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Bill's Conversion Spreadsheet
Excel Spreadsheet to convert between units of:
Length, Area, Volume, Dry Volume, Mass/Weight, Power, Energy, Force, Temperature, Velocity/Speed, Acceleration, Time, and Data.

Most conversions are exact.  Conversion factors that are not exact are listed to around 15 significant digits in most cases.  The formulas are written in such a way as to make the conversion method and assumptions easily understood.  The conversion formulas may be copied from the spreadsheet for use elsewhere.  Excel has some conversion factors built in, but these are NOT used, so as to make the formula's generic, with known assumptions.

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Units of Measurement

Length

Area

Volume
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

All calculations in and out of the English Fluid section are based on exactly 231 cubic inches per gallon.  This includes teaspoon, tablespoon, fluid ounce, cup, pint, quart, and gallon.

Dry Volume
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

Mass
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

av = avoirdupois

Power
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

Energy
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

Force
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

Temperature
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

Velocity
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

Acceleration
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

Time
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

Data
Formulas for converting between each of these units can be found in Bill's Conversion Spreadsheet.

Binary vs. Decimal.  The same metric prefixes are used for data quantities as for other units, with slightly different meanings as can be seen above.  This often leads to a ton of confusion.  The decimal units are actually "official", but the binary units are used by convention.  The International System of Units (SI) downright frowns on the usage of the prefixes for binary units.  It suggested alternatives, which are borderline silly.

Memory chips are generally built in quantities of powers of 2 because that is the most efficient way for computers to address them.  When you buy a 256MB memory DIMM, the actual quantity of data is 256*2²⁰, or 256 x 1024 x 1024 bytes.  When a new technology of memory arrives, it is generally 2 or 4 times the size of the previous generation.  In-between sizes are not convenient, and not supported by computers.

Disk Drives:  There is no particular reason to build disk drives in powers of 2 quantities of data.  An individual sector on the disk is generally a power of 2, such as 1024 bytes, but the number of sectors on the disk has no particular need for any alignment.  In fact, on modern high density disks, the number of sectors per track varies, with more being on the outer tracks than the inner tracks.

Disk drives, by convention, use base 10 when specifying the data size.  Undoubtedly this convention was initiated by the marketing department.  An 80GB drive is 80,000,000,000 bytes (approximately) and not 85,899,345,920 bytes (80*1024*1024*1024).  The manufacturer would call the latter an 85G drive.  85G looks better on the box than 80G.

Microsoft Windows (including XP) uses the binary sizes when specifying file sizes and disk sizes.  I.E. 1KB=1024 bytes, 1MB = 1024*1024 bytes.  When displaying the properties of a disk drive, both the decimal and binary numbers are displayed, with the decimal being fully listed without prefixes.  For example a drive advertised as 60GB might be listed as:
    Capacity:  60,003,868,672 bytes    55.8 GB.
The decimal number is the exact number of bytes.  It is slightly over 60 billion since they rarely are built to an exact decimal value.  The 55.8GB would be 55.8 * 2³⁰.  It is rounded to 3 significant digits.

This is why your 60G drive only shows up as 55.8GB.

Flash Memory is actually semiconductor memory, but often used like a disk drive.  It is commonly used by digital cameras, MP3 players, and USB "Flash Disks".  By convention, these use decimal sizes.  Compact Flash, Smart Media, Memory Stick, and Secure Digital are examples of flash memory cards.  Six more are probably being dreamed up as I type this.

Prefixes

Symbols from yotta (Y) through mega (M) should be capitalized.  From Kilo (k) to yocto (y) are lower case.  Note that several (m, p, z, and y) have different meaning when upper vs lower case.
The symbol for micro is the greek letter "µ" (pronounced Mew).  Often a lower case "u" in used instead for simplicity.

Information for this table taken from the Guide for the use of the International System of Units from the National Institute of Standards and Technology.

See reference below for powers of ten far in excess of this list.

Unit systems

International System of Units (SI)

SI is the modern metric system, used throughout the world.  It is the official unit system for the United States, and required for use by federal agencies.  Details of this system can be found in the National Institute of Standards and Technology web site.  Reference below take you directly to the document.  The abbreviation is SI.

The system defines 7 base units, and additional derived units.  The base unites are:

MKS

The MKS system is what is used by SI.  It stands for meter, kilogram, second, as base units

CGS

Stands for centimeter, gram, second.  Generally used where small quantities are measured.

English System

The system in common use in the United States, but pretty much nowhere else.  Some of the units include:

avoirdupois

A system of weights used in the English system.  The common usage of ounce and pound uses avoirdupois measurements.

Troy

A system of weights used in the English system, generally for precious metals.

Roman Numerals

I, X, C, and M are used in multiples, up to 3. 
A lower denomination in front of a higher denomination subtracts the value.  I.E. IX subtracts 1 from 10.
An overbar multiplies by 1000.  Two overbars multiplies by 1,000,000

Notes

US Survey units.

In 1959 the inch was set to be exactly 2.54 centimeters.  Prior to that, it was set at 39.37 inches per meter. The difference is only 0.0002%, or 2 parts per million, with the new definition being slightly smaller.  The "US Survey" numbers are still officially used where measurements are derived by "geodetic surveys" within the United States.  Presumably this was done to avoid having the definition of land owned and traded change with the unit change, even though the difference amounts to 111 square feet per square mile, or 25 square inches per acre.

The International inch, foot, mile, etc. are used unless "US Survey" is specifically listed.

The Acre is only used as a unit of land, and is only defined in terms of "US Survey" measurements, being 43560 square US Survey feet, or 640 acres per US Survey Square mile.

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