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OSHA Technical Manual (OTM)
Section X: Chapter 1

Effective Date: 1/20/1999 Directive Number: TED 0100015 [TED 10.15A] 
Metric System Conversion
 Introduction
 The Metric System
 Conversion
 References
 INTRODUCTION
 Use of metric measurement standards in the United States has been authorized by law since 1866. In 1988, Congress enacted legislation (Public Law 00418) to establish the metric system as the preferred system of weights and measures for all domestic trade and commerce. This legislation also required the use of metric measurement standards in all Federal activities. On July 25, 1991, the President issued Executive Order 12770, which reiterated the order to implement the metric system "as the preferred system of weights and measures for United States trade and commerce." This executive order directed all Federal agencies to implement "metrification" to the extent economically feasible by September 30, 1992.
 In 1988, when Congress enacted metrification legislation, the U.S. was the only major industrial nation not using the metric system. As a result, domestic industries were often at a competitive disadvantage and were sometimes even excluded from the international marketplace. In its deliberations, Congress also found that the inherent simplicity of the metric system could lead to major cost savings, further enhance our international competitive position, and provide substantial advantage to the Federal Government in its own operations. Commercially, metric measure is sometimes used as a household measure in the U.S., e.g. twoliter soft drinks. Most food labels list both metric and English measure. Metric measure labeling has opened markets for business products in areas which use metric measure daily.
 OSHAs safety compliance operations and industrial hygiene efforts have an advantage in metrification because the biological, chemical, and physical sciences have long used the metric system. Students of these subjects have been using metric weights and measures along with daily use of the "English" system of measures for decades. Timeweighted averages (TWA), permissible exposure limits (PEL), and sampling and reporting forms all make use of the metric system. For example, the industrial hygiene Air Sampling Report, OSHA91(S) (Rev. 1/84) requires data as metric measures and weights. Also examine OSHA reference Table Z1A
(29 CFR 1910 Subpart Z).
 THE METRIC SYSTEM
 Base Units
 Originally, there were only two basic reference points, the meter for length and the gram for weight. The list of reference points has been expanded for the International System (SI) to include the kilogram (instead of the gram) for mass, the second for time, the ampere for electric current, the degree Kelvin for temperature, and the candela for light intensity (see Table X:11).
 The reference objects for these International standards units are maintained for comparisons and checked periodically against other international references by the National Institute for Standards and Technology (NIST). The base units used in the metric system are given in Table X:11 below.
TABLE X:11. METRIC BASE UNITS
Unit
Measurement 
Unit
Name 
Symbol 



Length 
meter 
m 
Mass 
kilogram 
kg 
Time 
second 
s 
Electric current 
ampere 
A 
Thermodynamic
temperature 
Kelvin 
K 
Amount of substance 
mole 
mol 
Luminous intensity 
candela 
cd 

 Powers of Ten
 The metric system entails the use of multiples or powers of ten to describe magnitudes greater or less than the basic units of meter, gram, ampere, and so forth. The prefixes for the multiples and submultiples of basic units are presented in Table X:12. For example, mass or weight can be presented as:
 the kilogram is 1000 grams; and
 the milligram is 1/1000 gram.
In addition, there are:
 100 centimeters to one meter; and
 1000 meters to one kilometer.
Also, volume can be presented as:
 10 deciliters in one liter;
 10 liters in one decaliter; and
 10 decaliters in one hectoliter.
TABLE X:12. PREFIXES AND SYMBOLS FOR DECIMAL MULTIPLES AND SUBMULTIPLES OF UNITS^{1}
Power of 10 
Prefix 
Symbol 



10^{12} 
tera 
T 
10^{9} 
giga 
G 
10^{6} 
mega 
M 
10^{3} 
kilo 
k 
10^{2} 
hecto 
h 
10^{1} 
deca* 
da 
10^{1} 
deci 
d 
10^{2} 
centi 
c 
10^{3} 
milli 
m 
10^{6} 
micro 
m 
10^{9} 
nano 
n 
10^{12} 
pico 
p 
10^{15} 
femto 
f 
10^{18} 
atto 
a 

*Also "deka."
^{1} Conférence générale des Poid et Mesures, Comtes rendus de séances de la 11e Conférence générale des Poids et Mesures, Paris 1960, GauthierVillars, Paris 1961, page 87; Conférence générale des Poids et Mesures, Comtes rendus des séances de la 12e Conférence générale des Poids et Mesures, Paris 1964, GauthierVillars, Paris, 1964, page 94. 

