Psychrometric Calculator: Psychro Calc Pro
All properties of moist air, in seconds. A psychrometric calculator for HVAC and industrial process engineers. Enter any two known values and get dry-bulb and wet-bulb temperature, dew point, relative humidity, humidity ratio, enthalpy and density instantly.
Interface
Psychrometric calculator for Mac
Psychro Calc Pro is a psychrometric calculator for macOS, built for HVAC and industrial process engineers. You enter any two independent properties of moist air. The app works out the complete air state and plots it on a live psychrometric chart. You can also type values with the Mac keyboard. The interface is in English, and button and message names on this page are quoted as they appear in the app.
What it calculates
You pick any two of dry-bulb temperature (Tdb), wet-bulb temperature (Twb), dew point (Tdp), relative humidity (RH), humidity ratio (W) and enthalpy (h). The results table lists dry bulb, wet bulb, dew point, relative humidity, humidity ratio, enthalpy and density, together with the altitude used for the calculation. Density is an output only, and altitude is a separate setting rather than part of the input pair. Every successful calculation is saved automatically with its altitude, and the last 50 can be reloaded from the History screen.
Calculation basis
Saturation pressure is calculated with the Hyland–Wexler equations and the water vapor enhancement factor; barometric pressure is derived from altitude. Every formula and reference is listed on the app's About screen. Results are intended for review by a qualified engineer; the app is not a substitute for approval or a certificate of compliance.
Dew point, wet bulb and enthalpy calculation
In practice you often hold two measurements and need a third quantity: dew point from dry- and wet-bulb readings, or enthalpy from temperature and relative humidity. Psychro Calc Pro converts between them in one step. The wet bulb is the thermodynamic (adiabatic saturation) wet-bulb temperature. Below 0 °C the dew point is computed against saturation over ice, so it is a frost point.
Valid input pairs
Any two different parameters out of the six form a valid pair, with one exception: dew point plus humidity ratio. Both describe only how much water vapour the air holds and say nothing about temperature, so the pair cannot fix the air state and the app reports "NO SOLUTION". Entering the same parameter twice has no solution either. Input temperatures must lie between −100 and +200 °C, and neither wet bulb nor dew point may exceed the dry bulb. Physically impossible combinations, such as a wet bulb above the dry bulb, are flagged with an "IMPOSSIBLE" message.
SI and IP units
The SI/IP switch on the toolbar changes the unit system: °C and °F, g/kg and gr/lb, kJ/kg and Btu/lb, kg/m³ and lb/ft³, metres and feet. Switching keeps the calculation on screen: your values are converted and the result is calculated again; your altitude stays too. IP enthalpy uses a 0 °F dry-air reference. The results table, the PDF and the CSV export use the selected units, and so do the chart, its info box and the History list.
Why altitude matters
The properties of moist air depend on atmospheric pressure, and pressure falls with altitude. At the same dry-bulb temperature and relative humidity, lower pressure means each kilogram of dry air carries more water vapour, so the humidity ratio comes out higher than at sea level. Many psychrometric calculations assume a fixed sea-level pressure. Saturation pressure is calculated with the Hyland–Wexler equations and the water vapor enhancement factor; barometric pressure is derived from altitude.
You set altitude with the Alt button, in metres or feet depending on the selected units. It stays in effect while the app is open; when the app is reopened, the altitude is 0 m. A new altitude applies to the next calculation and does not recalculate the results already on screen.
Example: 35 °C / 25 % at 850 m
Take outdoor air at 35 °C and 25 % relative humidity at a site 850 m above sea level. Run it once at sea level and again at 850 m, and the atmospheric pressure at 850 m is clearly lower, while the humidity ratio for the same dry bulb and relative humidity is higher than the sea-level value. The Altitude row in the results table shows which altitude each result was calculated for, so the two runs are easy to compare side by side.
Psychrometric chart: live and zoomable
On the Mac, the calculated point is drawn on a live chart. It shows the saturation curve and lines of constant relative humidity, enthalpy, wet bulb and specific volume, and a box beside the point lists T, RH, W and h. Zoom with the mouse wheel or by pinching on a trackpad, and pan by dragging. A double-click or ⌘0 returns to the automatic view. As you zoom in, the axes and every line family are redrawn for the new range.
