AC temperature split calculator

Enter return-air and supply-air temperatures to calculate their cooling temperature difference in °F and °C. See what the measurement can and cannot tell you.

Use two air readings during cooling

Measure moving air with an air-temperature probe. A laser thermometer aimed at the grille measures its surface instead. Both readings must use the same unit.

Return minus supply. A grille-to-grille reading is not automatically the temperature drop across the indoor coil.

MEASURED RETURN MINUS SUPPLY

A temperature difference needs operating context

The calculation is return-air temperature minus supply-air temperature. It measures the difference between your two readings. It does not calculate the expected split for your equipment or diagnose a cooling fault.

75°F return, 55°F supply

75 − 55 = 20°F, equivalent to 11.1°C. A temperature difference uses the 1.8 conversion factor without a 32-degree offset.

24°C return, 14°C supply

24 − 14 = 10°C, equivalent to 18°F. Switching the tool’s unit converts both individual readings and keeps the same physical difference.

70°F return, 75°F supply

70 − 75 = −5°F. The supply reading is warmer. Check measurement direction and operating mode; the number alone does not reveal why.

Take a homeowner observation, not a cabinet test

  1. Confirm that the system is actively cooling. Allow readings to settle; the DOE diagnostic guide uses about 15 minutes of cooling operation before its measurements.
  2. Use an air-temperature thermometer/probe at safe, accessible return and nearby supply grilles. Record the exact locations. Wait for stable readings and keep the normal filter in place.
  3. Do not reach through grilles, drill ducts, open equipment cabinets or touch wiring or refrigerant components to use this calculator.
  4. Record both readings and the operating conditions. Duct gains or leakage can affect grille measurements, so they need not equal the indoor coil split.

Why this calculator has no “healthy” band

Cooling removes both heat and moisture. The expected dry-bulb temperature drop depends on entering-air conditions, airflow and the actual equipment. Manufacturer performance tables show different splits at different airflow and humidity conditions. A result inside a common rule-of-thumb range cannot prove correct charge or efficient operation.

If cooling is inadequate or visible ice is present, take your measurement record to a qualified technician. This tool does not tell you which component to repair.

Temperature-split questions

Can I use a laser thermometer?

An infrared thermometer reads a surface. A grille-surface temperature is not the flowing-air temperature required here. Use a suitable air-temperature probe instead.

Does a 20°F split mean my AC is working correctly?

No. It describes the difference between the entered readings. Humidity, airflow, duct effects and equipment-specific data are needed for interpretation; matching a target alone can still miss a refrigerant issue.

Does this cover a furnace, car AC or evaporative cooler?

No. The measurement guidance is for ducted central AC cooling. Furnace temperature rise, vehicle systems, evaporative coolers and other equipment need their own documented procedures.

Sources and limits

Primary references checked October 14, 2026. The DOE guide addresses qualified diagnostics; this page uses its measurement principles without equipment-access procedures. Inputs are calculated in your browser.