Area and volume in both inchpound and the metric system are calculated similarly. Area is length times width with the measure of each side in the same units for both length and width for square feet (feet^{2}) or square meters (m^{2}). Volume is likewise length times width times height: cubic yards (yard^{3}) or cubic meters (m^{3}). The metric measure of a liter is defined as a volume of 1000 cubic centimeters (cc) or a volume of 10 cm by 10 cm by 10 cm.
 In physics, some measurements are defined by several metric units. These derived units are named, generally, for the scientists who first defined the measurement. The metric expression as a combination of metric units is presented in Table X:13 below.
TABLE X:13. DERIVED SI UNITS WITH SPECIAL NAMES

Unit
name 
Symbol 
Formula 
Expressions as
metric base units 

Frequency 
hertz 
Hz 
1/s or s^{1} 
s^{1} 
Force 
newton 
N 
m • kg • s^{2} 
m • kg • s^{2} 
Energy, Work 
joule 
J 
N • m 
m^{2} • kg • s^{2} 
Power 
watt 
W 
J/s 
m^{2} • kg • s^{3} 
Electric charge 
coulomb 
C 
A • s 
A • s 
Electric potential 
volt 
V 
W/A 
m^{2} • kg • s^{3} • A^{1} 
Electric resistance 
ohm 
W 
V/A 
m^{2} • kg • s^{3} • A^{2} 
Electric capacitance 
farad 
F 
C/V 
m^{2} • kg^{1} • s^{4} • A^{2} 
Magnetic flux 
weber 
Wb 
V • s 
m^{2} • kg • s^{2} • A^{1} 
Pressure, Stress 
pascal 
Pa 
N/m^{2} 
m^{1} • kg • s^{2} 
Conductance 
siemens 
S 
A/V 
m^{2} • kg^{1} • s^{3} • A^{2} 
Magnetic flux density 
tesla 
T 
Wb/m^{2} 
kg • s^{2} • A^{1} 
Inductance 
henry 
H 
Wb/A 
m^{2} • kg • s^{2} • A^{2} 
Luminous flux 
lumen 
lm 
cd • sr 
cd • sr 
Illuminance 
lux 
lx 
lm/m^{2} 
m^{2} • cd • sr 

 CONVERSION
 The Conversion Process
 The conversion equivalent for an inch is 1 inch equals 2.54 centimeters or 25.4 millimeters. The measure and units to be converted are multiplied by the conversion factor to yield the new measure and units. Do include the units in the conversion calculation and cancel the units as part of the calculation process (Table X:14).
 Each conversion factor has the metric units per inchpound units and should be included in the calculation converting inchpound units to metric units until the use of the conversion factor is ingrained in the user. During a calculation, like units divided by like units equal one and cancel themselves, as shown in Table X:14 below.
 Because of the number of conversion factors presented in Federal Standard 376A (May 5, 1983), include with the numeric conversion factor both metric and inchpound unit labels during the calculations. Cancel like units during the calculation. Then only the new measurement unit labels will remain (see Table X:14).
TABLE X:14. AN EXAMPLE OF AN INCHPOUND TO SI METRIC CONVERSION
Original inchpound
measure 
Conversion 
Calculated
metric measure 

1 inch 
2.54 cm/inch
1 inch • 2.54 cm/inch
1 inch • 2.54 cm/inch 



2.54 cm
(Or 25.4 mm) 

