AC Delta T Calculator: 20 Degree Drop
Enter return and supply air temperatures to see your AC's temperature drop (delta T). 20F is a common rule of thumb; the unit spec and a technician decide.
This free AC delta T calculator from LiveFixPro takes two numbers you enter, the return air temperature and the supply air temperature in °F, and shows the difference between them: the temperature drop. A drop of about 20°F is a common rule of thumb. The unit's own specifications and a licensed technician decide the rest.
It's made for homeowners and for HVAC pros who want a quick subtraction. It asks for no personal details. It runs in your browser, and nothing you enter is uploaded or stored. A licensed HVAC technician should handle any inspection or repair of equipment. This tool reports a number only. It is not a diagnosis and not a quote.
What the calculator asks for
• Return air temperature (°F). The tool's label says: air going into the system.
• Supply air temperature (°F). The tool's label says: air coming out of the vents.
Use °F, from -20 to 200, with up to one decimal place.
What you get back
The tool subtracts the supply reading from the return reading and shows the temperature drop in °F. It then states how that number compares with the common 20°F rule of thumb, in one of three ways:
• About 20°F, in line with the common rule of thumb. "About 20°F" here means within 1°F of 20°F.
• Below the common 20°F rule of thumb.
• Above the common 20°F rule of thumb.
If the supply reading is higher than the return reading, the drop is negative, and the tool suggests checking that each number went in the right box.
Use Copy, Download CSV or Print to take the result with you, and Clear to start again. If a field is blank, isn't a plain number or is outside the range, the tool tells you what to fix.
Why 20°F?
The 20°F figure is a common rule of thumb, not a requirement and not a rule for every unit. Specifications differ by unit, so check the unit's own documentation and ask a licensed technician. This is the difference between two air temperatures. It is not a thermostat setting.
What this tool is not
It reports a number and where that number sits against the rule of thumb. It doesn't diagnose anything, and it doesn't tell you what to do about the result. The result says nothing about your equipment, so a licensed technician is the right person to ask. Ask a licensed technician before opening or repairing any equipment.
Using the result
• Write down your two readings so you can share them with a technician.
• Check the unit's documentation. The unit's own specifications come first.
• Treat the result as a number, not a verdict. Estimates only, not a forecast.
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FAQ
How does an AC delta T calculator work?
It subtracts the supply air temperature from the return air temperature and shows the difference in °F. It then compares that number with the common 20°F rule of thumb.
Why 20°F?
20°F is a common rule of thumb. It is not a rule for every unit. The unit's own specifications and a licensed technician decide the rest.
Does the result tell me anything about my unit?
No. The tool reports a number and whether it is above, at or below the rule of thumb. It does not diagnose, so a licensed technician is the right person to ask about your equipment.
What does about 20°F mean here?
It means within 1°F of 20°F. It is still only a comparison with a common rule of thumb, not a requirement.
Is my information stored?
No. The tool asks for no personal details and nothing is uploaded or stored.
Check your readings
Use the calculator above, then ask a licensed technician about your equipment. If you run an HVAC business, book a free consult if you'd like help with your phones.
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