Section: 1 | Conversion Factors |

How to Cite this Reference

CONVERSION FACTORS

The following table gives conversion factors from various units of measure to SI units. It is reproduced from NIST Special Publication 811, Guide for the Use of the International System of Units (SI) (Ref. 1). The table gives the factor by which a quantity expressed in a non-SI unit should be multiplied in order to calculate its value in the SI. The SI values are expressed in terms of the base, supplementary, and derived units of SI in order to provide a coherent presentation of the conversion factors and facilitate computations (see the table “International System of Units” in this section). If desired, powers of ten can be avoided by using SI prefixes and shifting the decimal point if necessary.

It should be noted that the SI was changedin 2019 (Ref. 2), resulting in minor changes in the last few significant figures in a few quantities. This table does not reflect those changes yet because the conversion factors listed therein are almost always using fewer signficiant figures.

Conversion from a non-SI unit to a different non-SI unit may be carried out by using this table in two stages, e.g.,

1 cal_th= 4.184 J

1 Btu_IT = 1.055056 E+03 J

Thus,

1 Btu_IT = (1.055056 E+03 ÷ 4.184) cal_th= 252.164 cal_th

Conversion factors are presented for ready adaptation to computer readout and electronic data transmission. The factors are written as a number equal to or greater than one and less than ten with six or fewer decimal places. This number is followed by the letter E (for exponent), a plus or a minus sign, and two digits that indicate the power of 10 by which the number must be multiplied to obtain the correct value. For example:

3.523 907 E-02 is 3.523 907 × 10^–2

0.035 239 07

Similarly:

3.386 389 E+03 is 3.386 389 × 10³

3 386.389

A factor in boldface is exact; i.e., all subsequent digits are zero. All other conversion factors have been rounded to the figures given in accordance with accepted practice. Where less than six digits after the decimal point are shown, more precision is not warranted.

It is often desirable to round a number obtained from a conversion of units in order to retain information on the precision of the value. The following rounding rules may be followed:

If the digits to be discarded begin with a digit less than 5, the digit preceding the first discarded digit is not changed.

Example: 6.974 951 5 rounded to 3 digits is 6.97
If the digits to be discarded begin with a digit greater than 5, the digit preceding the first discarded digit is increased by one.

Example: 6.974 951 5 rounded to 4 digits is 6.975
If the digits to be discarded begin with a 5 and at least one of the following digits is greater than 0, the digit preceding the 5 is increased by 1.

Example: 6.974 851 rounded to 5 digits is 6.974 9
If the digits to be discarded begin with a 5 and all of the following digits are 0, the digit preceding the 5 is unchanged if it is even and increased by one if it is odd. (Note that this means that the final digit is always even.)

Examples:
6.974 951 5 rounded to 7 digits is 6.974 952
6.974 950 5 rounded to 7 digits is 6.974 950

Reference

Thompson, A., and Taylor, B. N., Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402, 2008.
Newell, D. B., and Tiesinga, E., The International System of Units (SI), NIST Special Publication 330, 2019 Edition, National Institute of Standards and Technology, Gaithersburg, MD, 2019 <https://doi.org/10.6028/NIST.SP.330-2019>.

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Conversion Factors to SI Units (Factors in Bold Face Are Exact)

