CFM / PSI / Bar Conversion & Selection Guide

Keep the quantity, its units and its reference conditions together.

Compressor enquiries often arrive in mixed units: a tool manual states CFM, a quotation uses cubic metres per minute, and pressure appears in PSI or bar. Converting these values can make documents easier to compare. The conversion must preserve what each number means.

CFM measures volumetric flow. PSI and bar measure pressure. There is no direct conversion from CFM to PSI or bar. A compressor selection requires both a flow requirement and a pressure requirement, together with evidence that the offered machine supports their combination.

This guide provides numerical conversions, explains the qualifiers that must travel with them, and shows how to avoid rounding a proposal into apparent compliance. For wider portable equipment enquiries, visit Portable Compressors.

Identify the quantity first

Before using a factor, decide which kind of change is needed.

The first three are unit conversions. The last two require additional information. Treating all five as simple multiplication by a familiar constant can produce a plausible number with the wrong meaning.

Flow conversion factors

Using the international foot, one cubic foot equals 0.028316846592 cubic metres. The following relationships preserve the original flow basis:

These factors change units. They do not turn a free-air figure into the volume flowing inside a pressurised hose, or establish the delivery of a machine at another pressure.

Quick flow table

Values are rounded for reading. Every entry represents the same original flow basis across its row.

Worked example: preserve the time unit

An illustrative document states 185 CFM. Its metric equivalent is:

185 × 0.028316846592 = 5.23861661952 m³/min, or approximately 5.239 m³/min.

Expressed per hour, the same flow is approximately 314.317 m³/h. Writing 5.239 m³/h would introduce a factor-of-60 error.

Carry the original document's flow definition and rating pressure into the converted entry. The calculation does not supply those missing details if the original document omitted them.

Pressure conversion factors

Use bar ≈ psi × 0.06894757 and psi ≈ bar × 14.503774. Also, 1 bar = 100 kPa.

The conversion preserves pressure reference: psig becomes bar(g), while psia becomes bar(a). It does not add or remove atmospheric pressure.

Apply the same basis to both columns. These values are conversions, not suggested operating settings. They establish neither a machine rating nor an acceptable tolerance.

For example, 100 psig is approximately 6.895 bar(g). If an enquiry specifies 7 bar(g), retain that original requirement rather than relabelling 100 psig as exactly equivalent.

Gauge and absolute pressure need different labels

Gauge pressure refers to the prevailing atmospheric pressure. Absolute pressure refers to an ideal vacuum. Their relationship is:

Absolute pressure = gauge pressure + atmospheric pressure, using consistent units.

For an illustrative atmospheric pressure of 1.00 bar(a), a reading of 7.00 bar(g) corresponds to 8.00 bar(a). Conversely, 7.00 bar(a) corresponds to 6.00 bar(g) under that same assumption.

The 1.00 bar value is an example, not a site measurement. Atmospheric pressure varies with location and weather. Use the applicable value where the distinction affects the calculation.

A bare “7 bar” entry leaves its reference unresolved. Ask for clarification instead of silently choosing gauge or absolute. This is particularly important when comparing compressor documents with process calculations or vacuum information.

Read FAD, ACFM, SCFM and normal-flow labels carefully

Free-air delivery, or FAD, expresses delivered flow referred to specified inlet conditions. CFM is a unit in which a flow may be expressed; the letters alone do not identify those conditions.

Labels such as ACFM, SCFM and Nm³/h add information about actual or reference conditions. Obtain the definition used by the document, including relevant pressure, temperature and humidity or gas-composition assumptions. Avoid assigning one universal reference state solely from an abbreviation.

A unit change can preserve the specified reference state. Changing from actual conditions to another reference state requires a suitable calculation with the necessary conditions established.

For procurement, retain the supplier's full definition beside the converted value. If two quotations use different conventions, have those differences reconciled before comparing capacities. A matching number of cubic metres per minute does not establish equivalence when the reference conditions differ.

