Exothermic Heat Calculator
Estimate the peak temperature and heat risk when pouring resin, based on volume, resin type, and ambient conditions.
About this calculator
This calculator estimates how hot a resin pour will get by modeling two things at once: how much total heat the curing reaction generates, and how well that heat can escape before it builds up. Total heat scales directly with pour volume and a per-ounce BTU factor unique to each resin family — polyester generates roughly 3.5 BTU/oz (highest exotherm), standard epoxy about 1.8, urethane about 1.0 — reflecting real differences in how vigorously each chemistry reacts. To estimate how concentrated that heat becomes, the model treats the pour as if it were a sphere, derives its surface area from that assumed shape, and computes a surface-area-to-volume ratio: a lower ratio means less surface for heat to radiate away relative to how much resin is generating it, so temperature rise concentrates rather than dissipates — which is exactly why doubling pour volume raises peak temperature more than proportionally.
The resulting temperature rise is added to your ambient room temperature to estimate peak internal temperature, which then maps to a risk tier: above 150°F is worth monitoring, above 200°F risks yellowing or warping, above 300°F risks cracking or smoking, and above 400°F is flagged as a fire or smoke risk requiring a smaller pour. Time-to-peak scales mildly with volume (larger pours take somewhat longer to reach their peak, not less). Because this treats every pour shape as an idealized sphere, an actual thin, wide pour like a tabletop will run cooler than this estimate suggests, while a tall, narrow pour in a deep mold will run hotter — shape matters as much as volume in real exotherm risk.
Inputs
Results
Estimated Peak Temp
166 °F
Risk Level
Low-Moderate — monitor closely
How to Use This Calculator
- Enter the pour volume in ounces — larger pours generate more total heat and risk cracking or yellowing.
- Enter the ambient temperature in °F; higher room temperatures accelerate the exothermic reaction.
- Set the mix ratio to match your resin's label (1:1, 2:1, or 3:1).
- Select the resin type: polyester (high exotherm), epoxy (medium), or urethane (low).
- Read the estimated peak temperature rise and time to peak to know when to monitor the pour.
- If peak temperature is unsafe, split into thinner layers or pre-cool the mold to reduce heat buildup.
How the result changes with Pour Volume
| Pour Volume | Estimated Peak Temp | Risk Level |
|---|---|---|
| 16 | 116 °F | Low |
| 24 | 141 °F | Low |
| 48 | 216 °F | Moderate — may yellow or warp |
| 80 | 320 °F | High — may crack, yellow, or smoke |
What each input means
- Pour Volume
- Total volume of mixed resin being poured at once in fluid ounces. Larger pours generate significantly more heat.
- Ambient Temperature
- Room temperature where curing takes place. Higher ambient temperatures add to peak temperature.
- Mix Ratio
- Resin-to-hardener ratio. More hardener (1:1) generally produces more heat than less hardener (3:1).
- Resin Type
- Polyester resins generate the most heat. Epoxies are moderate. Urethanes generate the least exothermic heat.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersPour Volume = 32, Ambient Temperature = 72, Mix Ratio = 1, Resin Type = 2 = 4 input(s) provided
- Calculate Estimated Peak TempEstimated Peak Temp166 = 166
- Calculate Risk LevelRisk LevelLow-Moderate — monitor closely = Low-Moderate — monitor closely
- Calculate Temperature RiseTemperature Rise94 = 94
- Calculate Time to PeakTime to Peak31 = 31
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does polyester resin get a much higher peak temperature estimate than urethane for the same pour size?
Each resin type has its own BTU-per-ounce heat generation factor built into the model — 3.5 for polyester, 1.8 for standard epoxy, 1.0 for urethane — reflecting real differences in how vigorously each chemistry's curing reaction releases heat. Total heat and the resulting temperature rise scale directly with that per-ounce factor, so polyester's total BTU output is roughly double epoxy's and 3.5 times urethane's at the identical pour volume.
Why does the calculator care about the pour's surface-area-to-volume ratio?
It estimates the pour as a sphere, derives that sphere's surface area, and computes surface area divided by volume — a lower ratio means less surface exists to radiate heat away relative to how much resin is generating it, so heat concentrates and temperature climbs faster. This is the specific mechanism behind why doubling pour volume raises peak temperature by more than double, not just proportionally.
Does the mix ratio input actually change the peak temperature result?
No — the engine reads pourVolumeOz, ambientTempF, and resinType, but the mixRatio input isn't used in the calculation itself. The interface note that more hardener (1:1) generally produces more heat than less (3:1) is a real chemistry consideration worth knowing, but it isn't reflected numerically in this calculator's peak temperature or risk outputs.
My pour is a thin, wide tabletop slab — is the peak temperature estimate accurate for my shape?
The model assumes an idealized sphere shape when calculating surface-area-to-volume ratio, so it doesn't distinguish pour geometry. A thin, wide pour like a tabletop has proportionally much more surface area to shed heat than a sphere of the same volume, so it will run cooler than this estimate suggests — while a tall, narrow pour in a deep mold has less relative surface area and will run hotter than predicted.
Related Calculators
The questions that sit next to this one — chosen by subject, including calculators filed under a different category.
Resin Curing Time Calculator
Estimate pot life, demold time, and full cure time for epoxy resin based on mass, ambient temperature, and resin type.
Resin & EpoxyRiver Table Calculator
Calculate the total epoxy needed for a river table based on table length, river width, thickness, and average void depth.
Chemical EngineeringHeat of Reaction Calculator
Calculate the standard enthalpy of reaction (ΔH) from formation enthalpies of reactants and products using Hess's Law.
Resin & EpoxyEpoxy Volume Calculator
Calculate the exact amount of resin and hardener needed for a rectangular mold based on dimensions and mix ratio.
More in Crafts, Hobbies & Games.