight — always lower than its share by volume because hardener is less dense than resin.
In both cases, notice the weight ratio is noticeably different from the volume ratio — the 1:1 volume mix becomes roughly 100:86 by weight, and the 2:1 volume mix becomes roughly 100:44 by weight. This is completely normal and matches real manufacturer documentation; one commonly cited industrial example specifies a 100:47 weight ratio for a product sold as a 2:1 volume ratio. If you measure that same product by weight using a 2:1 ratio instead of 100:47, you'd be significantly off, and the mix could remain soft or tacky indefinitely.
Epoxy Resin CalculatorEnter your mold dimensions, mix ratio, and component densities to get exact resin and hardener amounts by both volume and weight.Deep Pour vs. Thin Coat: Not Interchangeable
Epoxy cures through an exothermic chemical reaction — it generates its own heat as resin and hardener react. In a thin layer, that heat dissipates into the surrounding air fast enough to stay harmless. In a thick pour, the same reaction is trapped inside a much larger volume of curing resin with much less surface area to shed heat relative to that volume, so internal temperature can climb high enough to cause visible problems: yellowing, cloudiness, shrinkage cracks, or in extreme cases enough heat to warp or even scorch the mold.
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| Property | Thin coat / tabletop resin | Deep pour resin |
|---|---|---|
| Typical max layer thickness | 1/8" (3mm) per pour | 1-2 inches (25-50mm) per pour, some formulas more |
| Cure reaction speed | Faster, more exothermic per volume | Slower, engineered to release heat gradually |
| Clarity risk if misused | Low | High — using thin-coat resin in a deep mold risks cracking or clouding |
| Typical use case | Tabletop coatings, small jewelry, coasters | River tables, large castings, thick embedments |
Choosing the right resin type for your pour
Depth per pour, not total project size, is the variable that determines which resin formula you need.
Measure your pour depth
Depth per single pour, not the total finished piece thickness.
Under 1/8" (3mm)?
A thin-coat/tabletop resin is formulated for this and cures reliably.
1-2 inches (25-50mm)?
Use a deep-pour formula engineered to release heat gradually.
Deeper than that?
Layer in multiple deep-pour stages, letting each cure before the next — even deep-pour resin has a per-pour depth limit.
This is why river tables and other thick castings specifically call for deep-pour formulated resin rather than pouring a thin-coat product in multiple thick layers — deep-pour resins are chemically formulated to react more slowly and release heat more gradually, which is exactly what a large curing volume needs to avoid cracking. Some large projects instead pour a thin-coat resin in multiple thin layers over time (letting each layer cure before adding the next) specifically to avoid the heat-buildup problem entirely, rather than switching resin types.
Common Failure Modes and Their Causes
- Cloudiness or a hazy finish: often caused by moisture in the mold or ambient humidity during cure, or by mixing at the wrong ratio.
- Bubbles trapped in the cured piece: usually from insufficient mixing time, pouring too fast, or not using a heat gun or torch to pop surface bubbles before the resin gels.
- A tacky or soft surface that never fully hardens: almost always an incorrect mix ratio (measured by the wrong basis — volume vs. weight — or simply measured imprecisely).
- Cracking or excessive heat in a deep pour: using a thin-coat resin at a depth it wasn't formulated for, or pouring the full depth in a single application instead of layering.
- Amber yellowing over time: normal UV degradation for many standard resins — UV-resistant or UV-stabilized formulas exist specifically for pieces that will see sunlight.
Choosing the Right Measuring Tools
For weight-based measuring, a digital kitchen or postal scale accurate to at least 1 gram is sufficient for most hobby-scale pours — a scale that only reads in larger increments makes it hard to hit a precise ratio on smaller batches. For volume-based measuring, dedicated graduated mixing cups (not household measuring cups, which are rarely precise enough) reduce the error that compounds when scaling a ratio up to a large pour. Whichever method your product specifies, consistency in tooling matters as much as the ratio number itself — switching between two different measuring cups mid-project, for example, can introduce enough error to shift a large pour's cure quality.
