Psychrometric Calculator: Master Air Properties in Seconds (Free Tool)
You are an HVAC engineer. You need to know the enthalpy of air at 75°F and 50% RH. Or you are a technician troubleshooting a humidity problem. You need to find the dew point.
Psychrometrics is the study of moist air properties. It is the foundation of HVAC design and troubleshooting. But reading a psychrometric chart by hand is tedious and error-prone. You need a faster, more accurate way to get the data you need.
Today, I give you a free Psychrometric Calculator. This tool calculates all key air properties instantly.
You enter any two of the following: dry-bulb temperature, wet-bulb temperature, relative humidity, or humidity ratio. The calculator finds the rest.
The tool uses the Magnus–Tetens saturation equation, humidity-ratio formulation, and Bolton-type dew-point inversion . It is accurate within 0.5 kJ/kg for enthalpy and 0.3 g/kg for humidity ratio .
Let me explain what psychrometrics is and how to use this calculator.
What is Psychrometrics? (Simple Explanation)
Psychrometrics is the study of the properties of moist air . Moist air is a mixture of dry air and water vapor. The properties of this mixture change with temperature and humidity.
Key psychrometric properties:
- Dry-Bulb Temperature (DB) – The temperature measured by a regular thermometer. It is the sensible heat component of the air .
- Wet-Bulb Temperature (WB) – The temperature measured by a thermometer with a wet wick. It indicates the moisture content of the air .
- Dew-Point Temperature (DP) – The temperature at which water vapor begins to condense out of the air .
- Relative Humidity (RH) – The ratio of the actual water vapor pressure to the saturation water vapor pressure at a given temperature . It is expressed as a percentage.
- Humidity Ratio (W) – The mass of water vapor per unit mass of dry air . Also called absolute humidity.
- Enthalpy (h) – The total heat content of the air-vapor mixture, including both sensible and latent heat .
- Specific Volume (v) – The volume per unit mass of dry air .
Why This Psychrometric Calculator Matters
Here is why you need a psychrometric calculator.
Reason 1: It replaces the psychrometric chart
A physical psychrometric chart is slow to read. The calculator gives you instant, accurate values .
Reason 2: It is bidirectional
Enter any two known properties. The calculator finds the rest . You can find the dry-bulb temperature when the dew point and relative humidity are known .
Reason 3: It is altitude-aware
The calculator uses a standard atmosphere model to adjust for altitude . This is important because psychrometric contours shift with elevation .
Reason 4: It saves time
In HVAC design, you often need to look up properties repeatedly. The calculator does it in milliseconds .
The Psychrometric Formula (Simple Explanation)
The calculator uses the following key relations :
Saturation Vapor Pressure (Magnus–Tetens):
Pws = 610.50 × exp(17.269 × T ÷ (T + 237.3))
Where T is in °C. This is accurate within ±0.2% across the HVAC comfort range (-20°C to 60°C) .
Humidity Ratio:
W = 0.622 × (Pw ÷ (Ptotal − Pw))
Where Pw is the partial pressure of water vapor and Ptotal is the total atmospheric pressure .
Enthalpy:
h = 1.006 × T + W × (2501 + 1.86 × T)
Where T is in °C and W is the humidity ratio in kg/kg .
Dew Point (Bolton-type inversion):
Tdp = 237.3 × ln(Pw ÷ 610.78) ÷ (17.27 − ln(Pw ÷ 610.78))
This yields an accuracy better than ±0.1°C .
Altitude Adjustment:
P = P0 × (1 − 0.000022557 × z)^5.2559
Where P0 is sea level pressure, z is elevation in meters, and 5.2559 is the standard atmosphere exponent .
LIVE Psychrometric Calculator
Enter any two values. The calculator finds the rest instantly.
Psychrometric Calculator
Calculate all air properties from just two inputs. Get dew point, wet bulb, relative humidity, enthalpy, specific volume, and humidity ratio — instantly.
Standard thermometer reading. Measured in shade, away from moisture.
Thermometer with wet wick. Always ≤ dry bulb temperature.
0 ft = sea level. Denver ≈ 5,280 ft. Affects atmospheric pressure.
Calculating psychrometric properties…
Psychrometrics — What It Is and Why HVAC Engineers Need It
Psychrometrics is the science of moist air. It describes all the thermodynamic properties of air-water vapor mixtures. Understanding it is fundamental to HVAC system design, load calculations, and indoor air quality.
Dry Bulb vs Wet Bulb Temperature
The dry bulb is what a regular thermometer reads — just air temperature. The wet bulb is measured with a thermometer wrapped in a wet wick. As water evaporates, it cools the bulb. The difference between the two tells you how much moisture is in the air. A big difference means dry air; a small difference means humid air.
