Compressor Power Calculator: Gas, Shaft & Driver Power, Discharge Temperature & Multistage Intercooling (2026)

COMPRESSOR POWER · API 617 / API 618 / API 619 · ASME PTC 10 · UPDATED OCTOBER 2026

By Muhammad Raza, Aienginear.com · Compressor terminology per API 617 (9th ed., 2022), API 618 (6th ed., 2024) and API 619 (5th ed., R2024); performance-test framework per ASME PTC 10-2022 and ISO 1217:2009/Amd 1:2016; standard reference conditions per ISO 13443 · Calculator v1.2.0, 225 built-in self-tests · Reading time about 16 minutes

How to use the calculator

1. Compressor Setup. Choose reciprocating, centrifugal, rotary screw or a general thermodynamic compressor, then the process: isentropic, adiabatic, polytropic, isothermal or a user exponent n. Pick the calculation mode (power, discharge temperature, flow limit or driver check).
2. Gas Properties. Use a library gas, a full mole composition, a specific gravity or your own MW, k and Z. Select ideal, Peng-Robinson, SRK, DAK or user Z, and choose the Z-corrected screening basis or the EOS enthalpy-entropy basis for high-pressure work.
3. Flow. Enter mass, molar, standard or actual flow. For MMSCFD, SCFM, Sm³/h or Nm³/h, confirm the reference pressure and temperature; the tool never assumes them silently.
4. Suction and discharge. Every pressure carries an explicit (a) or (g) unit, and gauge values are converted with the local atmospheric pressure you enter or an ISA estimate from site altitude.
5. Efficiencies and stages. Enter the thermodynamic efficiency once, then mechanical, gear, coupling and driver efficiencies. Set 1 to 6 stages with equal, user or optimised ratios and four intercooler models.
6. Machine and driver. Add cylinder bore, stroke and clearance, or impeller diameter and speed, then the driver type, margin and available site rating.
7. Results & Warnings. Read the stage table, power train and every warning, each with what it means, why it matters and what to check.
8. Report & Export. Print the branded engineering report, export CSV or JSON, save up to five cases (A to E) and keep the project file.

What is new in version 1.2.0

Version 1.2.0 is a standards and publishing update. The numerical engine is unchanged, so every result matches the previous release to the last digit, and the built-in regression snapshot confirms it each time the self-test runs. The changes are in what the tool references, how it sits inside this page and how much it tells you about its own limits.

How compressor power is calculated

Every compressor sizing method follows the same chain. First find the specific work (head) the gas must receive per kilogram, then multiply by the mass flow, and finally divide by each efficiency between the gas and the power supply. The calculator keeps those steps separate so that an efficiency can never be applied twice.

T1 is the absolute suction temperature in kelvin, PR the ratio of absolute discharge to absolute suction pressure, k the ideal-gas heat-capacity ratio Cp/Cv and Zavg the mean compressibility between suction and discharge. These are the constant-k, Z-corrected screening equations. When you enter a gas composition the calculator integrates temperature-dependent Cp instead, and on the EOS basis it uses Peng-Robinson or SRK enthalpy and entropy directly, so the formulas above are a hand-check, not the only route.

Isentropic, polytropic and isothermal compression

Three reference paths describe how gas behaves between suction and discharge. They differ in how heat and irreversibility are treated, and each comes with its own efficiency. Mixing a polytropic head with an isentropic efficiency is one of the most common errors in compressor spreadsheets.

• Isentropic (reversible adiabatic). No heat transfer and no friction. It is the reference for reciprocating and screw compressors, where adiabatic efficiency is taken as the isentropic efficiency.
• Polytropic. The real path is split into many small steps of equal efficiency. Polytropic efficiency barely changes with pressure ratio, which is why centrifugal and axial compressor data are quoted this way.
• Isothermal. All compression heat is removed as it is produced. It needs the least work and is used only as a figure of merit.

Efficiency definitions and typical values

Four separate losses sit between the gas and the electricity meter or fuel line. Thermodynamic efficiency covers aerodynamic or valve and flow losses inside the machine. Mechanical efficiency covers bearings, seals, packing and crosshead friction. Gear and coupling efficiencies apply only where those parts exist. Driver efficiency converts shaft power back to electrical, fuel or steam input. The ranges below are indicative starting points for early studies. Replace them with vendor values as soon as you have a quotation.

Efficiency has a strong effect on both power and temperature. Changing only the adiabatic efficiency in the worked example gives the results below.

