Measurement Units

Energy products are measured in physical units by mass, volume, and energy content. Two families of unit matter to energy statistics: original (“natural”) units — the unit specific to a product, used at the point where an energy flow is actually measured, chosen for the simplest measuring instrument (mass for coal, volume for crude oil) — and common units, usually energy units, needed whenever quantities of different products must be compared or aggregated, as in an energy balance. Converting from an original to a common unit requires a conversion factor, and in the absence of country- or product-specific information (density, gravity, calorific value), this conversion is a recurring source of discrepancy in energy statistics [IRES, Ch. IV, paras. 4.1–4.3, PDF p. 54, 2018]. See Calorific Values for the conversion factor that bridges mass/volume quantities to energy content.

The International System of Units (SI)

Original and common units are both grounded in the International System of Units (SI), the modern metric system providing a logical, interconnected framework for measurement in science, industry and commerce [IRES, Ch. IV, para. 4.4, PDF p. 54, 2018]. See International System of Units for SI’s governance and adoption history. SI’s seven base units are:

Physical quantity	Base unit
length	metre
mass	kilogram
time	second
electric current	ampere
thermodynamic temperature	kelvin
luminous intensity	candela
amount of substance	mole

[IRES, Ch. IV, para. 4.6, PDF p. 55, 2018]

Derived units are formed by combining base units according to the algebraic relations linking the corresponding quantities — products of powers of the base units. A derived unit whose product includes no numerical factor other than one is a coherent derived unit (e.g. the newton, N = kg·m/s²) [IRES, Ch. IV, para. 4.7, footnote 25, PDF p. 55, 2018].

SI uses a standard set of prefixes to indicate multiples or fractions of a unit:

Factor	Name	Symbol	Factor	Name	Symbol
10¹	deca	da	10⁻¹	deci	d
10²	hecto	h	10⁻²	centi	c
10³	kilo	k	10⁻³	milli	m
10⁶	mega	M	10⁻⁶	micro	μ
10⁹	giga	G	10⁻⁹	nano	n
10¹²	tera	T	10⁻¹²	pico	p
10¹⁵	peta	P	10⁻¹⁵	femto	f
10¹⁸	exa	E	10⁻¹⁸	atto	a
10²¹	zetta	Z	10⁻²¹	zepto	z
10²⁴	yotta	Y	10⁻²⁴	yocto	y

[IRES, Ch. IV, para. 4.8, PDF p. 55, 2018]

Original units

Original units are those employed at the point of measurement of a product flow, chosen to suit the product’s physical state (solid, liquid or gas) and to require the simplest measuring instruments. Typical examples: mass units (kilograms, metric tons) for solid fuels; volume units (barrels, litres) or mass units (metric tons) for oil; volume units (cubic metres) for gases. The units actually used vary by country and reflect historical practice and local conditions [IRES, Ch. IV, para. 4.9, footnotes 26–28, PDF p. 55, 2018].

Questionnaires for energy statistics sometimes require data in a unit other than the original/natural one. Crude oil and oil products are often requested by mass rather than volume, since heating value by weight displays less variation than heating value by volume. Gases and wastes are often requested in terajoules or other energy units, since their production processes (rather than chemical composition) usually define them, and different compositions of the same nominal gas or waste carry different energy content per unit volume; the collection of waste statistics in an energy unit is based on measured or inferred heat output used directly for heat raising [IRES, Ch. IV, para. 4.10, PDF pp. 55–56, 2018].

Mass units

Solid fuels such as coal and coke are generally measured in mass units. The SI unit for mass is the kilogram (kg); metric tons (1000 kg) are most commonly used for coal and its derivatives. Other mass units in national use include the pound (0.4536 kg), short ton (907.185 kg) and long ton (1016.05 kg) — all approximate conversions [IRES, Ch. IV, para. 4.11, footnote 29, PDF p. 56, 2018].

Table 1 — Mass equivalents (multiply the value in the FROM row by the cell to convert INTO the column unit):

FROM \ INTO	Kilograms	Metric tons	Long tons	Short tons	Pounds
Kilograms	1.0	0.001	0.000984	0.001102	2.2046
Metric tons	1000	1.0	0.984	1.1023	2204.6
Long tons	1016	1.016	1.0	1.120	2240.0
Short tons	907.2	0.9072	0.893	1.0	2000.0
Pounds	0.454	0.000454	0.000446	0.0005	1.0

Example: 1 metric ton = 0.984 long tons [IRES, Annex B, Table 1, PDF p. 162, 2018].

