Dew Point Calculator

Use our dew point calculator to calculate dew point (°F) from air temperature and relative humidity. Also returns actual vapor pressure (psi) and absolute humidity (lb/cu ft). Includes the dew point formula, how dew point is calculated, and step-by-step instructions.

Air temperature (°F)

Relative humidity (%)

Results

Dew point (°F)

—

Actual vapour pressure (psi)

—

Absolute humidity (lb/cu ft)

—

Steps: saturation vapor pressure from temperature → multiply by RH → dew point from vapor pressure → vapor density from ideal gas law.

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What Are Dew Points?

Dew point is the temperature at which air becomes saturated with water vapor. If the air cools to the dew point, water vapor can start condensing into liquid water (dew, fog, or cloud droplets).

Dew point is a direct indicator of how much moisture is in the air. Higher dew points feel more humid and sticky, while lower dew points feel drier.

This calculator finds dew point from air temperature (°F) and relative humidity (%). It also estimates actual vapor pressure (psi) and absolute humidity (lb/cu ft), which are related ways to describe moisture in the air.

Dew Point Formula

A common way to calculate dew point uses a Magnus-type approximation. The formula is typically applied in Celsius, then converted back to Fahrenheit.

Fahrenheit to Celsius =

T(°C) = (T(°F) − 32) × 5/9

Convert the input air temperature to °C for the dew point equation.

Magnus helper value =

γ(T, RH) = ln(RH/100) + (a × T) / (b + T)

Use T in °C and RH as percent. Common constants: a = 17.27, b = 237.7.

Dew point (Celsius) =

Td(°C) = (b × γ) / (a − γ)

Computes dew point in °C.

Celsius to Fahrenheit =

Td(°F) = Td(°C) × 9/5 + 32

Convert dew point back to °F.

Saturation vapor pressure (Magnus form) =

e_s = 6.112 × expa₂ × T) / b₂ + T))

T in °C, e_s in hPa. Common constants: a2 = 17.67, b2 = 243.5.

Actual vapor pressure =

e = (RH / 100) × e_s

e is in hPa if e_s is in hPa.

Convert vapor pressure to psi =

eps_i = e(hPa) × 0.0145038

1 hPa ≈ 0.0145038 psi.

Absolute humidity (vapor density) =

ρ_v = e(Pa) / (R_v × T(K))

Use e in Pa (e(Pa) = e(hPa) × 100), T(K) = T(°C) + 273.15, and R_v = 461.5 J/(kg·K). Result is kg/m^3.

Convert kg/m^3 to lb/cu ft =

ρ_v(lb/ft^3) = ρ_v(kg/m^3) × 0.06242796

1 kg/m^3 ≈ 0.06242796 lb/ft^3.

= Air temperature (°F, converted to °C and K in formulas)

= Relative humidity (%)

= Dew point temperature (°F)

e_s

= Saturation vapor pressure (hPa)

= Actual vapor pressure (hPa, then converted to psi and Pa)

ρ_v

= Absolute humidity (kg/m^3, then converted to lb/cu ft)

R_v

= Water vapor gas constant = 461.5 J/(kg·K)

Example: calculate dew point from temperature and relative humidity

T = 80°F, RH = 60% → Td ≈ 65.2°F

Higher relative humidity pushes dew point closer to air temperature.

How to Calculate Dew Point

1
Enter the air temperature (°F).
2
Enter the relative humidity (%).
3
The calculator estimates dew point (°F) using a Magnus-type dew point equation.
4
It also calculates actual vapor pressure (psi) and absolute humidity (lb/cu ft) from the same inputs.

Frequently Asked Questions

How is dew point calculated?

A common method uses air temperature and relative humidity in a Magnus-type approximation to estimate the temperature where the air would be saturated (the dew point).

What happens when dew point is reached?

When air cools to the dew point, it becomes saturated and condensation can begin. That can produce dew on surfaces, fog near the ground, or cloud formation depending on conditions.

How do you calculate dew point from relative humidity?

You need both relative humidity and air temperature. The same RH at different temperatures can produce different dew points, so RH alone is not enough.

Is there a dew point formula in Fahrenheit?

Most standard dew point equations are written for Celsius. This calculator converts your Fahrenheit temperature to Celsius internally, computes dew point, then converts the result back to °F.

Can I calculate dew point in Excel?

Yes. You can implement the same steps in Excel using ln(), exp(), and the Magnus constants, then convert between °F and °C as needed.

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