Contrary to popular opinion, gas is not generally sold per unit of volume, but rather per unit of energy that can be produced by burning the gas. End-use consumers of gas are interested in the heat energy that combusting the gas will generate. Since the heat energy of the gas is related to the relative proportion of “lighter” methane versus “heavier” ethane, propane, butane, pentane, and other components, heat energy is not a constant value between different gas sources.
The heat energy of a particular gas stream is measured by units of calorific value, which is defined by the number of heat units released when a unit volume of the gas burns. Typical units of calorific value are British thermal units (Btu), joules (J), and kilocalories (kcal).
Most industrial and residential customers receive gas via a pipeline connection and a gas meter that measures the volume of gas delivered. This volume measurement is subsequently converted, using the average calorific value per volume factor, into number of energy units consumed by the end user and multiplied by the price per unit of energy to determine the billed amount.
Worldwide, the cost of gas to the customer is commonly specified in dollars per British thermal unit. A British thermal unit is the energy required to raise the temperature of 1 pound of water by 1°F. For larger industrial customers, the abbreviations MBtu (thousand or 103 Btu) or MMBtu (million or 106 Btu) are more commonly used. In the United Kingdom, gas is charged to residential customers at a price per therm, which is equivalent to 100,000 Btu.
Gas volumes are usually measured in multiples of cubic feet (ft3) or cubic meters (m3). Gas reserves are expressed in billion cubic feet (bcf) (109), or trillion cubic feet (tcf) (1012), or, in the case of countries using the metric system, billion cubic meters (bcm). Gas volume produced or consumed is often expressed in million cubic feet (MMcf), (106), and Mcf (thousand cubic feet). Gas volume can also be expressed in million cubic feet per day (MMcfd), sometimes written as MMscfd to denote standard conditions, and its metric counterpart, billion cubic meters per day (bcmd). (M is commonly used to designate 1,000, which is based on the Roman numeral system. Thus MM denotes 1,000 x 1,000, or 1 million (106). In the metric system, k also refers to 1,000. The energy industry uses both M and k. Some companies use the lower case m to denote 1,000; thus mmcfd would be equal to MMcfd.
As stated earlier, conversion from volume to energy requires knowledge of the average calorific value of the particular gas. Natural gas from different fields, and sometimes different reservoirs in the same field, can have different proportions of hydrocarbon components and thus varying calorific values. A factor of 1,000 Btu/ft3 is commonly used.
Crude oil has a calorific value of 5.4 MMBtu to 5.8 MMBtu per barrel (bbl) of oil, depending on the composition of the oil. It is often necessary and useful to convert gas volume into energy equivalent barrels of oil using barrel of oil equivalent (boe) units. This is commonly done when both oil and gas are found and produced in the same reservoir, making it easier to estimate the total reserves or production volumes.
Online Gas Unit Converter
Natural gas units can be confusing. To make conversions easier, Natgas.info has created an app for the iPhone/iPad and Android systems, as well as this online gas units converter below.
To use the online converter tool on this site, see below (if the unit converter does not show below, there is an issue with Adobe Flash, please click on the Gas Unit Converter on left column of page). To use, first select the appropriate energy content per cubic feet of gas and per barrel of oil. If you do not know the exact factors to use, the factors 1 million Btu = 1000 cf and 1 barrel of oil equivalent = 5,800 cf is a common approximation.
Next, select whether the units to be converted are volumes (cubic feet, meters, etc) or rates (cubic feet per day) or end products (volumes of GTL or LNG liquids or watts of electricity). The first calculator is for volumes, and the second calculator can be used for rates and end products.
Lastly, pick the input units ("Units In"), enter the input value, and then pick the desired output unit ("Unit Out"). Click on "Calculate" to make the conversion.