Measurement, units of measurement, and conversions. METRIC RULES!

## Unit Prefixes

The following table lists the prefixes for decimal powers.

Number Multiple Prefix Symbol Cardinal
US/International
Note
1 000 000 000 000 000 000 000 000 1e+24 yotta Y septillion/quadrillion yotta = Greek for 8 = 1000^8
1 000 000 000 000 000 000 000 1e+21 zetta Z sextillion/-- zetta = sept = French for 7 = 1000^7
1 000 000 000 000 000 000 1e+18 exa E quintillion/trillion exa = Greek for 6 = 1000^6
1 000 000 000 000 000 1e+15 peta P quadrillion/--
1 000 000 000 000 1e+12 tera T trillion/billion
1 000 000 000 1e+09 giga G billion/milliard
1 000 000 1e+06 mega M million
1 000 1e+03 kilo k ⇒ K thousand
100 1e+02 hecto h ⇒ H hundred
10 1e+01 deka da ⇒ D ten
1 1e±00
0.1 1e-01 deci d
0.01 1e-02 centi c
0.000 1 1e-03 milli m
0.000 000 1 1e-06 micro μ
0.000 000 000 1 1e-09 nano n
0.000 000 000 000 1 1e-12 pico p
0.000 000 000 000 000 1 1e-15 femto f
0.000 000 000 000 000 000 1 1e-18 atto a   atto = atten = Greek for 18
0.000 000 000 000 000 000 000 1 1e-21 zepto z   zetta = sept = French for 7 = 1/1000^7
0.000 000 000 000 000 000 000 000 1 1e-24 yocto y   yocto = Greek for 8 = 1/1000^8

Hecto (102), deka (10-1), deci (10-1), and centi (10-2) are the only prefixes that are not powers of 103 or 10-3. They are uncommon except for c (centi) which is only commonly used with meter, as in 1 cm = 10-2 m.

The SI symbols should be consistent with case but the official standard does not do this. Positive powers should be uppercase and negative powers lowercase. "k" should be "K", "h" should be "H", and "da" is outright clumsy and should be "D". This practice is officially done with Z & z, as well as Y and y. Since this is my website, I will, as of this day [2004-08-16], make it the law on my site!

The symbol for micro (μ) is somewhat clumsy since it isn't an ASCII character but at least it's an ANSI character (181, xB5) and at least it's a lowercase mu (μ, 956, x3BC) instead of an uppercase Mu (Μ, 924, x39C).

## Miscellany

• I have a number of personal measurements (clothing sizes, etc.) in About Me.
• The metric system of units is the same as SI (System Internationale). SI is also known as MKS (meter-kilogram-second) to contrast it with CGS (centimeter-gram-second) which is very similar to SI but it has different derived units. EG: In CGS, the unit of force is dyne = 1 g*m/s2, but in MKS, the unit of force is Newton = N = 1 Kg*m/s2, hence 1 dyne = 10-5 N. CGS is still used in fields such as electromagnetism and astronomy largely out of convenience.
• The UK System (aka the IS or Imperial System) used to be defined by standards held in London. Canada used to use the IS. The IS differs from the USS only by their units of measurement for volume. IS is now defined by 3 exact measurements.
• 1 yard ::= 0.9144 m
• 1 pound ::= 0.45359237 Kg
• 1 gallon ::= 4 quarts ::= 8 pints ::= 16 cups ::= 32 gills ::= 4.54609 L
• The US System (USS) differs from the IS only by their units of measurement for volume. The USS is defined by 4 exact measurements.
• 1 yard ::= 0.9144 m
• 1 pound ::= 0.45359237 Kg
• 1 gallon (gal, liquid) ::= 4 quarts (qt, liquid) ::= 8 pints (pt, liquid) ::= 16 cups (liquid) ::= 32 gills ::= 3.785411784 L
• 1 bushel (bu, dry) ::= 4 peck (pk) ::= 8 gallons (dry) ::= 32 quarts (dry) ::= 64 cups (dry) ::= 2150.42 in^3 ::= 35.23907016688 L
• The English Units [W] system is older than IS and has a many fun units.
• The Chinese System is closer to metric system than the UK or US system.
• 1 Km = 2 li = 0.621 miles
• 1 m = 3 chi = 3.281 feet
• 1 nautical mile = 1.852 Km = 3.704 li = 1.150 miles
• 1 square Km = 100 hectares = 4 square li = 0.386 square miles
• 1 square m = 9 square chi = 10.764 square feet
• 1 qing = 100 mu = 6.6667 hectares
• 1 mu = 60 square zhang = 6.6667 acres
• 1 Kg = 2 jin = 2.205 pounds
• 1 L = 1 sheng = 0.220 gallons
• An erlang is a unit of measurement of resource usage in queueing theory. EG: How many registers must be available to serve customers at peak and off-peak hours?
• Density = mass/volume. EG: Kg/m^3 or lb/ft^3. Specific gravity = specific density = The ratio of the density of the item to the density of water at specific conditions. Density and specific gravity are conveniently related in the metric system because water's density at specific conditions is 1000 Kg/m^3 = 1 g/cm^3. In contrast water's specific density is also 62.428 lb/ft^3, which is not very convenient. So if you know that the density of African padauk is 737 Kg/m^3, then its specific gravity is 0.737. For comparison, here are the specific gravities of certain substances:
• 00.0012 = Air. Dry air at 20 C at sea level.
• 00.10 = Balsa
• 00.40 = Pine
• 01.00 = Water
• 01.27 = Kamagong
• 02.70 = Aluminum
• 07.86 = Iron
• 19.10 = Uranium.
• 19.30 = Gold.
• 22.65 = Iridium
• There are many SI units related to photometric light. Here are a few of them
• Luminous efficacy. lm/W.
• Luminous emittance. lux = lx = lm/m^2
• Luminous energy. lm*s.
• Luminous flux. lumen = lm = cd*sr.
• Lumnious intensity. candela = cd = lm/sr. The SI base unit. A common candle emits roughly 1 cd. A 100 W bulb emits roughly 120 cd.
• Luminance. nit = cd/m^2. The nit is a deprecated name but it's convenient.
• A typical computer screen emits 50-300 cd/m^2.
• An HDTV screen emiths 450-1000 cd/m2.
• Some Panasonic Toughbook computers can emit up to 1000 cd/m2.
• For viewing in sunlight, a screen should emit at least 800 cd/m^2. eBook readers can be read in sunlight because they rely on reflecting light rather than emitting it.
• The sun has a luminance of roughly 1.6e9 cd/m^2 at noon.
• Dirac = One word per hour; The fewest number of words that someone with the power of speech could utter in company. In honor of Paul Dirac.

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