Gauge vs absolute pressure and pressure ratio

Every compression equation uses absolute pressure. A gauge reading must first have the local atmospheric pressure added, and that matters most at low suction pressure. The calculator asks for the unit of every pressure, bar(g) or bar(a), psig or psia, and converts gauge inputs with the atmospheric pressure you enter. If you only know the site altitude, it can estimate atmospheric pressure from the ISA model, but only when you ask it to.

Standard flow vs actual flow (MMSCFD, Sm³/h, ACFM)

MMSCFD, SCFM, Sm³/h and Nm³/h are really mass-flow units in disguise: they state how much gas would occupy a volume at agreed reference conditions. Cylinders and impellers, however, see the actual volume at suction pressure and temperature. In the worked example, 10 MMSCFD is about 9,741 kg/h, but only about 2,288 m³/h of actual gas at 5.51 bar(a) and 35 °C.

The reference conditions also matter. The table below keeps the number “10 million cubic feet per day” and changes only the reference pressure and temperature. The ISO 13443 standard conditions are 15 °C and 101.325 kPa; US practice is usually 60 °F with 14.696 psia, while gas-sales contracts often use 14.73 psia. Normal conditions at 0 °C give almost 6 % more gas than the US convention.

Real-gas effects and the compressibility factor Z

Head and power scale almost directly with the average compressibility factor. Natural gas at moderate pressure has Z a little below 1, so assuming ideal gas overstates power. The calculator offers four Z methods and two thermodynamic bases. The Z-corrected screening basis keeps an ideal-gas k and averages Z across each stage. The EOS basis evaluates real-gas enthalpy and entropy from Peng-Robinson or SRK, which also moves the predicted discharge temperature, and is the better choice above roughly 30 bar(a). Above that pressure the tool raises an advisory but never switches your basis for you.

Multistage compression and intercooling

A high overall pressure ratio is split into stages with coolers between them. Cooling the gas back toward suction temperature lowers the head of the next stage, keeps discharge temperature within lubricant, valve and seal limits, and on reciprocating machines keeps volumetric efficiency and rod load in range. For an ideal gas with perfect intercooling, equal stage ratios of PR^1/N give the least total power. Real coolers lose pressure, so the calculator raises each stage ratio slightly to recover it, or lets you set the interstage pressures yourself.

Compressor discharge temperature

On the constant-k screening basis the ideal (isentropic) discharge temperature is T2s = T1 · PR^(k−1)/k, and the actual adiabatic discharge temperature is T2 = T1 + (T2s − T1) / ηis, both in kelvin. On a polytropic basis T2 = T1 · PR^(n−1)/n. With a gas composition, the calculator solves the entropy integral of Cp(T)/T for the isentropic state and then the enthalpy integral for the actual state; on the EOS basis it uses the equation-of-state entropy and enthalpy instead.

Discharge temperature is often the real limit on stage ratio. High temperature thins lubricants, shortens valve and packing life, speeds up fouling and can exceed seal, material or gas-service limits. Allowable values depend on the project specification, the gas and the vendor, so the review limit in the calculator is an input you set, not a hidden constant.

Worked example: three-stage natural gas compressor

The default project in the calculator is a gas-gathering duty: 10 MMSCFD (60 °F, 14.696 psia) of an illustrative lean natural gas with MW 19.49, compressed from 4.5 bar(g) and 35 °C to 70 bar(g) in a three-stage reciprocating compressor. Adiabatic efficiency is 80 %, mechanical efficiency 95 %, motor efficiency 95 %, and each intercooler returns the gas to 49 °C with a 0.5 bar pressure drop. Z comes from Peng-Robinson.

1. Absolute pressures. P1 = 4.5 + 1.013 = 5.513 bar(a); P3 = 70 + 1.013 = 71.01 bar(a); overall PR = 71.01 / 5.513 = 12.88.
2. Stage ratio. The cube root of 12.88 is 2.34, but two coolers each lose 0.5 bar, so the stages must each run at 2.39 to arrive at 71.01 bar(a).
3. Mass flow. 10 MMSCFD at 60 °F and 14.696 psia with standard-condition Z of 0.9966 is 2.706 kg/s, or 9,741 kg/h.
4. Stage 1 hand check. R = 8.314 / 19.49 = 0.4265 kJ/(kg·K). With k = 1.245, Zavg = 0.984 and T1 = 308.15 K, the constant-k head is 0.984 × 0.4265 × 308.15 × 5.084 × (2.388^0.1967 − 1) = 122.8 kJ/kg. The engine, integrating variable Cp, gives 123.1 kJ/kg: the two agree within 0.3 %.
5. Stage 1 power and temperature. Actual head = 123.1 / 0.80 = 153.9 kJ/kg; gas power = 2.706 × 153.9 = 416 kW; discharge temperature 107.1 °C (constant-k hand check 106.9 °C).
6. All stages. Summing the three stages gives 1,257 kW of gas power. Shaft power = 1,257 / 0.95 = 1,323 kW. Driver input = 1,323 / 0.95 = 1,393 kW.
7. Driver. Required rating = 1,323 × 1.10 = 1,456 kW, so the next IEC 60072-1 preferred output is 1,600 kW, leaving about 21 % margin at this case.
8. Benchmarks. Specific power is 1,323 / 10 = 132.3 kW per MMSCFD, and the single-reference isothermal power of 824 kW is 65.6 % of the gas power, a useful figure of merit when comparing cooled multistage options.

