Psychrometric Calculator
Calculate moist-air properties from dry-bulb temperature plus relative humidity, wet bulb, dew point, humidity ratio, or enthalpy.
Calculator is for informational purposes only. Terms and Conditions
Choose the air properties
Select the unit system and the two known air properties used to define the moist-air state.
Enter the known values
Only the second property label and unit changes when you switch known-property pairs.
Visual Check
The simplified chart plots dry-bulb temperature and humidity ratio with a live state point.
Solution
Live result, quick checks, warnings, and full solution steps.
Quick checks
- Check—
Show solution steps See the pressure basis, equations, substitutions, and assumptions
- Enter values to see the full solution steps and checks.
Source, Standards, and Assumptions
Calculation basis, constants, assumptions, and limitations.
Psychrometric properties are calculated using standard moist-air engineering relationships with simplified saturation vapor pressure correlations.
- Assumptions will appear after a valid calculation.
On this page
Calculator Guide
How to Use the Psychrometric Calculator
The Psychrometric Calculator above estimates HVAC moist-air properties from dry-bulb temperature and another known property such as relative humidity, wet bulb, dew point, humidity ratio, or enthalpy. If you searched for “psychometric calculator,” the HVAC term you likely want is “psychrometric calculator.”
Use the calculator to find humidity ratio, dew point, wet bulb temperature, relative humidity, enthalpy, vapor pressure, specific volume, density, and condensation risk. The sections below explain the formulas, units, example calculation, chart interpretation, assumptions, and common input mistakes.
Quick Answer
A psychrometric air-property calculation usually needs dry-bulb temperature, one independent humidity property, and pressure or altitude. From those values, the calculator estimates the full air state: humidity ratio, relative humidity, dew point, wet bulb, enthalpy, vapor pressure, specific volume, and density.
When not to rely on a simplified result
Do not use a simplified psychrometric result as the only basis for final HVAC equipment selection, code compliance, lab calibration, safety-critical dehumidification, or warranty-sensitive design. For final design, confirm the result with applicable standards, manufacturer data, field measurements, and qualified engineering judgment.
Inputs and Outputs Used by the Calculator
The calculator defines a moist-air state using dry-bulb temperature, pressure, and one independent humidity property. Once the state is known, the remaining psychrometric properties can be estimated from the same relationships used on a psychrometric chart.
| Type | Value | What It Means | Common Units |
|---|---|---|---|
| Input | Dry-bulb temperature | The ordinary air temperature used as the main temperature coordinate. | °F, °C |
| Input | Relative humidity, wet bulb, dew point, humidity ratio, or enthalpy | The second independent property used to define the moisture condition of the air. | %, °F, °C, grains/lb, g/kg, Btu/lb, kJ/kg |
| Input | Altitude or barometric pressure | Corrects the air-property relationships for the total pressure at the site. | ft, m, psia, kPa, inHg |
| Output | Humidity ratio | The mass of water vapor per mass of dry air. This is often the most useful HVAC moisture result. | lb/lb, grains/lb, kg/kg, g/kg |
| Output | Dew point, wet bulb, enthalpy, vapor pressure, and specific volume | Secondary properties used for condensation checks, cooling coils, humidification, dehumidification, and chart interpretation. | Mixed IP or SI units |
If you are also checking indoor comfort, the Relative Humidity Calculator can help isolate the relationship between actual vapor pressure and saturation vapor pressure.
Psychrometric Calculator Formula
The core psychrometric relationship calculates humidity ratio from water vapor pressure and total barometric pressure. Other inputs, such as relative humidity or dew point, are first converted into vapor pressure.
Humidity Ratio Formula
This equation gives humidity ratio \(W\), the mass of water vapor per mass of dry air. The constant 0.621945 is the approximate molecular weight ratio of water vapor to dry air.
Relative Humidity to Vapor Pressure
Here, \(\phi\) is relative humidity as a decimal, and \(P_{ws}\) is the saturation vapor pressure at the dry-bulb temperature.
Moist Air Enthalpy Approximation
This common SI approximation gives moist air enthalpy in \(kJ/kg\) dry air when \(T_{db}\) is in °C and \(W\) is in \(kg/kg\) dry air.
Source note
For professional chart work, compare simplified calculator results with recognized psychrometric references such as ASHRAE psychrometric chart resources. Public educational resources such as Penn State Extension’s psychrometric chart guide are also useful for understanding chart axes and air-property relationships.
