Dew point is the temperature at which air becomes saturated and moisture begins to condense. When any surface cools to or below the dew point, water condenses on it. It's a direct measure of how much moisture is in the air, independent of the air temperature.

How do you calculate dew point from temperature and humidity?

Use the Magnus formula: compute α = ln(RH/100) + (a·T)/(b+T), then dew point Td = (b·α)/(a−α), with constants a = 17.27 and b = 237.7 when temperature is in °C. Convert °F to °C first, then convert the result back to °F. It's accurate to about ±0.4°C from 0–60°C.

When does condensation form?

Condensation forms when a surface temperature drops to or below the air's dew point. For example, if the indoor dew point is 55°F, any duct, window, or pipe surface at 55°F or colder will sweat. Keeping surfaces above the dew point — or lowering the dew point with dehumidification — prevents it.

What relative humidity promotes mold?

Sustained indoor relative humidity above about 60% encourages mold growth. Keeping indoor RH in the 40–60% range is a common comfort and health target. Dew point matters too: high dew points mean cold surfaces will sweat and feed mold.

Dew Point & Relative Humidity Calculator

Free dew point calculator for HVAC. Find the dew point from dry-bulb temperature and relative humidity (or solve RH from dry-bulb and dew point), with condensation-risk guidance.

How the dew point calculation works

Dew point is the temperature at which air becomes saturated and water starts to condense. It’s a direct measure of the moisture in the air. This tool uses the well-validated Magnus formula:

α = ln(RH ÷ 100) + (a × T) ÷ (b + T)

Dew Point = (b × α) ÷ (a − α)

with constants a = 17.27 and b = 237.7 when temperature is in °C. Fahrenheit inputs are converted to Celsius, solved, and converted back. The formula is accurate to about ±0.4°C across the 0–60°C range. The calculator also runs it in reverse to find relative humidity from dry-bulb temperature and a measured dew point.

Worked example

At 75°F and 50% RH, the dew point is about 55°F — so any surface at 55°F or colder (a cold supply duct, a chilled-water pipe, a window) will sweat.

Why dew point matters

Condensation forms whenever a surface cools to or below the dew point. That’s the cause of sweating ducts, window fogging, and hidden moisture in wall cavities.
Mold risk climbs when indoor RH stays above ~60%. A comfortable, healthy target is roughly 40–60% RH.
Controlling dew point (through dehumidification or by keeping surfaces warm) is how you stop condensation problems.

Quantifying the moisture load that comes with that humidity? Use the Sensible & Latent Heat Calculator.

How FieldPad helps in the field

Check the dew point here, then document humidity and condensation findings in FieldPad against the equipment and the client. Turn a humidity or sweating-duct diagnosis into a client-ready estimate, all on one offline record that travels with the job.

Dew Point & Relative Humidity Calculator

How the dew point calculation works

Worked example

Why dew point matters

How FieldPad helps in the field

Stop Re-Running These Numbers by Hand

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Sensible, Latent & Total Heat Calculator