Path Load → Airflow → Duct Size → Comfort Check

Psychrometric Calculator

Enter dry bulb temperature and relative humidity to read dew point, wet bulb, enthalpy, humidity ratio, and ASHRAE 55 comfort zone status.

Psychrometrics

Dry bulb + RH → all comfort and system parameters

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Educational estimate only. Psychrometric values use established formulas (Magnus, Stull 2011, ASHRAE) and are suitable for residential planning and learning. Not a substitute for certified engineering analysis or code-required load documentation.

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What a Psychrometric Calculator Tells You

Psychrometrics is the study of moist air — how temperature and humidity interact. Enter a dry-bulb temperature and relative humidity above, and the calculator works out the properties that actually determine comfort and system performance: dew point, wet bulb temperature, humidity ratio, and enthalpy (the total heat energy in the air). None of these show up on a thermostat, but HVAC equipment is designed around them.

The short version: two rooms can both read "75°F" on a thermostat and feel completely different depending on humidity. Psychrometrics is the math that explains why.

Dew Point vs. Wet Bulb: What's the Difference

Dew point is the temperature at which air becomes fully saturated and starts condensing moisture — it's why your AC's evaporator coil "sweats" and drains water. A dew point above roughly 60°F starts to feel muggy indoors, regardless of the air temperature.

Wet bulb temperature is what a thermometer reads if you wrap a wet cloth around the sensor and let evaporation cool it — it's always lower than or equal to dry-bulb (actual air) temperature, and the gap between them tells you how much evaporative cooling potential the air still has. HVAC technicians use wet bulb readings to check whether a system is removing the right amount of moisture, not just heat.

Relative Humidity at a Fixed 75°F

Same air temperature, five different humidity levels — this is why "75°F" can feel comfortable or sticky depending on what the thermostat can't show you.

Bar chart 30% 30% 40% 40% 50% 50% 60% 60% 70% 70%
Comfortable range (ASHRAE 55) Feels sticky / muggy

Why Humidity Matters as Much as Temperature

ASHRAE's comfort standard (ASHRAE 55) doesn't just define a temperature range — it defines a temperature-and-humidity zone. A home at 75°F and 40% relative humidity feels comfortable. The same 75°F at 65% humidity feels warm and sticky, and most people will bump the thermostat down to compensate, burning more energy to fix a problem that's actually about moisture, not heat. Run your numbers above to see where you land against the ASHRAE 55 comfort zone.

Frequently Asked Questions

What is the dew point and why does it matter for HVAC?
Dew point is the temperature at which air becomes saturated — the point where moisture starts condensing on surfaces. In HVAC, dew point matters because: if supply air reaches a surface below its dew point, condensation forms (and mold can follow). Typical residential AC systems cool supply air to 55°F, so surfaces below 55°F will condensate — relevant for under-slab ducts and in very humid climates.
What is wet bulb temperature and how is it used?
Wet bulb temperature accounts for both heat and moisture in the air — it's what you'd measure with a thermometer wrapped in wet cloth. It's used for: sizing cooling towers, evaluating heat stress risk (WBGT), and determining the actual cooling capacity of evaporative coolers. On a psychrometric chart, constant wet bulb lines run diagonally.
What is enthalpy in HVAC?
Enthalpy is total heat content per pound of air, measured in BTU/lb. It includes both sensible heat (temperature you feel) and latent heat (moisture). HVAC engineers use enthalpy to calculate the total (sensible + latent) cooling load. When you see 'total capacity' on an equipment spec, it's based on entering and leaving air enthalpy.
What is a comfortable humidity ratio?
ASHRAE 55 defines the comfort zone as roughly 30–60% relative humidity at temperatures between 68–79°F. At or above 60% RH, most people feel clammy; below 30% RH, air is uncomfortably dry. In grains per pound, the sweet spot is typically 50–70 gr/lb. The psychrometric calculator above shows your current conditions relative to the ASHRAE 55 comfort zone.
What is specific volume in psychrometrics?
Specific volume (ft³/lb) is the volume of one pound of dry air at given temperature and humidity conditions. It's the inverse of air density. Standard conditions (70°F, sea level, 50% RH) give about 13.5 ft³/lb. Specific volume is used to convert between volume flow rate (CFM) and mass flow rate (lb/min) — important for precise duct and coil sizing.