What the Variables Mean
Each variable describes either the air temperature, the pressure of water vapor, or the moisture content of the air. The most important unit rule is to keep all pressure values in the same absolute pressure basis before using the humidity ratio formula.
\(W\): Humidity Ratio
Mass of water vapor per mass of dry air. In U.S. HVAC work, the result is often shown as grains of moisture per pound of dry air, where \(7000\) grains equals \(1\) pound.
\(P_v\): Vapor Pressure
Partial pressure of water vapor in the air. This value is found from relative humidity, dew point, or humidity ratio depending on the selected input pair.
\(P\): Barometric Pressure
Total absolute air pressure. Use actual barometric pressure when available, or use altitude as an approximation for standard atmosphere pressure.
\(T_{db}\): Dry-Bulb Temperature
The measured air temperature. It controls the saturation vapor pressure, so it strongly affects relative humidity, wet bulb, enthalpy, and dew point interpretation.
How to Use the Calculator
Start with the measured air condition you actually know. For most users, the easiest mode is dry-bulb temperature plus relative humidity. HVAC users may also work from wet bulb, dew point, humidity ratio, or enthalpy.
Select IP or SI units
Use IP for °F, psia, Btu/lb dry air, ft³/lb dry air, and grains/lb. Use SI for °C, kPa, kJ/kg dry air, m³/kg dry air, and g/kg.
Choose the known property pair
Select dry bulb plus relative humidity, wet bulb, dew point, humidity ratio, or enthalpy. The calculator uses the selected pair to determine vapor pressure and moisture content.
Enter altitude or pressure
Use manual pressure if you know actual barometric pressure. Use altitude when you only need a standard-atmosphere approximation.
Review the air-property table
Check humidity ratio, dew point, wet bulb, enthalpy, vapor pressure, specific volume, and condensation risk. If a result looks unrealistic, recheck units and input consistency before using it.
Which input mode should you choose?
Use dry bulb plus relative humidity when you have a humidity sensor reading. Use dry bulb plus dew point when checking condensation risk. Use dry bulb plus wet bulb when working from field psychrometer data. Use dry bulb plus humidity ratio when working from HVAC design documents. Use dry bulb plus enthalpy when comparing air-process energy.
How to Interpret Psychrometric Results
A psychrometric result tells you more than whether the air “feels humid.” It tells you the moisture content, condensation temperature, cooling or heating energy content, and how the air state would appear on a psychrometric chart.
What to do with the result
Use humidity ratio for moisture load, dew point for condensation risk, wet bulb for evaporative cooling checks, and enthalpy for heating or cooling process comparisons.
What changes the result most?
Dry-bulb temperature changes saturation vapor pressure nonlinearly, while vapor pressure determines actual moisture content. Together, these two values drive most of the calculated properties.
Sanity check
At 75°F and 50% RH near sea level, a humidity ratio near 65 grains/lb dry air is reasonable. A result hundreds of grains/lb at normal room conditions usually means a unit or input error.
Condensation interpretation
If a surface temperature is below the calculated dew point, moisture can condense on that surface. This is why dew point is often more useful than relative humidity for ducts, windows, roofs, cold pipes, and dehumidification checks.
Input Checklist Before You Trust the Answer
Psychrometric calculations are sensitive to units and physical consistency. A small input mistake can create a result that looks precise but does not describe the actual air state.
Temperature pair is possible
Wet bulb and dew point should not exceed dry bulb for ordinary unsaturated air. If they do, check whether the values were reversed or entered in the wrong unit system.
Relative humidity is 0–100%
Relative humidity above 100% represents supersaturation or bad input for most HVAC calculations. Near 100%, expect condensation risk to be high.
Pressure is absolute
Use absolute barometric pressure, not gauge pressure. If using inHg, psia, or kPa, confirm the value represents atmospheric pressure.
Humidity ratio unit is clear
Do not confuse \(lb/lb\) with grains/lb. A humidity ratio of \(0.0092\,lb/lb\) is about \(64.6\) grains/lb, not \(0.0092\) grains/lb.
Worked Example
This example uses the most common workflow: dry-bulb temperature plus relative humidity at standard sea-level pressure.
Step 1: Find vapor pressure
Step 2: Calculate humidity ratio
Step 3: Convert to grains per pound
Final answer
Humidity ratio: about \(0.00923\,kg/kg\), or about \(64.6\,grains/lb\) dry air. This is reasonable for 75°F indoor air at 50% relative humidity near sea level.