 Conversion Precision
 Some conversion factors have been developed to seven significant digits, e.g. Federal Standard 376A includes the conversion factor for the foot (survey) as used in the United States as:
1 ft (survey) = 0.3048006 m
 When using a conversion factor, round resulting calculations to the number of significant digits of the original inchpound measurement or to fewer significant decimal positions.
 Precision is not increased by converting from inchpound to metric by a multipledigit conversion factor.
 Calculations or conversion of a measurement from inchpound to metric measure requires the application of three rules of calculation, often done by habit.
 Significant number of digits. A distance shown as 61 miles has two significant digits, implying measurement accuracy to a whole number of miles. A distance written as 61.6 miles is three significant digits, two plus one decimal digit and implies accuracy to onetenth of a mile. The number of digits presented usually reflects the accuracy of the actual measurement. The figures in 61 miles or 61.2 miles may represent an automobiles odometer measurement. A surveyors measurement may be represented as 61.243 miles from measurement stake point to stake point: five significant digits including three decimals.
 Number of carried digits.
 If the conversion number will result in a larger number, 1.609 km/mi, carry only the total number of first number digits, e.g.:
 61 mi × 1.609 km/mi calculates to 98.149 km but is presented as 98 km.
 61.2 mi × 1.609 km/mi calculates to 98.4708 km but is presented as 98.5 km.
 61.243 mi × 1.609 km/mi calculates to 98.539987 km but is presented as 98.540 km.
 If the conversion number will result in a smaller number, carry only the total number of first number digits plus one extra digit, e.g.:
 4 lb × 0.453 lb/kg calculates to 1.812 kg but is presented as 1.8 kg (for measurement by a household scale).
 4.3 lb × 0.453 lb/kg calculates to 1.9479 kg but is rounded up and is presented as 1.95 kg (for measurement by a postal scale).
 4.33 lb × 0.453 lb/kg calculates to 1.96149 kg is presented with no rounding as 1.961 kg (for measurement by a laboratory scale).
 "Rounding" of numbers. To "round off" calculated numbers to a minimum number of significant digits, follow these rules:
 When a number such as 13.34 is to be rounded to 13.3+, the digit 4 is less than 5 so the number is rounded to
an unchanged 13.3.
 When a number as 13.37 is to be rounded to 13.3+, the digit 7 is equal to or greater than 5 so the number is
rounded or increased to a changed 13.4.
 Conversion Equivalents
 The document to be used as a primary reference for inchpound to metric conversions is Federal Standard 376A, Preferred Metric Units for General Use by the Federal Government, May 5, 1983. A selected number of conversion factors are presented in Tables X:15 through X:115 below.
 The basic units of the measurement system are by definition to manmade reference standards for mass or weight (the kilogram) and length (the meter). Some units of measure are mathematically derived, as plane and solid angle measures (from geometry), or derived from astronomical phenomena, time, or are not related to either mass or length measurement references. The units for these measures have not changed. A measure of frequency, cycles per second (cps), has been renamed Hertz to honor the German physicist H. R. Hertz.
 Measures of light were defined in physics originally in terms of the metric system and have not changed. If the measure of light was converted to the inchpound system, and includes inchpound area measures as part of its definition, a conversion to metric equivalents is necessary. These exceptions to light measurements are presented in the appropriate table.
 Metrification or establishment of the measure system as a base of ten is extended to temperature measures.
 G.D. Fahrenheit, the German physicist, defined a scale of measure for his alcohol thermometer in 1709 and used the same scale again in 1714 for his mercury thermometer. On this scale, water freezes at 32° and boils at 212°. The base of ten temperature scale definition of Anders Celsius, the Swedish astronomer, is used in the metric system for temperature measures. The point at which water freezes or becomes a solid is 0° Celsius and the point of transition of water from a liquid to a gas is at 100° Celsius.
[These definitions assume that the atmospheric pressure is at a standard of 760 mm of mercury pressure.]
 The relationship between the arbitrary Fahrenheit scale and the Celsius scale used within the metric system is:
degreesFahrenheit32
9 
x 5 = 
degrees Celsius 

TABLE X:15. Length
1.609347 kilometers (km)
1 mile (mi) [statute]

1.609344 kilometers (km)
1 mile (mi) [international]

1.828 meters (m)
1 fathom (fm)

0.9144 meter (m)
1 yard (yd)

0.3048006 meter (m)
1 foot (ft) [survey]

0.3048 meter (m)
1 foot (ft) [international]

2.54 centimeters (cm)
1 inch (in)

2.54 millimeters (mm)
1 inch (in)

25.4 µm
1 mil

1 µm
1 micron

0.0254 µm
1 µin

0.1 nm
1 angstrom (Å)




The equation must be applied for the conversion. For every ninedegree interval on the Fahrenheit temperature scale (°F), the Celsius temperature scale (°C) changes only five units. Please note that at the freezing point for water, the Celsius scale is at zero but the Fahrenheit scale is at 32°. It is convenient to remember that only at 40° are both scales equivalent, or :
40°C = 40°F