Worked example: rounding changes the apparent result

Assume a hypothetical requirement of 14.200 m³/min FAD and an offer of 500 CFM FAD. For this example, both refer to identical conditions, the same required pressure and the same delivery boundary.

The offer converts to 14.158423296 m³/min. The numerical difference is:

14.200 − 14.158423296 = 0.041576704 m³/min, approximately 0.042 m³/min below the stated requirement.

Rounding the offer to one decimal place produces “14.2 m³/min,” which visually matches the requirement. That display does not establish that the offer satisfies the unrounded minimum.

This example does not decide whether such a difference is acceptable in practice. That depends on the governing requirement, rating or measurement tolerances and the responsible technical review. The point is to preserve the difference until those questions are resolved.

Keep full calculation precision during comparison and round only for presentation. Retain original values so another reviewer can reproduce the result.

Do not use pressure ratios to invent compressor output

A question such as “How much CFM will this machine provide if the pressure changes?” concerns machine performance. It is different from asking how a known quantity of gas occupies volume under different conditions.

Dividing a nameplate CFM value by the pressure setting does not establish a new compressor rating. Nor does a simple inverse-pressure calculation establish the output available at a lower setting. Obtain the manufacturer's supported performance information for the actual configuration.

The same distinction applies when someone asks for “CFM at 10 bar.” Clarify whether they need the compressor's FAD at that working pressure or the physical volumetric flow at specified conditions in a line. Those are different descriptions and can serve different engineering purposes.

This guide supplies no general ACFM-to-SCFM multiplier or compressor derating formula. Those calculations require inputs and methods appropriate to the question being answered.

Keep volume, flow and storage separate

A receiver marked in litres describes a volume. L/min describes a flow rate. The presence of litres in both descriptions does not make them interchangeable.

Similarly, multiplying flow by time can produce a volume on the same stated basis, but it does not by itself calculate receiver filling time or useful stored air. Such an assessment also needs the relevant pressure range, temperature assumptions, compressor behaviour and simultaneous demand.

When an enquiry mixes receiver size with compressor delivery, record each separately. This avoids turning a storage description into an unsupported claim about continuous air supply.

Use a conversion record that preserves meaning

A useful record contains more than the calculated number:

For example, converting a supplier's 500 CFM entry is useful when the result remains attached to its FAD definition and working pressure. Recording only “14.2” removes the information needed to interpret it.

This record can accompany an enquiry or quotation comparison. It helps the customer, supplier and technical reviewer work from the same requirement even when their documents use different units.

Frequently asked questions

How many PSI is 185 CFM?

There is no direct conversion. Specify the required pressure separately and confirm the machine's supported flow at that pressure.

Is 1 CFM equal to 28.3 litres per minute?

Approximately. The more precise factor is 28.316846592 L/min per CFM, preserving the flow basis. It is not 28.3 litres per second.

Is 7 bar exactly 100 PSI?

No. On the same pressure basis, 7 bar is approximately 101.53 PSI. Retain the specified requirement when comparing ratings.

Can SCFM be converted to m³/min?

The volume unit can be changed while preserving the stated standard conditions. Label the result with those conditions; the conversion alone does not produce actual line flow.

Does a conversion establish compressor suitability?

It establishes numerical equivalence within the stated conversion. Selection still requires applicable performance, duty, air quality, site conditions and complete supply assessment.

What should I send if a quotation's units are unclear?

Send the original specification and quotation, including table headings, notes and model identification. Preserve any existing qualifiers rather than rewriting the ambiguous entry before review.

Discuss your compressor requirement

Reddy Compressors has operated since 2002 from Secunderabad/Hyderabad, with Telangana and Andhra Pradesh as priority service areas. Wider Indian project support is considered against confirmed supply arrangements and the assignment.

Provide the original duty information so the enquiry can preserve its technical meaning while identifying suitable equipment and scope. High-pressure supply is subject to supplier confirmation; emergency coordination carries no fixed response-time guarantee.