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: $t_d \approx t - (100 - \varphi)/5$.

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 extract 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

Simple approximation (±1 °C for φ > 50%)

$$t_d \approx t - \frac{100 - \varphi}{5}$$

Example: t = 22 °C, φ = 50% → $t_d \approx 22 - \dfrac{50}{5} = 22 - 10 =$ 12 °C

Magnus–Tetens equation (accurate approximation)

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

Example: t = 22 °C, φ = 50%:

• $\gamma = \ln(0.5) + \dfrac{17.625 \cdot 22}{243.04 + 22} = -0.6931 + 1.463 = 0.770$
• $t_d = \dfrac{243.04 \cdot 0.770}{17.625 - 0.770} \approx$ 11.1 °C

The accuracy of the Magnus–Tetens equation is ±0.3 °C over the range −40 to +60 °C.

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

t [°C]	φ = 30%	φ = 40%	φ = 50%	φ = 60%	φ = 70%	φ = 80%
10	−6.0	−2.6	0.1	2.7	4.9	6.8
15	−2.1	1.6	4.7	7.4	9.6	11.7
20	2.0	6.1	9.3	12.1	14.4	16.5
22	3.7	7.9	11.2	14.0	16.4	18.5
25	6.3	10.5	13.9	16.8	19.2	21.4
30	10.6	15.0	18.5	21.5	24.0	26.2

The dew point in the Mollier diagram

The Mollier h-x diagram is the European counterpart of the psychrometric chart. In it, the dew point is immediately visible: from the air-state point, draw a vertical line (constant $x$) down to the saturation curve (φ = 100%). The intersection is the dew point. Open the interactive diagram

Surface condensation — a worked example

Office: t = 24 °C, φ = 55% → dew point = 14.2 °C.

An exterior window with a U-value of 0.8 W/(m²·K) (quality insulating double/triple glazing): at an outdoor temperature of −12 °C and an indoor temperature of 24 °C, the inner glass surface temperature (from the temperature factor $f_{Rsi}=1-U\cdot R_{si}$, with $R_{si}=0.13\ \text{m}^2\text{·K/W}$) works out to roughly 20 °C, comfortably above the 14.2 °C 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-value (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 = 20 °C, the dew point rises by 1 °C for every increase of about 3–4% in φ. So going from φ = 50% ($t_d \approx 9.3$ °C) to φ = 70% ($t_d \approx 14.4$ °C) is a difference of about 5 °C.

What dew point is “safe” for chilled-water piping? For pipes carrying a chilled medium on a 6/12 °C flow/return regime (chilled water), the insulation surface must stay above the room’s dew point. At t = 26 °C, φ = 55%, $t_d \approx 16$ °C, so the insulation must keep its surface above 16 °C.

How does the dew point differ from the wet-bulb temperature? The dew point is the condensation temperature at constant humidity. 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 mould 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 mould needs to take hold and spread quickly.