Related Tools
If your resin project sits on or inside a wooden piece, the woodworking board foot & movement calculator and wood shrinkage & radial calculator help account for how the wood itself will move around your cured resin. For a finished piece that needs a painted or sealed surface instead, the paint calculator covers that coverage math.
Sources to Verify or Cite
- TotalBoat, measuring epoxy by weight vs. volume: https://www.totalboat.com/blogs/totalboat/how-to-measure-epoxy-by-weight-vs-volume
- Meridian Adhesives, converting mix ratios from weight to volume: https://meridianadhesives.com/product-information-insights/how-to-convert-epoxy-mix-ratios-from-weight-to-volume/
- Your specific product's technical data sheet (TDS) for its exact density and recommended measuring method — always the authoritative source over general guidance.
Frequently Asked Questions
Why is my epoxy ratio 1:1 by volume but not 1:1 by weight?
Resin and hardener are different chemical formulations with different densities. A 1:1 mix ratio by volume assumes equal-sized containers, but since resin and hardener weigh different amounts per milliliter, the same 1:1 volume mix converts to a different ratio by weight — commonly around 100:83 to 100:86.
Can I use deep pour resin for a thin coating?
Generally yes, though deep pour resin often cures more slowly than a dedicated thin-coat product, so it may take longer to reach full hardness for a thin surface coating. The bigger risk runs the other direction — using thin-coat resin in a deep mold.
Why did my deep resin pour crack or turn cloudy?
This is usually heat-related: epoxy curing is an exothermic reaction, and in a thick pour that heat can't dissipate fast enough, causing internal stress, cracking, or cloudiness. Use a resin specifically formulated for deep pours, or pour in multiple thin layers instead of one thick pour.
How do I know if my resin ratio is by volume or by weight?
Check the product's technical data sheet — reputable manufacturers specify both, but the primary recommended measurement method (and which scale or measuring cups to use) is listed there. Never assume; the two ratios are rarely the same number.
My resin stayed tacky and never fully hardened — why?
This is almost always an incorrect mix ratio, most commonly from measuring a weight-ratio product by volume (or the reverse), or from an imprecise scale/measuring tool. Double-check the data sheet's specified ratio and measurement method.
What causes bubbles in cured epoxy?
Insufficient mixing time, pouring too vigorously, or not degassing the surface with a heat gun or torch before the resin begins to gel. A slower, more thorough mix and a brief pass with a heat source right after pouring resolves most bubble issues.
What is the maximum depth for a single deep pour?
Most deep-pour formulas support 1-2 inches (25-50mm) per pour, though this varies by product — always check the specific manufacturer's maximum recommended depth per pour rather than assuming, since exceeding it risks the same heat-buildup problems as using a thin-coat resin too deep.
Do I need a scale or can I use measuring cups?
Either works if your product's data sheet specifies that measurement method and you use precise tools — a digital scale accurate to at least 1 gram for weight, or dedicated graduated mixing cups (not household measuring cups) for volume.
Why does my cured resin look yellow after a few months?
This is typically normal UV degradation for many standard epoxy formulas, especially in pieces exposed to sunlight. UV-resistant or UV-stabilized resin formulas exist specifically to slow this yellowing for pieces that will see regular sun exposure.
Final Summary
A resin's mix ratio by volume and by weight are two different numbers, both determined by the resin and hardener's specific densities — always check the data sheet rather than assuming. Separately, match your resin formulation (thin-coat vs. deep-pour) to your actual pour depth, since heat buildup during cure is the root cause of most cracking and clouding failures in thick castings.
Written by
Do The Calculation Team
Do The Calculation Editorial Board
The Do The Calculation Editorial Board is comprised of software engineers, finance analysts, and technical contributors focused on building clean, accurate, and easy-to-use calculator tools.