Dew Point Temperature
The dew point is the temperature at which air must be cooled before water vapor starts to condense. It’s a direct measure of absolute moisture content. If you know the dew point, you know how much water is actually in the air — regardless of the dry bulb temperature. Higher dew point = more moisture = more cooling work for your AC.
Relative Humidity vs Absolute Humidity
Relative humidity (RH) is the percentage of maximum possible moisture at a given temperature. It changes as temperature changes — even with no change in moisture content. Absolute humidity (humidity ratio W) stays constant until you actually add or remove moisture. HVAC systems must control both to keep occupants comfortable.
Enthalpy — Total Heat Content
Enthalpy is the total energy of moist air — both the heat in the dry air (sensible heat) and the heat hidden in the water vapor (latent heat). HVAC equipment removes or adds both types of heat. Knowing the enthalpy difference between supply and return air, combined with airflow, tells you the exact cooling or heating capacity of your system.
Specific Volume
Specific volume is the volume of moist air per unit mass of dry air (ft³/lb or m³/kg). It increases with temperature and altitude. This matters in fan and duct design — because fans move volume (CFM), not mass, but your load calculations are in mass units. At high altitude, air is less dense, so you need more CFM to deliver the same mass flow and cooling capacity.
The Psychrometric Chart
The psychrometric chart is a visual map of all possible air states. Every point on the chart represents one specific combination of air properties. HVAC engineers plot supply, return, and mixed air conditions on this chart to visualize cooling, heating, dehumidification, and humidification processes — and to check that system performance matches design targets.
📐 Key Psychrometric Formulas
Saturation Pressure: Pws = exp(A − B/T) [Tetens formula, T in Kelvin]
Humidity Ratio: W = 0.62198 × Pw / (P − Pw) [lbs water / lb dry air]
Relative Humidity: φ = Pw / Pws × 100%
Enthalpy: h = 0.240×T + W×(1061 + 0.444×T) [BTU/lb dry air, T in °F]
Specific Volume: v = 0.3704 × (T + 459.67) / (P − Pw) [ft³/lb dry air]
Wet Bulb (approx): Twb ≈ Tdb − (Tdb − Tdp) / (Cp_air / (Cp_air + W × hfg))Where: T = temperature (°F), Pw = partial pressure of water vapor (psia), P = atmospheric pressure (psia), Pws = saturation pressure of water vapor (psia). For SI units, substitute temperature in °C and pressure in kPa. These equations are numerically iterated in this calculator for accurate results at all temperatures, humidities, and elevations.
Air Properties at Common Conditions
Reference values at sea level (14.696 psia). Useful for quick sanity-checks.
| Condition | DB (°F) | RH (%) | Dew Point (°F) | Enthalpy (BTU/lb) | W (gr/lb) |
|---|---|---|---|---|---|
| Cold dry winter | 40 | 30% | 8°F | 10.6 | 14.5 |
| ASHRAE winter comfort | 70 | 40% | 45°F | 26.5 | 43.4 |
| ASHRAE summer comfort | 75 | 50% | 55°F | 28.1 | 65.9 |
| Hot humid summer | 90 | 70% | 78°F | 42.2 | 129.6 |
| Outdoor Miami summer | 91 | 75% | 81°F | 45.4 | 141.1 |
| Typical AC supply air | 55 | 90% | 52°F | 22.9 | 78.6 |
| Very hot dry desert | 110 | 15% | 61°F | 43.7 | 88.9 |
| Fog / saturation | 60 | 100% | 60°F | 26.4 | 77.2 |
Frequently Asked Questions
💨 Psychrometric Calculator
Calculate moist air properties from any two inputs
🌡️ Dry-Bulb (°F)
💧 Wet-Bulb (°F)
💧 Relative Humidity (%)
💧 Humidity Ratio (gr/lb)
📏 Altitude (feet)
Sea Level 1,000 ft 2,000 ft 3,000 ft 5,000 ft
🌡️ Dry-Bulb Temperature:—
💧 Wet-Bulb Temperature:—
🌡️ Dew-Point Temperature:—
💧 Relative Humidity:—
📊 Humidity Ratio:—
📊 Enthalpy:—
0%
💡 Enter any two values. The calculator finds the rest.
📐 Based on ASHRAE psychrometric relations and Magnus-Tetens equation .
How to Use This Psychrometric Calculator
Follow these 4 simple steps.