Reverse check
Using \(W=0.00923\) in the rearranged vapor pressure relationship gives \(P_v \approx 1482\,Pa\). Dividing that by \(P_{ws}\approx2965\,Pa\) gives \(\phi \approx 0.50\), confirming the 50% relative humidity input.
How to Visualize a Psychrometric State Point
A psychrometric chart places dry-bulb temperature on the horizontal axis and humidity ratio on the vertical axis. Relative humidity curves sweep through the chart, and the 100% RH boundary is the saturation curve.
The calculator finds one air state point, then reports the properties that would be read from the corresponding psychrometric chart location.
Reference Checks for Psychrometric Results
Psychrometric values are condition-specific, so there is no single “good” humidity ratio, dew point, or enthalpy. Instead, compare results against the input condition, the pressure basis, and whether the answer makes physical sense.
| Result Pattern | Likely Meaning | What to Check |
|---|---|---|
| Dew point close to dry bulb | Air is near saturation. | Check for condensation risk and confirm RH is near 100%. |
| Very low humidity ratio | Air is dry or cold. | Verify RH and temperature units, especially °F vs °C. |
| Wet bulb higher than dry bulb | Input combination is usually impossible. | Recheck measurement, unit selection, and whether values were reversed. |
| Huge change after altitude entry | Pressure correction is affecting the calculation. | Confirm altitude units and actual barometric pressure if available. |
Chart note: Psychrometric charts are pressure-specific. A standard sea-level chart is a good learning reference near sea level, but high-elevation sites should use pressure-corrected calculations or a chart for the proper pressure.
Design Notes and Practical HVAC Ranges
Psychrometric calculations are commonly used for HVAC comfort, cooling coil checks, dehumidification, humidification, ventilation, and building moisture risk. The calculator result is a starting point, not a full equipment design.
Comfort checks
Indoor comfort is often discussed using dry-bulb temperature and relative humidity, but humidity ratio and dew point are better indicators of actual moisture content and condensation risk.
Cooling and dehumidification
Cooling below the dew point can remove moisture. Compare entering and leaving humidity ratios to understand moisture removal.
Heating only
Sensible heating raises dry-bulb temperature but does not add moisture. Relative humidity usually drops even when humidity ratio stays nearly constant.
Final design
For coil selection, ventilation design, and energy modeling, verify results with equipment data, project conditions, and applicable HVAC standards.
Units and Conversions
Psychrometric calculations mix temperature, pressure, mass ratio, heat content, and volume. Unit consistency is critical because the formulas use absolute pressure and dry-air basis quantities.
Temperature
Use °F or °C as selected. For the SI enthalpy approximation, dry-bulb temperature is entered in °C.
Pressure
Use absolute pressure. Common references are \(14.696\,psia\), \(101.325\,kPa\), or \(29.921\,inHg\) near standard sea-level conditions.
Humidity ratio
\(1\,lb/lb = 7000\,grains/lb\). In SI, \(1\,kg/kg = 1000\,g/kg\).
Enthalpy
\(1\,Btu/lb\) dry air is approximately \(2.326\,kJ/kg\) dry air. Keep the dry-air basis consistent.
For vapor-pressure-specific calculations, the Vapor Pressure Calculator can help when you need to focus on pressure-temperature relationships directly.
Psychrometric vs Psychometric Calculator
The spelling matters. A psychrometric calculator is the HVAC and thermodynamics tool used for moist air properties. A psychometric calculator often refers to psychology, testing, scoring, or measurement, although many users accidentally use that spelling when looking for HVAC air-property calculations.
Use psychrometric for HVAC
- Dry bulb and wet bulb calculations
- Dew point and condensation checks
- Humidity ratio and enthalpy
- Psychrometric chart state points
Do not confuse the intent
- Psychometric testing scores
- Personality or aptitude measurement
- Survey scale reliability
- Psychology assessment metrics
On this page, the calculator is for HVAC moist air properties. The article keeps the common misspelling visible while using the correct engineering term as the main page topic.
Common Mistakes
Most wrong psychrometric results come from entering a physically inconsistent air state or mixing unit systems.
Do
- Use dry-bulb temperature as the normal air temperature.
- Use actual barometric pressure when available.
- Check whether dew point is below dry bulb.
