Dew Point Calculator — Free Online Calculator
Dew point tells you when moisture in the air will condense on a cool surface — the key number for diagnosing duct sweating, window fogging, and latent load on the coil. Enter dry-bulb and relative humidity to get dew point (or work backward from a measured dew point to find RH), then optionally check a surface temperature for condensation risk.
Enter your air conditions
Air temperature measured with a standard dry-bulb thermometer.
From a hygrometer or psychrometer reading.
Measured or calculated dew point temperature.
Duct, window, or wall surface — flags condensation if at or below dew point.
Dew point
Relative humidity
55% RH
Comfort band
Comfortable
Condensation check
Enter a surface temperature to check for condensation risk.
See the breakdown
Dew point is a psychrometric property — pair with sensible/latent heat when sizing dehumidification or diagnosing coil load. Insulate ducts when surface temp drops near dew point.
Common dry-bulb / RH → dew point
Magnus approximation at typical indoor conditions. Use the calculator above for other values.
| Dry-bulb (°F) | RH (%) | Dew point (°F) | Typical context |
|---|---|---|---|
| 68°F | 45% | 46°F | Cool-season indoor — low latent load |
| 72°F | 50% | 52°F | ASHRAE comfort midpoint |
| 75°F | 55% | 58°F | Typical summer return air |
| 80°F | 60% | 65°F | Humid climate — elevated latent load |
| 85°F | 70% | 74°F | Deep South return — duct sweating risk in attics |
Sources & standards: ASHRAE Fundamentals (psychrometric properties) · ACCA Manual J (moisture load) · ENERGY STAR HVAC Quality Installation (comfort verification).
The formula, explained in plain English
Dew point comes from the relationship between dry-bulb temperature, relative humidity, and the saturation vapor pressure of water. The Magnus approximation is accurate enough for field psychrometrics and is the same relation inverted to solve for RH when you know dew point.
Why dew point beats RH alone
Two rooms at 50% RH but different temperatures have different dew points. Dew point tracks the actual moisture content — the number that matters when a 55°F supply duct runs through a 95°F attic.
Latent load on the coil
Higher dew point means more water vapor to remove. Use the Sensible & Latent Heat Calculator to split total coil BTU when return air is humid.
Duct insulation target
Insulation keeps the duct surface above ambient dew point. If the outer jacket of flex in an attic drops below the surrounding air's dew point, it sweats regardless of charge.
Pair with Delta T
High humidity widens the latent share of coil capacity and can change the sensible temperature split. Check both psychrometrics and airflow before adjusting refrigerant charge.
Worked examples
Three scenarios: typical comfort conditions, a humid return-air sample, and a duct-sweat check with a surface temperature.
Comfortable indoor — 75°F DB, 55% RH
Result: typical summer return air — latent load is moderate and RH sits in the ASHRAE comfort band.
Humid climate — 80°F DB, 60% RH
Result: elevated moisture load. Consider 350 CFM/ton airflow and verify the coil is not short-cycling on sensible load alone.
Duct sweat check — 75°F DB, 55% RH, 52°F duct surface
Result: the duct jacket is below dew point — add R-8 insulation and a vapor barrier, or relocate duct into conditioned space.
Frequently asked questions
Common questions about dew point, relative humidity, and condensation on HVAC surfaces.
What is dew point in HVAC?
Dew point is the temperature at which water vapor in the air condenses into liquid. In HVAC work it tells you when cool surfaces — supply ducts in a hot attic, chilled beams, or windows — will sweat. If a surface is at or below the air's dew point, moisture forms on it.
What is the dew point at 75°F and 55% RH?
At 75°F dry-bulb and 55% relative humidity, the dew point is about 58°F using the standard Magnus approximation. That means any surface cooler than 58°F in that air mass will be at risk of condensation.
What indoor humidity level is comfortable?
ASHRAE comfort guidance targets 30–50% RH in occupied spaces. Below 30% feels dry; above 60% feels clammy and drives higher latent load on the coil. Dew point is often a better condensation metric than RH alone because it stays tied to the actual moisture content.
Why do supply ducts sweat in summer?
Cold supply air lowers the duct surface temperature. If that surface drops to or below the surrounding air's dew point — common on uninsulated flex in a vented attic — water condenses on the duct. Fix with proper insulation, vapor barrier, and sealing, not just more airflow.
How does dew point relate to superheat and subcooling?
They measure different things. Dew point is an air-moisture property (psychrometrics); superheat and subcooling are refrigerant-side charge checks. High indoor humidity raises latent load on the coil, which can widen the cooling temperature split — pair dew point with the Delta T Calculator when diagnosing comfort complaints.
Is the Magnus formula accurate enough for field work?
Yes for typical HVAC conditions. The Magnus (August–Roche–Magnus) approximation is accurate to within about ±1°F dew point for dry-bulb temperatures between roughly 32°F and 122°F and RH between 5% and 95%. For lab-grade psychrometrics, use ASHRAE reference tables — for duct sweating and comfort checks, Magnus is standard field practice.
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