of 0.04%. A standard cubic foot in the US Customary System is approximately equivalent to 0.02833 standard cubic meters in the SI system. In the natural gas industry, where quantities are often expressed in standard cubic feet, large multiples of standard cubic feet are generally not expressed with metric prefixes, but rather with prefixes based on roman numerals, where the s for "standard" is often omitted. Common units of gas volumes include ccf (hundred standard cubic feet), Mcf (thousand standard cubic feet), and MMcf (million standard cubic feet). The M refers to the Roman numeral for thousand, while two M's represent one thousand thousands, or one million. Bcf (billion standard cubic feet), Tcf (trillion standard cubic feet), Qcf (quadrillion standard cubic feet), etc., are also used. Compressed or liquefied gases in refillable cylinders The National Conference on Weights and Measures, a US-based non-profit organization working in cooperation with the US National Institute of Standards and Technology, has defined a set of standards in a regulation entitled the "Uniform Regulation for the Method of Sale of Commodities". This regulation defines a standard cubic foot, for compressed or liquefied gases in refillable cylinders other than LPG by, "A standard cubic foot of gas is defined as a cubic foot at a temperature of 21 °C (70 °F) and a pressure of 101.325 kilopascals [kPa] (14.696 psia)". Industrial gases Yet other definitions are in use for industrial gas, where, in the US, a standard cubic foot for industrial gas use is defined at and 14.696 psia (101.325 kPa), while in Canada, a standard cubic meter for industrial gas use is defined at and 101.325 kPa (14.696 psia). Converting actual volumes to standard volumes An actual volume can be converted to a standard volume using the following equation: Vs = Va × Fp × Ft × (Fpv)2 Where, Vs: standard volume Va: actual volume (sometimes shown as Vr for registered volume) Fp: pressure factor (sometimes shown as Pm for pressure multiplier) Fp: absolute pressure / standard pressure = (line gauge pressure + atmospheric pressure)/base pressure Ft: temperature factor (sometimes shown as Tm for temperature multiplier) Ft: absolute standard temperature / absolute line temperature = [273.15 + standard temperature (°C)] / [273.15 + line temperature (°C)] or [459.67 + standard temperature (°F)] / [459.67 + line temperature (°F)] Fpv: super compressibility factor (often omitted or shown as equaling 1) Example: How many standard cubic feet