Volume units

Volume units are original units for most liquid and gaseous fuels, and some traditional fuels. The SI unit for volume is the cubic metre, equivalent to a kilolitre or 1000 litres. Other volume units include the British/Imperial gallon (~4.546 litres), the United States gallon (~3.785 litres), the barrel (~159 litres) and the cubic foot (also used for gaseous fuels). Given oil markets’ preference for the barrel, the barrel per day is commonly used in the petroleum sector for cross-time-frequency comparison (e.g. monthly vs. annual crude oil production), though other volume-per-time units serve the same purpose for other products [IRES, Ch. IV, para. 4.12, footnote 30, PDF p. 56, 2018].

Table 2 — Volume equivalents:

FROM \ INTO	U.S. gallons	Imperial gallons	Barrels	Cubic feet	Litres	Cubic metres
U.S. gallons	1.0	0.8327	0.02381	0.1337	3.785	0.0038
Imp. gallons	1.201	1.0	0.02859	0.1605	4.546	0.0045
Barrels	42.0	34.97	1.0	5.615	159.0	0.159
Cubic feet	7.48	6.229	0.1781	1.0	28.3	0.0283
Litres	0.2642	0.220	0.0063	0.0353	1.0	0.001
Cubic metres	264.2	220.0	6.289	35.3147	1000.0	1.0

Example: 1 barrel = 0.159 cubic metres [IRES, Annex B, Table 2, PDF p. 162, 2018].

Density, specific gravity and API gravity

Since liquid fuels are measured by either mass or volume, converting between the two requires knowing density, defined as mass divided by volume:

Density = mass / volume

[IRES, Ch. IV, para. 4.13, PDF p. 56, 2018]

Specific gravity is a dimensionless ratio of the density of the fuel to the density of water at a specified temperature — equivalently, the ratio of the mass of a given volume of fuel (e.g. oil) at 15°C to the mass of the same volume of water at that temperature:

Specific gravity = densityfuel / densitywater = massfuel / masswater

[IRES, Ch. IV, para. 4.14, PDF p. 56, 2018]

Under SI/metric units, volume is obtained by dividing mass by density (and mass by multiplying volume by density); other measurement systems require consulting conversion-factor tables [IRES, Ch. IV, para. 4.15, PDF p. 56, 2018].

API gravity, a standard adopted by the American Petroleum Institute, is another common measure of the gravity or density of liquid fuels. It relates to specific gravity by:

API gravity = (141.5 / specific gravity) − 131.5

[IRES, Ch. IV, para. 4.16, PDF pp. 56–57, 2018]

API gravity is used in practice to characterize crude oil — see Oil for its use in distinguishing heavy from light crude oil.

Energy units

Energy, heat, work and power are related but distinct concepts. When force moves an object over a distance, work is done, heat is released (under any but ideal conditions), and energy is transformed; energy is the capacity to do (and often the result of doing) work, and heat can be both a by-product of work and a form of energy in its own right. The coherent SI derived unit of energy, heat and work is the joule (J), precisely defined as the work done when a constant force of 1 newton moves a body of 1 gram over 1 metre. Common multiples used in energy statistics are the megajoule, gigajoule, terajoule and petajoule [IRES, Ch. IV, para. 4.17, PDF p. 57, 2018].

Other energy units in use: the kilogram calorie (kilocalorie, kcal) or its multiples in the metric system; the British thermal unit (Btu) or its multiples; the ton of coal equivalent (tce); the ton of oil equivalent (toe); and the kilowatt hour (kWh) [IRES, Ch. IV, para. 4.18, PDF p. 57, 2018].

