facebook pixel

The Metric International System of Units

The International system of units is the up to date form of the metric system decided by international agreement. This metric system is the measurement of units defining 7 base units, and is the measurement system mostly used worldwide today in everyday commerce, science and industry. The abbreviation SI which is derived from the original French name le Système international d'unités is the more commonly know term

M = meter - length s = second - time A = ampere - electric current K = kelvin - thermodynamic temperature mol = mole - amount of substance cd = candela - luminous intensity kg = kilogram

The metric system was originally developed by a group of scientists who had been employed by Louis XVI of France to develop a more rational system of measurement. The new government then continued with the system after the French Revolution. The metric system was first promoted to the US by Thomas Jefferson and was legally passed in 1866, however today the US is still one of the 3 countries who have not totally converted to using SI.

The process over the years to reach a conclusion has been unsteady, and the US still uses a mix of customary units and metric units with metric increasingly being used in science, commerce, industry and schools. The UK reluctantly officially adopted SI but still refuses to replace old systems altogether. The argument is that the metric system is much simpler to use and to understand as all the units are devised by 10

The SI was conceived in 1960 from the older metric system and there has been a few variants since, as the units are defined through an international agreement between 54 nations. Annual meetings are held at the BIPM the International Bureau Metrology Center in France by the International Committee for Weights and Measures.

Conversion Factors

The conversion between units used in different systems is found from the basic definition of the units or by convention. The conversion of units to another is better achieved by using a conversion tool. Josh Madison created and wrote a freeware program that is a virus free conversion tool you can download safely 'convert.exe'.

This example below gives you an example of the relationship between SI and the US customary units. The correct way is to multiply the value on the left (US) by the conversion factor and then round off to the number according to the digits you want. As an example - feet to inches:
1 foot = 0.3048 m
#6 ft = 3.23088 m and then rounded off to 3.23
Never round off the figure value before doing the conversion as this will interfere with the accuracy.
You can find a complete guide to the SI system in ASTM E 380 metric practice.

Helpful Metric Equivalent Conversions and Prefix's of the Measurement of Units

Contract plans:
Degrees and bearings - no change
1 station - 1,000 meters
Mileposts - same for now as they are still under review, you can also state kilometerposts.
Elevations - meters (m)
Commercial industry standards

  • Gals/hr or min (pumps) - liters per second (L/s
  • Gallon tanks 500 - cubic meter(m3/liter (L)
  • Gallon drums 55 - cubic meter (m3) or liter (L) (liquid)
  • Sack of cement in pounds - kilogram (kg)
  • Reinforced steel (diameter) -millimeters (mm)
  • Reinforced steel LBS/FT - kilogram per meter (kg/m)
  • Bushel - cubic meter (m3)

General interim:

  • Acre - (ha) hectare
  • Cubic feet and cubic yard - (m3) cubic meter
  • Gallon and mgallon - (L) liter, (m3) cubic meter
  • Hundred weight - (kg) kilogram
  • Linear foot - (m) meter
  • Mboard feet - (m3) cubic meter
  • Mile - (km) kilometer
  • Nautical mile - (same) nautical mile
  • Pound - (kg) kilogram
  • Square feet/square yard - (m2) square meter
  • Ton - (t) tonne

Special provisions:

  • Gauge metal thickness - millimeters (mm)
  • Fahrenheit - kelvin (k) kelvin or © degree Celsius
  • Water depth in Fathoms - (m) meters
  • Feet/pounds (torque) - (N-m) newton/meter
  • Pounds/square inches (pressures) - (kPA) kilopascal (Mpa) megapascal (used for very large numbers)
  • Inch linear - (mm) millimeter
  • Kips/ksi tension - (kPa) kilopascal or (Mpa) megapascal
  • Pounds/acre (erosion) - (kg)/(ha)kilogram/hectare
  • Pounds/cubic feet (density) - (kg/m3) kilogram per cubic meter
  • Mil (thickness) - (um)micrometer
  • Foot/pounds/seconds horsepower - (W) watt

Pressures and force:

  • Kip per square inch - megapascal (Mpa)
  • Pound per square foot - kilogram per square meter (kg/m3)
  • Pound per square foot - pascal (Pa)
  • Inch per square foot - pascal (Pa)
  • Kip (1000lb) - kilogram (kg)
  • Kip (1000lb) - Newton (N)
  • Pound - kilogram (kg)
  • Pound - Newton (N)