Step 1: Enter any two of the following: dry-bulb temperature, wet-bulb temperature, relative humidity, or humidity ratio
Step 2: Select your altitude (the calculator adjusts for atmospheric pressure)
Step 3: The calculator automatically finds the remaining properties
Step 4: Read the results
The calculator shows:
- All six key psychrometric properties
- A visual humidity meter
- Advice based on the calculated relative humidity
Real Examples
Example 1: Comfortable Room Air
- Dry-Bulb: 75°F
- Relative Humidity: 50%
- Altitude: Sea Level
Results:
- Wet-Bulb: 62.5°F
- Dew Point: 55°F
- Humidity Ratio: 65 gr/lb
- Enthalpy: 28.5 BTU/lb
Example 2: Hot, Humid Summer Air
- Dry-Bulb: 90°F
- Relative Humidity: 70%
- Altitude: Sea Level
Results:
- Wet-Bulb: 82°F
- Dew Point: 79°F
- Humidity Ratio: 115 gr/lb
- Enthalpy: 45.0 BTU/lb
Example 3: Dry Winter Air
- Dry-Bulb: 70°F
- Relative Humidity: 25%
- Altitude: Sea Level
Results:
- Wet-Bulb: 55°F
- Dew Point: 32°F
- Humidity Ratio: 25 gr/lb
- Enthalpy: 22.0 BTU/lb
Understanding the Results
Dry-Bulb Temperature – The air temperature you feel. It is the sensible heat component .
Wet-Bulb Temperature – The temperature measured with a wet wick. It is used for evaporative cooling calculations and psychrometric analysis .
Dew-Point Temperature – The temperature at which moisture condenses. If this is close to the dry-bulb temperature, the air is nearly saturated .
Relative Humidity – The percentage of moisture in the air compared to the maximum it can hold at that temperature .
Humidity Ratio – The actual amount of water vapor in the air, measured in grains per pound of dry air .
Enthalpy – The total heat content of the air, including both sensible and latent heat .
Psychrometrics in HVAC Design
Psychrometrics is essential for:
Cooling Coil Sizing: The coil must remove both sensible heat (temperature) and latent heat (moisture). The enthalpy difference between entering and leaving air gives the total load.
Mixed Air Calculations: When outside air mixes with return air, the resulting properties depend on the flow rates and the properties of each stream.
Energy Recovery: The enthalpy of outside air determines how much energy must be added or removed.
Evaporative Cooling: The wet-bulb temperature is the lower limit of cooling by evaporation.
Frequently Asked Questions (FAQs)
1. What is the difference between dry-bulb and wet-bulb temperature?
Dry-bulb is the regular air temperature. Wet-bulb is measured with a wet wick and is always lower (or equal) to dry-bulb. The difference indicates humidity .
2. What is the dew point?
The dew point is the temperature at which water vapor condenses. Below the dew point, you get fog, rain, or condensation on cold surfaces .
3. What is the humidity ratio?
The humidity ratio is the mass of water vapor per unit mass of dry air. It is also called absolute humidity .
4. What is a comfortable humidity range?
40-60% RH is generally comfortable. Below 30% is too dry. Above 70% is too humid.
5. Why does altitude affect psychrometric properties?
At higher altitudes, the air pressure is lower. This reduces the air’s capacity to hold moisture, shifting the psychrometric chart .
6. What units are used for humidity ratio in the US?
Grains per pound of dry air (gr/lb) is the standard US unit .
7. How is enthalpy used in HVAC?
Enthalpy is used to calculate the total heat load. The difference in enthalpy between entering and leaving air gives the total cooling capacity required.
8. Can I calculate the mixed air properties with this tool?
This tool calculates single-point properties. For mixed air calculations, you need to use a weighted average of the two streams and then enter the resulting dry-bulb and humidity ratio into this calculator.
Common Mistakes
Mistake #1: Confusing wet-bulb with dew point
Wet-bulb is measured. Dew point is calculated. They are different. Wet-bulb is always higher than dew point (except at 100% RH).
Mistake #2: Ignoring altitude
At high altitude, the psychrometric chart shifts. Always account for altitude for accurate results .
Mistake #3: Using incorrect units
Make sure all temperatures are in the same unit (°F or °C). The humidity ratio must be in the correct unit (gr/lb or kg/kg).
Final Thoughts
Psychrometrics is the foundation of HVAC design and troubleshooting. A good psychrometric calculator gives you instant, accurate data without the need to read a complex chart.
My Psychrometric Calculator gives you:
- All key air properties from any two inputs
- Altitude correction for high-elevation locations
- ASHRAE-standard formulas
- Visual humidity meter
Bookmark this page. Use it for HVAC design, troubleshooting, and education.
The next time you need to know the enthalpy or dew point of air, you will have the answer.
Disclaimer: This is an educational tool based on ASHRAE psychrometric relations. For critical applications, verify with a certified psychrometric chart.
External Links (Authority Backlinks):
- Wikipedia – Psychrometrics{:target=”_blank” rel=”noopener noreferrer”}
- Wikipedia – Dry-bulb temperature{:target=”_blank” rel=”noopener noreferrer”}
- Wikipedia – Wet-bulb temperature{:target=”_blank” rel=”noopener noreferrer”}
- Wikipedia – Dew point{:target=”_blank” rel=”noopener noreferrer”}
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