- Convert humidity ratio correctly between grains/lb and kg/kg.
- Use dew point for condensation checks.
Don’t
- Do not enter gauge pressure as atmospheric pressure.
- Do not enter wet bulb higher than dry bulb.
- Do not assume relative humidity equals moisture content.
- Do not use sea-level assumptions for high-elevation work without checking pressure.
- Do not use simplified results as final design approval.
Troubleshooting Unrealistic Results
If the answer looks wrong, check physical consistency before assuming the formula failed. Psychrometric formulas can return impossible values when the input pair does not describe real unsaturated air.
Humidity ratio is negative
The input pair is nonphysical, commonly because enthalpy is too low for the dry-bulb temperature or a unit conversion is wrong.
Relative humidity is above 100%
The air state is supersaturated or the dew point, wet bulb, humidity ratio, or pressure input is inconsistent.
Dew point is higher than dry bulb
Check whether dew point and dry bulb were reversed, or whether one is in °F and the other is in °C.
Enthalpy looks too high
Check humidity ratio units first. Entering grains/lb as lb/lb can make the enthalpy result extremely unrealistic.
Assumptions and Limitations
This calculator is best used for educational calculations, preliminary HVAC checks, and troubleshooting. It uses simplified moist-air relationships, so the result should be treated as an engineering estimate rather than a certified design value.
Ideal gas basis
The formulas assume dry air and water vapor can be treated using standard ideal moist-air relationships over common HVAC ranges.
Saturation correlation
Saturation vapor pressure is estimated using a simplified correlation. Small differences from chart or software results are normal.
Wet-bulb approximation
Wet-bulb calculations may use an approximate constant-enthalpy relationship and can differ from a full ASHRAE procedure.
Pressure approximation
Altitude mode uses an atmospheric approximation. Use measured barometric pressure for better field accuracy.
Professional judgment note
For HVAC design, moisture control, laboratory conditioning, healthcare spaces, industrial drying, cold storage, and building-envelope risk, confirm the calculator result using project requirements, field measurements, manufacturer data, and applicable engineering standards. Related thermal-load work may also require a separate Heat Transfer Calculator check.
Key Terms
These terms help connect the calculator inputs, formulas, chart, and result interpretation.
Dry-Bulb Temperature
The ordinary air temperature, used as the main temperature input in most psychrometric calculations.
Wet-Bulb Temperature
A temperature related to evaporative cooling and moisture content. It is usually lower than dry bulb for unsaturated air.
Dew Point
The temperature at which the current water vapor content would reach saturation and condensation could begin.
Humidity Ratio
The mass of water vapor per mass of dry air. It is often the most useful moisture-content output for HVAC calculations.
Enthalpy
The heat content of moist air per unit mass of dry air, commonly used in heating, cooling, and air-process analysis.
Psychrometric Chart
A chart that shows relationships among dry bulb, wet bulb, relative humidity, dew point, humidity ratio, enthalpy, and specific volume.
FAQ
Is psychometric calculator the same as psychrometric calculator?
For HVAC air-property calculations, the correct engineering term is psychrometric calculator. Psychometric usually refers to measurement in psychology, but many users accidentally search psychometric calculator when they want moist air properties.
What two values do I need for a psychrometric calculator?
For a moist-air calculation, you usually need dry-bulb temperature plus one independent humidity property such as relative humidity, wet bulb temperature, dew point temperature, humidity ratio, or enthalpy. Pressure or altitude is also needed for more accurate results.
Can wet bulb temperature be higher than dry bulb temperature?
For ordinary unsaturated air, wet bulb temperature should not be higher than dry bulb temperature. If wet bulb is higher than dry bulb, the inputs are usually reversed, measured incorrectly, or entered with the wrong units.
Why does altitude affect psychrometric calculations?
Altitude affects barometric pressure, and barometric pressure is part of the humidity ratio and vapor pressure relationships. The same dry-bulb temperature and relative humidity can produce a different humidity ratio at a high-elevation site than at sea level.
How do I use dew point from the calculator?
Use dew point to check condensation risk. If a duct, window, pipe, roof deck, coil surface, or other surface is colder than the dew point, moisture can condense on that surface.
Is this calculator accurate enough for final HVAC design?
Use this calculator for education, quick checks, troubleshooting, and preliminary HVAC estimates. For final HVAC design, confirm the assumptions with applicable standards, manufacturer data, project requirements, and qualified engineering judgment.