Dew Point Calculation: Formulas, Tables, Online Calculator

The dew point (dew-point temperature, $t_d$) is the temperature to which air must be cooled to reach 100% relative humidity. It is calculated from the dry-bulb temperature $t$ and the relative humidity $\varphi$. The simplest rough formula in IP units: $t_d \approx t - 0.36\,(100 - \varphi)$.

Why we need the dew point

The dew point is a key quantity when designing:

• Cooling coils in air handling units — to condense out excess moisture, the coil must cool the air below its dew point as required
• Heat-recovery exchangers — condensation on the fins occurs below the dew point of the exhaust air
• Thermal bridges — a surface below the dew point of the indoor air is prone to mold growth
• Chilled-water piping — the insulation must prevent condensation on the pipe surface

Dew point calculation — formulas

A note on units before we start. The empirical coefficients in the equations below are defined for degrees Celsius, and that is how the math is carried out internally — IP is a display conversion, not a separate calculation path. So when you work these by hand from IP inputs, convert to °C first, solve, then convert the result back to °F. Only the rough rule of thumb has a clean IP form.

Simple approximation (±2°F for φ > 50%)

$$t_d \approx t - 0.36\,(100 - \varphi)$$

Example: t = 72°F, φ = 50% → $t_d \approx 72 - 0.36 \cdot 50 = 72 - 18 =$ 54°F

Magnus–Tetens equation (accurate approximation)

Coefficients apply to $t$ in °C:

$$\gamma(t, \varphi) = \ln\!\left(\frac{\varphi}{100}\right) + \frac{17.625 \cdot t}{243.04 + t}$$ $$t_d = \frac{243.04 \cdot \gamma}{17.625 - \gamma}$$

Example: t = 72°F, φ = 50%:

• Convert: 72°F = 22.22 °C
• $\gamma = \ln(0.5) + \dfrac{17.625 \cdot 22.22}{243.04 + 22.22} = -0.6931 + 1.4765 = 0.7834$
• $t_d = \dfrac{243.04 \cdot 0.7834}{17.625 - 0.7834} \approx 11.3\ \text{°C} =$ 52.4°F

Note the rule of thumb ran about 1.6°F high here — that is typical, and the gap widens fast below 50% RH.

The accuracy of the Magnus–Tetens equation is ±0.5°F over the range −40 to 140°F.

ASHRAE Hyland–Wexler (standard-grade accuracy)

ASHRAE Fundamentals 2021 defines the saturation vapor pressure with an equation containing six empirical coefficients (a combination of $1/T$, $T$, $T^2$, $T^3$, and $\ln T$ terms) — highly accurate, on the order of hundredths of a percent against reference data.

Table of dew points

Dew-point temperature [°F] at standard atmospheric pressure, computed per ASHRAE 2021.

t [°F]	φ = 30%	φ = 40%	φ = 50%	φ = 60%	φ = 70%	φ = 80%
50	21.3	27.4	32.2	36.8	40.7	44.2
60	29.0	35.8	41.5	46.2	50.3	53.9
70	37.3	44.7	50.7	55.6	59.9	63.7
75	41.6	49.2	55.2	60.3	64.7	68.5
80	45.9	53.7	59.8	65.0	69.5	73.4
90	54.6	62.6	69.0	74.4	79.0	83.1

The dew point on the psychrometric chart

On the psychrometric chart the dew point is immediately visible: from the air-state point, draw a horizontal line (constant humidity ratio $x$) left to the saturation curve (φ = 100%). The intersection is the dew point. In the Mollier h-x chart — the European layout, with the axes transposed — the same construction runs vertically instead.

Surface condensation — a worked example

Office: t = 75°F, φ = 55% → dew point = 58°F.

An exterior window with a U-factor of 0.14 Btu/(h·ft²·°F) (quality insulating double/triple glazing): at an outdoor temperature of 10°F and an indoor temperature of 75°F, the inner glass surface temperature (from the temperature factor $f_{Rsi}=1-U\cdot R_{si}$, with $R_{si}=0.74\ \text{h·ft}^2\text{·°F/Btu}$) works out to roughly 68°F, comfortably above the 58°F dew point, so condensation on the glass pane is not a risk. The risk remains at less-insulated spots with a markedly higher local U-factor (frame, edge of the glazing, spacer bar), where the surface temperature can drop below the dew point even in an otherwise high-quality window.

Where the risk exists, the fix is to lower the humidity below 40% in winter, or to improve the thermal insulation of the frame and glazing edge.

Frequently asked questions

How quickly does the dew point change with humidity? Roughly: at t = 70°F, the dew point rises by 1°F for every increase of about 2% in φ. So going from φ = 50% ($t_d \approx 50.7$°F) to φ = 70% ($t_d \approx 59.9$°F) is a difference of about 9°F.

What dew point is “safe” for chilled-water piping? For pipes carrying a chilled medium on a 42/54°F supply/return regime (chilled water), the insulation surface must stay above the room’s dew point. At t = 79°F, φ = 55%, $t_d \approx 62$°F, so the insulation must keep its surface above 62°F.

How does the dew point differ from the wet-bulb temperature? The dew point is the condensation temperature at constant humidity ratio. The wet-bulb temperature is lower than the dry-bulb temperature but higher than the dew point — it is the equilibrium temperature during evaporation. They coincide only at φ = 100%.

How is the dew point linked to mold growth in bathrooms? Showering drives the humidity in a bathroom up sharply, and the dew point climbs with it. Room corners are usually the coldest surfaces in the space: thermal bridges meet there and the air barely moves. As soon as the wall surface in the corner drops below the dew point, moisture condenses out of the air onto it. That permanently damp substrate is exactly what mold needs to take hold and spread quickly.