SCFM to ACFM and Nm³/h to m³/h: standard and actual flow, pumping speed and throughput

How to convert between standard and actual volume flow

A gas takes up less space at a higher pressure and more space at a higher temperature. For the same amount of gas, the ideal gas law therefore gives:

V₂ = V₁ · (p₁ / p₂) · (T₂ / T₁)

Here p is the absolute pressure and T the absolute temperature in kelvin (T = t in °C + 273.15, or T = (t in °F + 459.67) / 1.8). Index 1 is the condition in which you know the value, index 2 the condition you are looking for. The formula works the same way for volume flows in m³/h, l/min or cfm. For standard flow (index S) and actual flow (index A) it reads:

V_A = V_S · (p_S / p_A) · (T_A / T_S)

The calculator assumes that:

• the gas behaves like an ideal gas,
• it is the same gas with the same composition in both conditions,
• nothing condenses, such as water vapour, and nothing is dried out or added.

At high pressures a real gas deviates from an ideal gas. You then also need the compressibility factor Z of the gas in question; this calculator does not include it.

If the pressure is given as gauge pressure, for example in psig or bar(g), the calculator adds the ambient pressure. The preset value is 1,013.25 mbar; for exact results, enter the atmospheric pressure at the site.

Reference conditions compared: Nm³, Sm³, ANR and scfm

A figure in normal or standard cubic metres or feet is only clear when the reference condition is stated. The calculator can use these reference conditions:

Nm³ and Sm³

Under DIN 1343, normal conditions are 0 °C and 1,013.25 mbar; a normal volume, usually written as Nm³ in data sheets, refers to these conditions. ISO 13443 notes that in gas measurement, 0 °C and 101.325 kPa are commonly called “normal” conditions and 15 °C and 101.325 kPa “(metric) standard” conditions. There, a standard cubic metre (Sm³) usually refers to this metric standard condition, 15 °C and 101.325 kPa (in the calculator: “ISO 13443, natural gas”); other standards and data sheets also use “standard” for other conditions. ISO 13443 also puts the reference condition in the quantity symbol rather than in the unit: V(273.15 K, 101.325 kPa) in m³ instead of “Nm³”. Data sheets do not always use “normal” in the sense of DIN 1343, so check which pressure and temperature a figure refers to.

ANR under ISO 8778

Pneumatics uses the standard reference atmosphere of ISO 8778: 20 °C, 100 kPa and 65 % relative humidity. A flow that refers to it is marked “ANR” in brackets after the unit, for example l/min (ANR). The humidity is part of the definition, but this calculator converts the gas volume with pressure and temperature only; humidity does not enter the calculation. On that basis, 100 l/min (ANR) corresponds to 91.9591 l/min at normal conditions to DIN 1343.

ISO 1217: compressor flow

ISO 1217 covers acceptance tests of displacement compressors. It refers the actual volume flow rate to the temperature, pressure and composition of the gas at the compressor’s standard inlet point (clause 3.4.1) – an inlet condition, not a fixed standard condition. If no inlet conditions have been agreed, clause 6.2 applies the values of Annex F: 100 kPa, 20 °C and a relative water vapour pressure of 0. The Compressed Air Challenge recommends the same conditions for comparing air compressor ratings: 14.5 psia (1 bar), 68 °F (20 °C), dry.

scfm: which standard?

There is no single standard condition for scfm (standard cubic feet per minute). ISO 13443 notes that most non-metric reference conditions use a temperature base of 60 °F but different pressure bases. The table lists four US reference conditions from three sources; 40 CFR 98.6 allows either 60 °F or 68 °F. acfm (actual cubic feet per minute) is the flow at operating conditions.

Why a vacuum pump’s pumping speed is an actual volume flow

ISO 21360-1 describes the pumping speed (volume flow rate, symbol S or q_V) as the volume of gas that flows per unit of time from the test dome through the pump inlet. It therefore applies at the pressure at the inlet, the inlet pressure. Pumping speed is an actual volume flow, not a standard one.

So if a pump has a pumping speed of 100 m³/h at 10 mbar, it moves 100 m³ per hour at that pressure. That volume holds only about one hundredth of the gas the same volume would hold at atmospheric pressure. At 20 °C it corresponds to 0.919591 Nm³/h to DIN 1343.

The amount of gas a pump removes is therefore described by the throughput Q = p · S (ISO 21360-1, clause 3.9). Conversely, ISO 21360-1 takes the pumping speed as the quotient of throughput and equilibrium pressure. Because pumping speed changes with inlet pressure, data sheets often show it as a curve over inlet pressure.

