What Is Dew Point?
Dew point is the temperature at which air becomes saturated (100% RH) and water vapor starts condensing into liquid. It measures the actual moisture content of the air — unlike relative humidity, it doesn't change when air is simply heated or cooled. That makes it the single most useful number for condensation risk: any surface colder than the air's dew point will sweat.
How Dew Point Is Calculated
pw = (RH ÷ 100) × pws(Tdb) — water vapor partial pressure
Tdp = temperature where pws(Tdp) = pw — saturation inversion
W = 0.621945 × pw ÷ (P − pw) — humidity ratio
This calculator uses the ASHRAE Hyland–Wexler saturation-pressure formulations (the same equations behind professional psychrometric software) with separate water and ice branches, so it stays accurate from −20 to 60 °C and at any altitude via the pressure input.
Worked Example — Room Air at 25 °C, 50% RH
- Saturation pressure at 25 °C: pws = 3.169 kPa
- Vapor pressure: pw = 0.50 × 3.169 = 1.585 kPa
- Dew point = temperature where saturation pressure equals 1.585 kPa → 13.9 °C
- Humidity ratio W = 0.621945 × 1.585 ÷ (101.325 − 1.585) = 9.88 g/kg
- Wet bulb ≈ 17.9 °C, enthalpy ≈ 50.3 kJ/kg
So in a 25 °C / 50% office, an uninsulated chilled-water pipe at 7–12 °C is below the 13.9 °C dew point — it will drip. You can see this point plotted on our free printable psychrometric chart (PDF).
Where Dew Point Matters in MEP Design
- Chilled water pipe insulation: insulation thickness is selected so the outer jacket surface stays above the room dew point — the anti-sweat criterion.
- Duct and diffuser sweating: supply air at 12–14 °C can pull metal surfaces below the dew point of humid room or ceiling-void air.
- Cooling coil dehumidification: a coil only removes moisture when its surface temperature is below the entering air dew point (apparatus dew point concept).
- Cold walls & thermal bridges: in hot-humid climates, poorly insulated walls or window frames below indoor dew point grow mold.
- Airflow & load checks: pair with our TR to CFM Calculator and Online Ductulator for the rest of the quick HVAC workflow.
Dew Point Comfort Scale
| Dew Point | Perception |
|---|
| < 10 °C (< 50 °F) | Dry / very comfortable |
| 10 – 15.5 °C (50 – 60 °F) | Comfortable |
| 15.5 – 18.3 °C (60 – 65 °F) | Slightly humid |
| 18.3 – 21 °C (65 – 70 °F) | Humid / muggy |
| > 21 °C (> 70 °F) | Oppressive |
Frequently Asked Questions
What is dew point?
The temperature at which air becomes saturated (100% RH) and water vapor starts condensing. Any surface colder than the air's dew point will collect condensation — that's why cold pipes sweat. Air at 25 °C / 50% RH has a dew point of about 13.9 °C.
How do you calculate dew point?
Find the vapor partial pressure pw = RH × pws(T), then find the temperature whose saturation pressure equals pw — that's the dew point. This tool uses the ASHRAE Hyland–Wexler formulations for accuracy from −20 to 60 °C.
What is a comfortable dew point?
Below 10 °C (50 °F): dry and very comfortable. 10–15.5 °C (50–60 °F): comfortable. 15.5–18.3 °C: slightly humid. 18.3–21 °C: humid/muggy. Above 21 °C (70 °F): oppressive. Dew point describes comfort better than RH because it measures actual moisture.
Dew point vs relative humidity — what's the difference?
RH is a percentage relative to saturation at the current temperature, so it changes when air is heated or cooled even with the same moisture. Dew point measures actual moisture content and stays constant during sensible heating/cooling — making it the better condensation-risk indicator.
Why does dew point matter in HVAC design?
Chilled pipes, ducts and diffusers below the room dew point will sweat — insulation is sized to keep surfaces above it. Cooling coils dehumidify only when the coil surface is below the entering air dew point, and the supply air dew point controls space humidity.
Disclaimer: Calculated using ASHRAE psychrometric formulations at the entered barometric pressure. For critical applications (clean rooms, cold stores, museum environments), verify with calibrated instruments and project-specific analysis.