“King Henry Died Monday Drinking Chocolate Milk” is a mnemonic device for remembering the metric units, in descending order: kil-, hecto-, deka-, meter-, deci-, centi-, milli-.
 
The mnemonic “King Henry Died Monday Drinking Chocolate Milk” has been cited in print since at least 1994. There have been many kings named “Henry,” with the most famous being King Henry VIII of England (1509-1547). Chocolate milk became popular in the 1910s-1920s—long after the reign of most every “King Henry.”
   
The “M” for “meter” (or “Monday”) is sometimes replaced by “U” for “unit” or “B” for “base unit,” so that the mnemonic can be used for meters, grams and liters. “King Henry Died Unexpectedly Drinking Chocolate Milk,” “King Henry Doesn’t Usually Drink Chocolate Milk” and “King Henry Died By Doing Crystal Meth” are related mnemonics.
   
     
The Free Dictionary
Acronym Definition
KHDMDCM King Henry Died Monday Drinking Chocolate Milk (mnemonic for metric units of measure)
KHDMDCM Kilo, Hecto, Deka, Meter, Deci, Centi, Milli (metric measures)
KHDMDCM King Henry Doesn’t Mind Drinking Chocolate Milk (mnemonic for metric units of measure)
KHDMDCM Kangaroos Hop Down Mountains Drinking Chocolate Milk (mnemonic; metric measures)
KHDMDCM King Henry Died Mother Didn’t Care Much (mnemonic for metric units of measure)
KHDMDCM King Henry Danced Merrily Down Center Main (mnemonic for metric units of measure)
 
Utah Education Network
King Henry’s Story
Once upon a time in a faraway land there lived a king who loved chocolate milk. His name was King Henry.
(...)
kiloliter hectoliter dekaliter liter deciliter centiliter milliliter
(...)
King Henry Does Usually Drink Chocolate Milk
 
Wikipedia: List of rulers named Henry
There have been many monarchs with the name “Henry”. The years shown below are the regnal years.
 
Byzantine Empire
Henry of Flanders (1206-1216) (Latin Empire)
 
Scotland
Note there was no king of Scotland named Henry, although there was a king consort
Henry Stuart, Lord Darnley
 
England, Wales and Ireland
Henry I of England r. (1100 - 1135)
Henry II Plantagenet of England r. (1154 - 1189)
Henry III of England r. (1216 - 1272)
Henry IV of England r. (1399 - 1413)
Henry V of England r. (1413 - 1422)
Henry VI of England r. (1422 - 1461, 1470-1471)
Henry VII of England r. (1485 - 1509)
Henry VIII of England r. (1509 - 1547)
   
France
Henry I of France (1031 - 1060)
Henry II of France (1547 - 1559)
Henry III of France (1574 - 1589)
Henry IV of France (1589 - 1610)
Henry V of France (2-9 August 1830)
   
Google Books
Helping Your Child to Learn:
A proven system that shows parents how to help their children study and receive top grades in elementary and junior high school
By Gordon W. Green
New York, NY: Carol Pub. Group,
1994
Pg. 88:
An easy way for children to remember this progression is through the saying: King Henry Died Monday Drinking Chocolate Milk, where the first letter of each word in the sentence refers to the first letter of metric lengths from largest (kilometer) to smallest (millimeter.
 
Google Books
Classroom Learning & Teaching
By Robert L. Hohn
White Plains, NY: Longman Publishers USA
1995
Pg. 207:
King Henry Died Monday Drinking Chocolate Milk
Descending order of metric prefixes: kil-, hecta-, deka-, meter-, deci-, centi-, milli-.
 
Google Books
Teaching inclusive mathematics to special leaners, K-6
By Julie A. Sliva
London: Sage
2004
Pg. 38:
For example, a teacher may introduce the mnemonic, “King Henry Died Monday Drinking Chocolate Milk.” This mnemonic helps students remember the progression of the metric system: kilometer, hectometer, dekameter, meter, decimeter, centimeter, and millimeter.
 
The Hindu (India)
Saturday, Jul 10, 2004
Tell the tale
K.T. Rajagopalan
Memorising can be fun when you learn the trick. 
(...)
In Biology, the animal kingdom (and the plant kingdom) is divided and subdivided. The hierarchy is not easy to remember, but the sentence “Kids Prefer Cheese Over Fried Green Spinach” comes to our rescue. The initial letters represent the classification: Kingdom, Phylum, Class, Order, Family, Genus, Species. A variation is “King Philip Came Over For Good Sleep.” Talking of royalty, `King Henry Died Gently Drinking Chocolate Milk’ signifies Kilo, Hecta, Deca, Grams (or meters, litres, hertz, watts, bytes or whatever) Deci, Centi and Milli.
 
Google Books
CliffsTestPrep FTCE:
General knowledge test
By Jeffrey S. Kaplan and Sandra K. McCune
Hoboken, NJ: Wiley Pub.
2005
Pg. 290:
Use “King Henry Doesn’t Usually Drink Chocolate Milk,” which is a mnemonic for remembering the following metric prefixes:
kilo-, hecto-, deca-, unit measurement, deci-, centi-, milli-
In this problem, the unit measurement is meters.
   
Google Books
Every Good Boy Deserves Fudge:
The Book of Mnemonic Devices
By Rod L. Evans
New York, NY: Perigee
2007
Pg. 91:
King Hector’s daffy mother drinks chocolate milk.
King Henry died Monday during Christmas mass.
UNIT
Kilometer
hectometer
decameter
meter
decimeter
centimeter
millimeter
 
Telegraph (London)
Mnemony clever ways to remember stuff
12:01AM GMT 12 Dec 2007
(...)
The Metric System
To remember the order of kilometre (1,000 metres), hectometre (100 metres), decametre (10 metres), metre, decimetre (1/10 of a metre), centimetre (1/100 of a metre) and millimetre (1/1000 of a metre), say to yourself: King Henry Died Mightily Drinking Chocolate Milk, or Kippers Hardly Dare Move During Cold Months.