Which design suits your pressure range is explained on our technology pages, for example oil-lubricated rotary vane vacuum pumps, screw vacuum pumps, Roots vacuum pumps and liquid ring vacuum pumps. All designs are listed in the vacuum pump overview.

Throughput Q = p · S: units and sccm

Throughput is the product of inlet pressure and pumping speed. It states the amount of gas per unit of time as pressure times volume per time. The usual units convert as follows:

Mass flow controllers and many gas dosing systems work in sccm (standard cubic centimetres per minute) and slm (standard litres per minute; 1 slm = 1,000 sccm). ISO 21360-1 refers the standard volume flow rate, including sccm, to 0 °C and 101,325 Pa; this calculator uses the same values for slm. Check the data sheet of a flow controller for the reference condition its figures use. A throughput in mbar·l/s can only be converted to sccm if the gas temperature is known. From the ideal gas law:

q_V,std = Q · T₀ / (p₀ · T), with T₀ = 273.15 K and p₀ = 101,325 Pa

Pressure units such as mbar, Torr, psi or bar are converted by the vacuum unit converter.

Worked examples

Examples 1 and 3 are preset in the calculator above.

Example 1: Nm³/h to m³/h at 6 bar gauge

Given: 100 Nm³/h to DIN 1343. Wanted: the flow at 6 bar gauge, 1,013.25 mbar ambient pressure and 20 °C.

p_A = 6 bar + 1.01325 bar = 7.01325 bar (absolute)
V_A = 100 m³/h · (1.01325 bar / 7.01325 bar) · (293.15 K / 273.15 K) = 15.5055 m³/h

Example 2: scfm to acfm

Given: 100 scfm referred to 68 °F and 14.7 psia (one of the two reference conditions in 40 CFR 98.6). Wanted: acfm at 100 psig, 14.7 psia ambient pressure and 80 °F.

p₂ = 100 psi + 14.7 psi = 114.7 psia; T in kelvin = (t in °F + 459.67) / 1.8
V₂ = 100 cfm · (14.7 / 114.7) · (299.817 K / 293.15 K) = 13.1075 acfm (that is 22.2698 m³/h)

Example 3: throughput of a vacuum pump

A vacuum pump has a pumping speed of 100 m³/h at an inlet pressure of 10 mbar; the gas is at 20 °C.

S = 100 m³/h = 27.7778 l/s
Q = p · S = 10 mbar · 27.7778 l/s = 277.778 mbar·l/s = 27.7778 Pa·m³/s = 208.35 Torr·l/s
at a gas temperature of 20 °C: 15,326.5 sccm = 15.3265 slm = 0.919591 Nm³/h

More calculators and contact

More calculators are listed in the calculator overview, including the vacuum unit converter and the calculator for the boiling point of water under vacuum.

Ehrler & Beck sells and services vacuum technology, including equipment from Leybold, Atlas Copco, Busch, Becker, Edwards, Elmo Rietschle and Pfeiffer Vacuum. If you want to select or replace a vacuum pump, you can reach us through the contact form.

Sources

• DIN 1343:1990-01, Referenzzustand, Normzustand, Normvolumen; Begriffe und Werte (reference conditions, normal conditions, normal volume; concepts and values).
• ISO 21360-1:2020, Vacuum technology – Standard methods for measuring vacuum-pump performance – Part 1: General description; clauses 3.1, 3.2, 3.9, 3.10 and list of symbols.
• ISO 8778:2003, Pneumatic fluid power – Standard reference atmosphere; Table 1 and clause 4.2.
• ISO 1217:2009, Displacement compressors – Acceptance tests; clauses 3.1.23, 3.4.1, 3.4.3, 6.2 and Annex F. Public copy at Public.Resource.Org
• ISO 13443:1996, Natural gas – Standard reference conditions; clause 3 with notes 2, 4 and 5, Annex A.
• US Code of Federal Regulations, 40 CFR 98.6, definition “Standard conditions or standard temperature and pressure (STP)”. Text at Cornell LII
• U.S. Energy Information Administration, glossary entry “Cubic foot (cf), natural gas”. eia.gov
• NIST Handbook 130 (2026), Uniform Regulation for the Method of Sale of Commodities, section 2.16.2. doi.org/10.6028/NIST.HB.130-2026
• NIST Special Publication 811 (2008), Guide for the Use of the International System of Units, Table 11 and Appendix B.8: conversion factors for foot, inch, pound, pound-force, psi, torr, atmosphere and °F. nvlpubs.nist.gov
• Compressed Air Challenge, Fact Sheet #8 “Packaged Compressor Efficiency Ratings”, Rev. 1, May 2026. compressedairchallenge.org

All figures on this page are calculated with the formulas and conversion factors above and rounded to at most six significant digits.