Dew Point Calculator

Formula and method

T in°C and RH in percent: g=ln(RH/100)+17.62 T/(243.12+T); Td=243.12 g/(17.62−g). Fahrenheit input converts by(TF−32)× 5/9; dew point°F=Td× 9/5+32.

Worked example

At 25°C and 50%RH, water dew point is about 13.852°C(56.933°F). At 20°C/40%RH it is about 5.980°C; at 100%RH it equals air temperature.

Assumptions and limits

Source fixed to Sensirion Introduction to Humidity, Version 2.0/August 2009, p 1 Table 1/p 2 Eq 12: water coefficients 17.62 and 243.12°C. Other versions are not mixed.
Both air temperature and computed dew point must lie within−45 to 60°C. RH must exceed 0 and be at most 100%. Very low RH may put dew point outside the supported range; extrapolation is rejected.
A negative water dew point still refers to supercooled water; it is not automatically frost point. Water-RH and ice-RH differ. Frost point needs different coefficients and a consistent humidity reference.
Pressure enhancement is neglected for normal atmosphere. No compressed-air, gas-mixture, altitude correction or condensation-safety certification. Vapor-pressure approximation error is not a dew-point temperature accuracy guarantee; sensor errors propagate.

Frequently asked questions

What happens at RH 100?

Within the supported range, dew point equals air temperature. RH 0 requires ln(0) and is rejected rather than replaced by an arbitrary small value.

Can this size a compressed-air dryer?

Not directly. Pressure dew point depends on measurement pressure. Use manufacturer data and an explicit pressure basis; this is an atmospheric water-dew-point approximation.

References

Dew Point Calculator practical guide and common questions

What does Dew Point Calculator calculate?

Enter air temperature and RH referenced to water. Uses water coefficients from Sensirion’s original 2009 application note for a normal-atmosphere approximation, not pressure dew point or wet bulb. T in°C and RH in percent: g=ln(RH/100)+17.62 T/(243.12+T); Td=243.12 g/(17.62−g). Fahrenheit input converts by(TF−32)× 5/9; dew point°F=Td× 9/5+32.

When is this useful?

At 25°C and 50%RH, water dew point is about 13.852°C(56.933°F). At 20°C/40%RH it is about 5.980°C; at 100%RH it equals air temperature.

How do I enter the inputs?

Enter the inputs for Dew Point Calculator: Air temperature, Input temperature unit, Water-referenced RH (above 0 through 100). Calculate, check the units and assumptions, then copy or download the results.

How should I interpret the results?

Source fixed to Sensirion Introduction to Humidity, Version 2.0/August 2009, p 1 Table 1/p 2 Eq 12: water coefficients 17.62 and 243.12°C. Other versions are not mixed. Both air temperature and computed dew point must lie within−45 to 60°C. RH must exceed 0 and be at most 100%. Very low RH may put dew point outside the supported range; extrapolation is rejected. A negative water dew point still refers to supercooled water; it is not automatically frost point. Water-RH and ice-RH differ. Frost point needs different coefficients and a consistent humidity reference. Pressure enhancement is neglected for normal atmosphere. No compressed-air, gas-mixture, altitude correction or condensation-safety certification. Vapor-pressure approximation error is not a dew-point temperature accuracy guarantee; sensor errors propagate.

Where are my inputs processed?

Dew Point Calculator runs in this browser. Inputs are not uploaded to a calculation server; copying and CSV download require your action. The existing site analytics and advertising still load.