To convert from	to	Multiply by	Times power of 10
Continued on next page...
abampere	ampere (A)	1.0	E+01
abcoulomb	coulomb (C)	1.0	E+01
abfarad	farad (F)	1.0	E+09
abhenry	henry (H)	1.0	E–09
abmho	siemens (S)	1.0	E+09
abohm	ohm (Ω)	1.0	E–09
abvolt	volt (V)	1.0	E–08
acceleration of free fall, standard (g_n)	meter per second squared (m/s²)	9.806 65	E+00
acre (based on U.S. survey foot)^a	square meter (m²)	4.046 873	E+03
acre foot (based on U.S. survey foot)^a	cubic meter (m³)	1.233 489	E+03
ampere hour (A · h)	coulomb (C)	3.6	E+03
ångström (Å)	meter (m)	1.0	E–10
ångström (Å)	nanometer (nm)	1.0	E–01
apostilb (asb)	candela per meter squared (cd/m²)	3.183 098	E–01
are (a)	square meter (m²)	1.0	E+02
astronomical unit (au)	meter (m)	1.495 978 70700	E+11
atmosphere, standard (atm)	pascal (Pa)	1.013 25	E+05
atmosphere, standard (atm)	kilopascal (kPa)	1.013 25	E+02
atmosphere, technical (at)^b	pascal (Pa)	9.806 65	E+04

^aThe U.S. survey foot equals (1200/3937) m. 1 international foot = 0.999998 survey foot.
^bOne technical atmosphere equals one kilogram-force per square centimeter (1 at = 1 kgf/cm²).
^cThe Fifth International Conference on the Properties of Steam (London, July 1956) defined the International Table calorie as 4.1868 J. Therefore the exact conversion factor for the International Table Btu is 1.055 055 852 62 kJ. Note that the notation for the International Table used in this listing is subscript “IT”. Similarily, the notation for thermochemical is subscript “th.” Further, the thermochemical Btu, Btu_th, is based on the thermochemical calorie, cal_th, where cal_th = 4.184 J exactly.
^dThe kilogram calorie or “large calorie” is an obsolete term used for the kilocalorie, which is the calorie used to express the energy content of foods. However, in practice, the prefix “kilo” is usually omitted.
^eConversion factors for mercury manometer pressure units are calculated using the standard value for the acceleration of gravity and the density of mercury at the stated temperature. Additional digits are not justified because the definitions of the units do not take into account the compressibility of mercury or the change in density caused by the revised practical temperature scale, ITS-90. Similar comments also apply to water manometer pressure units. Conversion factors for conventional mercury and water manometer pressure units are based on ISO 31-3.
^fThe exact conversion factor is 1.638 706 4 E–05.
^gThe darcy is a unit for expressing the permeability of porous solids, not area.
^hThe centigrade temperature scale is obsolete; the degree centigrade is only approximately equal to the degree Celsius.
ⁱThis is a unit for the quantity second moment of area, which is sometimes called the “moment of section” or “area moment of inertia” of a plane section about a specified axis.
^jThe exact conversion factor is 10⁴/π.
^kThis conversion factor is based on 1 d = 86 400 s; and 1 Julian century = 36 525 d. (See The Astronomical Almanac for the Year 1995, page K6, U.S. Government Printing Office, Washington, DC, 1994.)
^lIn 1964 the General Conference on Weights and Measures reestablished the name “liter” as a special name for the cubic decimeter. Between 1901 and 1964 the liter was slightly
larger (1.000 028 dm³); when one uses high-accuracy volume data of that time, this fact must be kept in mind.
^mThe value of this unit, 1 nautical mile = 1852 m, was adopted by the First International Extraordinary Hydrographic Conference, Monaco, 1929, under the name “International nautical mile.”
ⁿFor converting fuel economy, as used in the U.S., to fuel consumption.
^oThe exact conversion factor is 4.535 923 7 E–01. All units that contain the pound refer to the avoirdupois pound unless otherwise specified.
^pIf the local value of the acceleration of free fall is taken as g_n=9.806 65 m/s² (the standard value), the exact conversion factor is 4.448 221 615 260 5 E+00.
^qThe therm (EC) is legally defined in the Council Directive of 20 December 1979, Council of the European Communities (now the European Union, EU). The therm (U.S.) is legally defined in the Federal Register of July 27, 1968. Although the therm (EC), which is based on the International Table Btu, is frequently used by engineers in the United States, the therm (U.S.) is the legal unit used by the U.S natural gas industry.
^rDefined (not measured) value.

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