• Calorie. The International Steam Table Calorie (IT calorie) was originally defined as 1/860 international watt-hour, later fixed exactly at 4.1868 joules — this is the definition used throughout the conversion tables in the IRES Chapter IV annex. The kilocalorie and teracalorie, its common multiples, are based on the IT calorie in the IRES context. A different definition, the gram calorie (heat needed to raise 1 gram of water 1°C from a reference temperature), equals 4.1855 joules at a 14.5°C reference temperature — other reference temperatures give other values [IRES, Ch. IV, para. 4.19, footnotes 31–32, PDF p. 57, 2018].
• Btu. The British thermal unit is a heat measure equal to the heat needed to raise 1 pound of water at 60°F by 1°F. Its common multiples are the therm (10⁵ Btu) and the quad (10¹⁵ Btu). The internationally agreed Btu value is currently 1055.06 joules [IRES, Ch. IV, para. 4.20, footnote 33, PDF p. 57, 2018].
• tce vs. toe. When coal was the principal commercial fuel, the ton of coal equivalent (tce) was the common energy unit; as oil grew in importance it was superseded by the ton of oil equivalent (toe). The toe is now defined as 41.868 gigajoules; the tce as 29.3076 gigajoules. It should not be assumed that one ton of coal contains one tce, or one ton of oil one toe, of energy — calorific values vary widely across coal types, crude oils and petroleum products [IRES, Ch. IV, para. 4.21, PDF p. 57, 2018].
• Watt and kilowatt hour. Power is the rate at which work is done (or heat released, or energy converted); the rate of one joule per second is a watt. A 100-watt light bulb draws 100 joules of electricity per second [IRES, Ch. IV, para. 4.22, PDF p. 57, 2018]. The kilowatt hour (kWh) is the energy equivalent of 1000 watts sustained over one hour: 1 kWh = 3.6×10⁶ joules [IRES, Ch. IV, para. 4.23, PDF p. 57, 2018]. Electricity is usually measured in kWh, letting users perceive electrical energy in terms of how long an appliance of a given wattage can run on it; heat quantities are usually measured in calories or joules instead [IRES, Ch. IV, para. 4.24, PDF p. 57, 2018].

Table 3 — Energy equivalents:

FROM \ INTO	TJ	Million Btu	GCal	GWh	ktoe	ktce
Terajoule (TJ)	1	947.8	238.84	0.2777	2.388×10⁻²	3.411×10⁻²
Million Btu	1.0551×10⁻³	1	0.252	2.9307×10⁻⁴	2.52×10⁻⁵	3.6×10⁻⁵
GigaCalorie (GCal)	4.1868×10⁻³	3.968	1	1.163×10⁻³	10⁻⁴	1.429×10⁻⁴
Gigawatt hour (GWh)	3.6	3412	860	1	8.6×10⁻²	1.229×10⁻¹
Ktoe	41.868	3.968×10⁴	10⁴	11.630	1	1.429
Ktce	29.308	2.778×10⁴	0.7×10⁻⁴	8.14	0.7	1

Example: 1 GWh = 3.6 TJ [IRES, Annex B, Table 3, PDF p. 162, 2018] — [IRES, Ch. IV, para. 4.25, PDF p. 57, 2018].

Common units

Because original units vary widely across products (metric tons, barrels, kilowatt hours, therms, calories, joules, cubic metres), comparing fuel quantities or estimating transformation efficiencies requires converting them to a common unit — a conversion that may need a distinct factor for each product [IRES, Ch. IV, para. 4.26, footnote 35, PDF p. 58, 2018].

The joule is the only energy unit that is itself part of the International System of Units, and it is the common unit generally used in energy statistics, although other energy units (toe, GWh, Btu, calories, etc.) are also seen in practice. It is recommended that the joule be used as the common unit [IRES, Ch. IV, para. 4.27, PDF p. 58, 2018].

It is further recommended that national and international agencies responsible for energy statistics — and any organization advising or working on their behalf — always clearly define the measurement units and common units used in their publications and disseminated data. The conversion factors and methods used to convert original physical units into the chosen common unit(s) should be described in energy statistics metadata and be readily accessible to users, and it should be made clear whether energy units are defined on a gross or net calorific basis [IRES, Ch. IV, para. 4.28, PDF p. 58, 2018]. See Calorific Values for the gross/net distinction.

Related

• International System of Units — SI’s governance and adoption history, the framework underlying every unit on this page
• Calorific Values — the conversion factor that turns a mass or volume quantity into energy content, and the gross/net basis flagged in the common-units recommendation above
• Default Calorific Values — Table 4.1’s default net calorific values, and the fuelwood/charcoal/waste unit mechanics that build on the mass, volume and energy units defined here
• Units Recommended for Dissemination — Table 4.4, which restates the joule-as-common-unit recommendation at the dissemination level (terajoules)
• IRES Chapter IV — Measurement Units and Conversion Factors — the chapter digest this page expands on
• IRES Annexes and References — Annex B, Tables 1–3, the source of the mass/volume/energy equivalence tables above
• SIEC Classification System — the product classification these units are applied against
• Coal, Oil, Biofuels — product pages that already use gross calorific value (GCV) or gravity as a SIEC classification threshold