Epoxy cure time & temperature calculator

The times on an epoxy label are measured at one temperature — usually around 70–77°F (21–25°C). WEST SYSTEM’s Epoxyworks articles give the rule of thumb: every 18°F (10°C) warmer roughly halves the times, and every 18°F cooler roughly doubles them. Start with Which epoxy?: tabletop coatings, deep-pour river-table resins and art resins run on very different clocks, so each choice uses one maker’s published figures — TotalBoat TableTop, EcoPoxy FlowCast, MAS Deep Pour X or ArtResin. Using something else? Pick My own label and type the numbers from your label. Then pick the time you’ll start mixing and you get a schedule in clock times.

The 18-degree rule

Epoxy cures by a chemical reaction, and like most reactions it speeds up as it warms. WEST SYSTEM’s Epoxyworks puts it as a rule of thumb: every 18°F (10°C) increase roughly halves the pot life and cure time, and every 18°F drop roughly doubles them. It’s approximate — real products don’t follow it exactly, especially far from the label temperature — but it’s a far better planning figure than the label time alone when your garage is at 55°F.

factor = 2 ^ ((label temp − your temp) ÷ 18°F)      (÷ 10 in °C)
estimated time = label time × factor

Example with the tabletop figures — TotalBoat TableTop’s 40 minutes of working time and at least 3 hours before the next coat, both at 70°F — in a 60°F basement: 70 − 60 = 10°F, so the factor is 2^(10 ÷ 18) ≈ 1.47. Expect roughly 59 minutes to work, about 4.4 hours before you can recoat and about 10 days to full cure instead of 7. The bigger catch is that TotalBoat’s application range starts at 65°F, and the calculator flags that too. Worked examples: how long epoxy takes to cure at 60°F, will epoxy cure at 50°F and epoxy working time at 90°F. For how dry-to-the-touch, recoat and full cure differ between tabletop, deep-pour and floor epoxies, see how long epoxy takes to cure.

Recoat window, demold and full cure

Labels give several different times, and the calculator scales each one by the same factor:

• Working time (pot life) — how long the mixed epoxy stays runny. After this it starts to thicken and won’t level.
• Earliest recoat — the next coat can go on. Before this, the first coat is too soft and the two can drag or wrinkle.
• Latest recoat without sanding — some labels give a window. Pour the next coat before it closes and the layers bond chemically; after it, the surface is too hard to grip, so scuff-sand it (and wipe off any blush) first.
• Demold / unclamp — for castings and river tables, when the piece can come out of the form.
• Full cure — hard enough for normal use, heat and cleaning sprays. Dry to the touch comes long before this.

Pick the time you’ll start mixing and the schedule turns those durations into clock times — for example, the 60°F example above with a 24-hour latest recoat on the label and a 9:00 am start gives “recoat from 1:25 pm to 8:15 pm (+1 day)”, where “+1 day” means the next day. The engine doesn’t know today’s date, so there are no weekdays; count the days on from pour day.

Which temperature to enter

The one that matters is the epoxy’s own temperature while it cures — and a thin coat takes on the temperature of what it’s sitting on. A concrete floor, a slab brought in from a cold shop or a tabletop by a drafty door can be several degrees colder than the air. Enter the room temperature and, if you can measure it, the surface temperature: the calculator uses the colder of the two.

Many labels also give a minimum or an application range, and it varies a lot by product. In the WEST SYSTEM range alone, 205 Fast Hardener is formulated to cure down to 40°F, 206 Slow and 207 Special Clear shouldn’t be used below 60°F, and 209 Extra Slow not below 70°F. TotalBoat lists 65–80°F for its TableTop epoxy; MAS asks for 60–80°F for Deep Pour X. Below the minimum, Epoxyworks warns, epoxy may eventually harden but not reach a complete cure — so enter the label minimum if there is one, and the calculator flags it.

At the hot end, anything above about 122°F (50°C) is only for a heat post-cure that a data sheet specifies after the epoxy has already set; the calculator warns there, and stops above about 150°F (65°C).

Humidity and the dew point

The dew point is the temperature at which moisture in the air starts to condense on a surface. A slab or tabletop at or near it gets a film of water you often can’t see, and epoxy over it can lose adhesion, cure cloudy or form amine blush — a greasy film on the surface. Sherwin-Williams’ flooring advice is to stay at least 5°F above the dew point before and throughout application and cure; the coatings trade uses 3°C in metric. Enter the humidity (and the surface temperature, ideally) and the calculator works the dew point out with the Magnus formula:

γ = ln(RH ÷ 100) + 17.62 × T ÷ (243.12 + T)
Td = 243.12 × γ ÷ (17.62 − γ)      (T and Td in °C)
coat only if surface ≥ Td + 3°C (5°F)

Example: a 60°F garage at 85% humidity has a dew point of about 55°F. A slab at 60°F is only about 4.5°F above it — inside the 5°F margin — so the calculator says not to coat. Run a dehumidifier or warm the slab, and avoid starting late in the day when the temperature is about to fall.

Why the bucket is different

Pot life on a data sheet is usually measured on a small mass of mixed epoxy in a cup — WEST SYSTEM uses 100 grams. Its 105/205 data sheet shows how big the gap is: 9–12 minutes of pot life in the cup at 72°F, but 60–70 minutes of working time spread as a thin film. The more you mix at once, the more exotherm (the heat the resin gives off as it cures) builds up, so a gallon in a bucket sets far faster than the label suggests while the same epoxy spread thin on a table stays workable longer. Pour big batches out of the bucket promptly — especially in a warm room.

Working in the cold

• Store resin and hardener indoors so they start warm; cold resin is thick and holds bubbles.
• Warm the room and the surface for several hours before pouring, and keep it warm through the cure.
• Don’t aim a heater at a fresh pour — it blows dust and can create hot spots. Fuel-burning heaters also add moisture to the air, which raises the dew point.
• Slow, cold cures can leave a greasy amine blush on some epoxies — see why epoxy stays sticky.