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Compressor Compression Ratio Calculator

Divide the absolute discharge pressure by the absolute suction pressure. This calculator adds the local atmospheric pressure to your gauge readings for you — corrected for elevation.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Measured as close to the compressor discharge as possible.

Measured at the compressor suction. A reading below zero is a vacuum.

Gauges read pressure above the local atmosphere, which is lower at altitude. Leave at 0 for sea level.

Result

Compression ratio
2.84

Compression ratio 2.84 : 1.

Absolute discharge pressure
254.7psia
Absolute suction pressure
89.7psia
Local atmospheric pressure
14.7psia
Show the working
  1. Atmospheric pressure at the site (standard atmosphere)
    14.696 psia
  2. Absolute pressure = gauge + atmospheric
    discharge 254.7 psia, suction 89.7 psia
  3. Compression ratio = absolute discharge ÷ absolute suction
    254.7 ÷ 89.7 = 2.84

Why absolute pressure matters

A gauge reads zero at atmospheric pressure, but about 14.7 psi of air pressure is acting on it at sea level. Any calculation that multiplies or divides pressures must use absolute pressure (gauge plus atmospheric), or the answer is meaningless.

At altitude the atmosphere is lower, about 12.2 psia at 5,000 feet, so the same gauge readings give a slightly higher ratio. Enter your elevation to correct for it.

What the ratio tells you

The higher the compression ratio, the more work the compressor does on each pound of refrigerant and the hotter it runs. High ratios come from high condensing pressure, low suction pressure, or both — for example a dirty condenser coil or a starved evaporator.

HVAC School reports compression ratios of about 2.3:1 to 3.5:1 as common for air conditioning near design conditions, and higher ratios for medium- and low-temperature refrigeration. Compare your reading with the equipment manufacturer’s data rather than a single number.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Discharge (high-side) gauge pressure
240 psi
Suction (low-side) gauge pressure
75 psi
Site elevation
0 ft

Result

Compression ratio
2.84
Absolute discharge pressure
254.7 psia
Absolute suction pressure
89.7 psia
Local atmospheric pressure
14.7 psia

Compression ratio 2.84 : 1.

Assumptions

  • Absolute pressure = gauge reading + local atmospheric pressure from the standard atmosphere at the site elevation.

Limitations

  • Gauge error matters most at low suction pressures, where a 1 psi error changes the ratio the most.
  • Normal ranges depend on the application; compare with the equipment manufacturer’s data.

Frequently asked questions

What is the compression ratio for 240 psig discharge and 75 psig suction?

At sea level, 254.7 ÷ 89.7 ≈ 2.84 : 1.

Where should I measure the pressures?

As close to the compressor discharge and suction as possible. Pressure drops in long lines make the ratio look different from what the compressor actually sees.

Can a compression ratio be too low?

Yes. A ratio well below normal for the conditions can point to a compressor that is not pumping properly, for example worn valves or scroll leakage. A technician should confirm with other readings.

Sources

  1. Why Compression Ratio Matters — B. Orr, HVAC School (2026). Compression ratio = absolute discharge pressure ÷ absolute suction pressure (gauge + about 14.7 psi at sea level); R-22 example 240 psig / 75 psig → 254.7 ÷ 89.7 ≈ 2.84; air conditioning ratios of 2.3:1 to 3.5:1 are common.
  2. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.

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