Reciprocating, centrifugal and screw compressor screening

Reciprocating (API 618 terminology). The tool works out swept volume and displacement for single- or double-acting cylinders, clearance volumetric efficiency ηv = 1 + C − C·(Zs/Zd)·PR^1/k − L, cylinder capacity against the required flow, mean piston speed, and gas rod load in compression and tension with a reversal check. Inertia loads, valve dynamics, pulsation (API 688) and torsional analysis remain OEM work.

Centrifugal (API 617 and ISO 10439-1 terminology). It reports polytropic head, head per impeller, tip speed, head and flow coefficients and tip Mach number. Surge and choke margins are measured only against curve points you enter; fan-law scaling is a similarity check, and no compressor map is ever generated for you. Running the worked example as a polytropic centrifugal duty gives about 1,388 kW at 78 % polytropic efficiency.

Rotary screw (API 619 terminology). Results are thermodynamic power with an ideal built-in volume-ratio check. Oil-injection cooling, leakage and slide-valve behaviour need vendor software.

Driver sizing and power margin

The required driver rating is the driver shaft power multiplied by one plus your margin. There is no universal margin: project specifications, the purchaser’s datasheet, the API standard for the machine and the type of driver all set it. For electric motors the calculator proposes the next preferred output from the IEC 60072-1 kW series or the NEMA MG 1 hp series, as a mechanical shaft-output size. Frame, poles, voltage, enclosure, hazardous-area marking and the IE efficiency class under IEC 60034-30-1:2025 come from the motor catalogue. Gas engines and gas turbines lose output with altitude and hot ambient air, so their site rating must come from the manufacturer; enter it as the available rating and the tool reports utilisation and remaining margin.

Performance comparison: test vs design

Enter design or guarantee data and measured suction and discharge conditions, and the Performance Comparison panel compares flow, pressure ratio, head, efficiency and power, with a fan-law similarity check and a specific-volume ratio check. ASME PTC 10-2022 and ISO 5389:2005 (turbocompressors) and ISO 1217:2009 with Amendment 1:2016 (displacement compressors) are the frameworks behind the comparison. It is a screening aid for spotting fouling, leakage or wear, not a certified acceptance test, which needs calibrated instruments, an uncertainty analysis, agreed test conditions and the test code’s own correction procedures.

Common compressor calculation mistakes

How this calculator compares

Most free compressor power calculators take one pressure ratio, one k and one efficiency and return a single number. That is fine for a rough check, but it hides the assumptions that cause the largest errors. The table shows what is typical of simple web calculators and spreadsheets against what this tool does.

Assumptions and limitations

• Single-phase gas only. Condensation in coolers, hydrates and liquid carry-over are not modelled, and the tool warns when cooler outlet conditions might condense heavy ends or water.
• Peng-Robinson and SRK use zero binary interaction parameters. AGA Report No. 8 and GERG-2008 (ISO 20765-2:2015) are not implemented; use a process simulator for custody-transfer accuracy.
• Reciprocating rod loads are gas loads only. Inertia loads, valve dynamics, pulsation and torsional behaviour need OEM and API 688 studies.
• Centrifugal curves are preliminary curves fitted to your points, not vendor maps; fan-law scaling is a similarity screen only.
• Screw compressor results are thermodynamic with an ideal built-in volume-ratio check.
• All limits and margins are your inputs. No universal discharge-temperature, rod-load or margin value is imposed.

Standards and references

The calculator uses the terminology and calculation structure of the documents below as a methodology basis. Editions and dates were re-checked against publisher and national-standards-body catalogues in October 2026. Selecting a standard in the tool does not make a result compliant: work to the edition named in your contract and buy the official text from the publisher.

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Frequently asked questions