What temperature should I see when distilling at 40% ABV?

At sea level (101.325 kPa), an ethanol-water vapour produces 40% ABV distillate at approximately 82.3°C vapour temperature. Thermometer placement in the lyne arm will match most closely.

Why does altitude affect distillation temperature?

At higher altitudes, atmospheric pressure is lower, which reduces the boiling point of ethanol-water mixtures. At 1000 m elevation, distillation temperatures are about 3–4°C lower than at sea level.

Can I use still temperature to time my cuts?

Temperature gives a useful guide but should not be your sole method. The ethanol-water VLE curve flattens significantly between 40–80% ABV — a 5°C change can represent a 15–20% ABV swing.

Where should I place my thermometer in a pot still?

For vapour temperature readings, place the probe in the vapour path — in the swan neck or lyne arm just above the pot. Thermometers in the pot liquid measure liquid temperature and will give incorrect ABV estimates.

Vapor Temperature to ABV Calculator

Vapor Temperature ↔ ABV

Type in either field — the other updates instantly

Vapor Temperature

°C

Valid range: 78.15°C – 100°C at sea level

⇄

Distillate ABV

Valid range: 0% – 97.2% ABV

Atmospheric Pressure

hPa

Standard sea level = 1013.25 hPa

Or enter altitude

Converts to pressure automatically

Vapor Temperature

—°C

Distillate ABV

—%

US Proof

—°

UK Proof

—°

Pressure correction

—°C

Based on ethanol-water VLE data at the specified atmospheric pressure. Results assume a pot still with no rectification. Actual distillate ABV may vary slightly with still design, heating rate, and column height.

Vapor Temperature Reference Table

At standard atmospheric pressure (1013.25 hPa / sea level). Values for a pot still with no rectification.

Vapor Temp (°C)	Vapor Temp (°F)	Distillate ABV	US Proof	Typical fraction

Why Atmospheric Pressure Matters

Water boils at 100°C at sea level, but at lower temperatures at altitude because atmospheric pressure is lower. The same applies to ethanol-water mixtures — the entire temperature-ABV curve shifts downward as altitude increases.

At 1000m elevation, atmospheric pressure is roughly 900 hPa and all boiling points drop by approximately 3.5°C. This means that at 1000m altitude, a vapor temperature of 83°C corresponds to a different distillate ABV than at sea level. If you use a sea-level table at altitude, every reading will be wrong by 2–4°C — which translates to 5–15% ABV error.

Pressure correction ≈ (1013.25 − P) × 0.037 °C/hPa

The calculator applies this correction automatically. Enter your local atmospheric pressure directly, or enter your altitude and it will calculate the pressure for you using the standard barometric formula.

Understanding the VLE Curve

The relationship between vapor temperature and distillate ABV comes from the vapor-liquid equilibrium (VLE) of ethanol-water mixtures. At any given temperature, ethanol and water are present in the vapor above the boiling liquid in a proportion determined by their relative volatilities and the non-ideal interactions between the two molecules.

Ethanol is more volatile than water — it prefers to be in the vapor phase. This is why a pot still enriches ethanol: the vapor that rises is always richer in ethanol than the liquid it came from. As distillation proceeds, the ethanol in the boiler is depleted, the liquid gets more watery, and the vapor temperature rises. This is exactly what your still thermometer measures over the course of a run.

The curve is not linear — the ABV drops slowly at first when temperatures are near the azeotrope (78.15°C / 97.2% ABV), then drops increasingly steeply as the run progresses toward pure water at 100°C.