TABLE X:16. Area
2.589998 square kilometers (km^{2})
1 square mile (m^{2})

0.4046873 hectare (ha)
1 acre

4046.873 square meters (m^{2})
1 acre

0.8361274 square meters (m^{2})
1 square yard (yd^{2})

0.09290304 square meters (m^{2})
1 square foot (ft^{2})

6.4516 square centimeters (cm^{2})
1 square inch (in^{2})

645.16 square millimeters (cm^{2})
1 square inch (in^{2})



TABLE X:17. Velocity
0.3048 meter/second (m/s)
1 foot/second (ft/s)

1.609347 kilometers/hour (km/h)
1 mile/hour (mi/h)



TABLE X:18. Volume
1233.489 cubic meters (m^{3})
1 acre foot (acre ft)

0.7645549 cubic meter (m^{3})
1 cubic yard (yd^{3})

0.1589873 cubic meter (m^{3})
1 barrel (bbl) [42 gallons]

0.02831685 cubic meter (m^{3})
1 cubic foot (ft^{3})

28.31685 liters (l)
1 cubic foot (ft^{3})

0.002359737 cubic meter (m^{3})
1 board foot (bd ft)

35.23907 liters (l)
1 bushel (bu)

3.785412 liters (l)
1 gallon (gal) [liquid]

0.9463529 liter (l)
1 quart (qt) [liquid]

0.4731765 liter (l)
1 pint (pt) [liquid]

29.57353 milliliters (ml or cc)
1 fluid ounce (fl oz)

16.38706 cm^{3}
1 in^{3}



TABLE X:19. Mass Weight
1.016047 metric tons (t)
1 long ton
[where 1 t = 1000 kg]

0.90718474 metric ton (t)
1 short ton
[where 1 short ton = 2000 pounds]

0.45359237 kilogram (kg)
1 pound (lb avdp)

31.10348 grams (g)
1 ounce (oz troy)

28.34952 grams (g)
1 ounce (oz avdp)

64.79891 milligrams (mg)
1 grain (gr)



TABLE X:110. Density
1.186553 metric ton/m^{3}
1 short ton/yd^{3}

16.01846 kg/m^{3}
1 lb/ft^{3}

119.8264 g/l
1 lb/gal

7.489152 g/l
1 oz/gal



TABLE X:111. Pressure
101.325 kPa
1 atmosphere (atm) [standard]

100 kPa
1 bar

6.894757 kPa
1 lbf/in^{2} [formerly psi]

3.38638 kPa
1 inch Hg [at 32^{º}F]

2.98898 kPa
1 foot H_{2}O [at 39.2^{º}F]

0.24884 kPa
1 inch H_{2}O [at 60^{º}F]

0.133322 kPa
1 mm Hg [at 0^{º}C]

0.1 kPa
1 millibar



TABLE X:112. Stress
6.894757 MPa
1 kip/in^{2} (formerly ksi)

0.006894757 MPa
1 lbf/in^{2} (formerly psi)

6.894757 kPa
1 lbf/in^{2} (formerly psi)

0.04788026 kPa
1 lbf/ft^{2}



TABLE X:113. Work
3.6 MJ
1 kWh

4.184 J
1 cal th

1.055056 kJ
1 Btu

1.355818 J
1 ft • lbf

105.4808 MJ
1 therm (U.S.)



TABLE X:115. Miscellaneous
Heat flow rate

1.055056 kW
1 Btu/s

Heat capacity

1.899108 kJ/^{º}C
1 Btu/^{º}F

Magnetic field
strength

79.57747 A/m
1 oersted (Oe)

Conductivity

100 S/m
1 mho/cm

Wavelength

0.1 nm
1 angstrom (Å)

Luminance

0.3183099 cd/cm^{2}
1 lambert (L)


0.1550003 cd/cm^{2}
1 cd/in^{2}


3.426259 cd/cm^{2}
1 ftlambert (ftL)


10.76391 1x
1 footcandle (fc)



TABLE X:114. Power
3.516800 kW
1 ton (refrigeration)

1.055056 kW
1 Btu/s

0.7456999 kW
1 hp (550 lbf/s)

0.746 kW
1 hp (electric)

0.2930711 W
1 Btu/h



 REFERENCES
General Services Administration. Federal Standard 376A, Preferred Metric Units for General Use by the Federal Government, May 5, 1983.
