Filament humidity calculator
Work out the room dew point and check whether a cold surface could collect condensation. Your room reading cannot tell you the humidity inside a sealed box.
Enable JavaScript to enter your readings. The formula and worked example below remain available without it.
What this calculation tells you
Dew point is the temperature at which the entered air reaches saturation. A surface at or below that temperature may collect condensation. This is useful when moving spools or storage boxes between a warm room and a cold garage.
The formula
Using the National Physical Laboratory's Magnus approximation (checked 2026-10-25):
γ = ln(RH / 100) + 17.62 × T / (243.12 + T)Td = 243.12 × γ / (17.62 − γ)
T is room temperature in °C; RH is relative humidity as a percentage. The tool accepts −45 to 60 °C and humidity above zero up to saturation. Below freezing, frost point differs from dew point over liquid water.
Worked example
At 20 °C and 50% relative humidity, the room dew point is 9.3 °C. A surface at 8 °C is below that dew point; a surface at 15 °C is above it. This example is calculated with the same engine used by the tool.
What it cannot tell you
The result does not measure filament moisture, predict print quality, or estimate humidity inside a sealed box. Box volume and desiccant weight alone do not establish drying capacity. Follow the desiccant maker's sizing and regeneration guidance.