Chart modes
The chart picks its range from the dry-bulb temperature of the point and labels the mode faintly inside the plot:
- STANDARD HVAC MODE: −10 °C to 60 °C. The temperature axis runs −10 to 55 °C and the humidity-ratio axis 0 to 35 g/kg.
- LOW TEMP MODE: below −10 °C. The axis is centred around the point in 5 °C steps and reaches down to −100 °C at most.
- INDUSTRIAL DRYING MODE: above 60 °C. The temperature axis runs −10 to 130 °C and the humidity-ratio axis 0 to 80 g/kg.
If the point would fall outside these axes, the range is extended.
PDF psychrometric report with your company logo
The Export PDF icon creates a one-page A4 report: a title with date, time and app version, the chart exactly as shown on screen (zoom included) on a white background, and a Parameter / Value / Unit table in the selected units. The report is saved without a save dialog and revealed in Finder. Export CSV saves the same results for use in Excel.
Company logo
Choose an image with Add Logo and it appears in the top-right corner of the report. The logo is kept between sessions; the app stores its own copy without the image's metadata.
What Psychro Calc Pro is not
Psychro Calc Pro is not an equipment selection or sizing program. It calculates and displays a single state of moist air. For analyses between two air states, mixing and similar tasks, use the separate product Psychro Process Pro.
Download from the Mac App Store
Get Psychro Calc Pro from the Mac App Store. It requires macOS 10.15 (Catalina) or later, and no account is needed.
7-day free trial
The trial starts the first time you open the app, lasts 7 days and has every feature unlocked. The trial record is kept in your Mac's Keychain, so deleting and reinstalling the app does not reset it.
One-time purchase
When the trial ends, the app asks for a one-time Lifetime purchase. Payment goes through the App Store in your local currency. If you bought the app before with the same Apple ID, Restore Purchase brings it back.
Frequently asked questions
Which two quantities can I enter?
You can enter any two different parameters from dry bulb, wet bulb, dew point, relative humidity, humidity ratio and enthalpy. The only exception is the pair of dew point and humidity ratio. Altitude is a separate setting, not part of the pair, and density is calculated as a result only. The app solves the remaining values from your two inputs.
Why can't dew point and humidity ratio be selected together?
Both describe the amount of moisture in the air and carry no information about temperature. With only the moisture content known, the dry-bulb temperature stays undetermined and the full air state cannot be found. The app answers this pair with "NO SOLUTION". To solve it, combine either one with a temperature, relative humidity or wet bulb instead.
What temperature and altitude range does it support?
Input temperatures must be between −100 °C and +200 °C, the range in which the saturation-pressure relation is valid. Altitude can be entered from −500 to 11,000 m. A value outside these ranges returns "ERR: OUT OF RANGE" and is not accepted, so you can simply enter a valid value and continue.
How does altitude affect the results?
Altitude changes atmospheric pressure, and the properties of moist air depend on that pressure. Saturation pressure is calculated with the Hyland–Wexler equations and the water vapor enhancement factor; barometric pressure is derived from altitude. It stays in effect while the app is open; when the app is reopened, the altitude is 0 m. The Altitude row in the results table shows what each result was based on.
How do I switch between SI and IP units?
Use the SI/IP switch at the right end of the toolbar. Switching keeps the calculation on screen and your altitude, converting the values; values you type afterwards are read in the selected units. The results table, PDF and CSV follow the selection, and so do the chart (°F and gr/lb axes), its info box and the History list.
How do the free trial and purchase work?
On the Mac, the free trial starts at first launch and lasts 7 days with every feature unlocked. After that, a one-time Lifetime purchase is needed. The trial record sits in the Keychain, so reinstalling does not reset it. After buying, you can unlock another Mac signed in with the same Apple ID through Restore Purchase.
Can I use it to select equipment?
No. The program does not select or size equipment. From the two parameters you choose, it calculates a single state of moist air and shows it on the chart. For analyses between two air states, or mixing, see the separate product Psychro Process Pro. Results are intended as engineering support.
Where are my PDF reports saved?
The report is saved without a save dialog, into the app's own Documents folder, and then shown selected in Finder. You can open that folder in Finder with Shift + ⌘ + G. File names start with PsychroCalcPro_ followed by a timestamp, so every report gets a new name and older files are never overwritten.
How to get it
Download free from the Mac App Store · 7-day free trial · Lifetime: one-time in-app purchase · Local price shown in the App Store
Mac App Store →Try it in your browser first: free online psychrometric chart →