If you can divide and multiply 2 digit numbers by 2, 3, and 10,
you can convert metric numbers! We're going to metrics,
eventually, whether you like it or not! Get with it! Learn with your
children! No business person or world traveler can afford not to know!

(go to this page for free conversions (metrics and more)):
http://www.convert-me.com/

Ditty(s) (EASY formulas follow):

Miles are kilometers* divided by 3 then times 2.
Water we know freezes at 32,
So Fahrenheit is 30 plus Celsius times 2*,
Centimeters are inches two and a half times a day.
Kilos are more than twice what pounds weigh.
But meters are feet, it's easy to see,
Divided by what is almost three,
Square klicks* are miles times almost three.
Gallons at liters are very sore,
Cause liters are gallons times almost four.
Square meters and feet call each other a sin;
Cause square feet are meters times over 10.
But quarts and liters are such very good friends,
They only differ by half of 10 ... percent that is.
And 10 percent is how small the stuff,
There is between good enough and rough.
So if financially faced with yards or meters
Earn 10 percent more with metric measures.
You'll never forget how good it is when
All your conversions are with the power of 10.

(may want to substitute klicks for kilometers)
(Children's/soldier's ending or begining at end of page)
*Fahrenheit and Centigrade ºF = approx. 30 + (ºC x 2)

Riddles:
How can I make my feet smaller? Convert them to meters!
How can I be greater than miles ahead? Convert them to kilometers!
How can I get more gas for my money? Convert my gallons to liters!
How can you make the area you have to clean smaller? Get someone who doesn't know metric to do it.

Here are the easy formulas (Children may like term "magic formulas"):
(all typically have 0.1% to 1.6% error when optional
"[...]" step is completed) Always less than 10% error.)

mi to km multiply by 3 and then divide by 2.
km to mi divide by 2 and then multiply by 3

ºF to ºC subtract 30 and divide by 2, [add 10% of quotient]
ºC to ºF multiply by 2, add 30, [subtract 10% of ºC]
(>10% error near zero or below - see examples below)

in to cm multiply by 2, add ½ of the inches
cm to in divide by 2, subtract 10% of centimeters

ft to m multiply by 3, divide by 10.
m to ft multiply by 3, [add 10%]

sq. ft to sq. m divide by 10, [subtract 10% of new number]
sq. m to sq. ft multiply by 10, [add 10% of new number]

sq. mi to sq. km multiply by 3, [subtract 10% of new number]
sq. km to sq. mi divide by 3, [add 10% of new number]

gal to liter multiply by 4 (or by 2 twice), [subtract 10% of new number]
lt. to gal divide by 4 (or by 2 twice).

qt and liter are equal. [subtract ½ of 10%]
lt. and qt are equal. [add ½ of 10%]

pt to ml multiply by 1000, divide by 2, (subtract ½ of 10% of new#)
ml to pt multiply by 2, divide by 1000, (add ½ of 10% of new#)

Examples:
(Steps in [... ] are optional for better accuracy)

Inches to Centimeters (cm) Multiply by 2, add ½ of inches

2 in = 5.08 cm (2 x 2.54)

1. 2 x 2 = 4
2. 2 / 2 = 1
3. 4 + 1 = 5 (1.6% error)

Cm to inches Divide by 2, subtract 10% of cm

60 cm = 23.62 in (60 / .3048)

1. 60 / 2 = 30
2. 10% of 60 = 6
3. 30 - 6 = 24 (1.6% error)

Feet to Meters Multiply by 3, divide by ten

4 ft = 1.219 m (4 x 0.3048)

1. 4 x 3 = 12
2. 12 / 10 = 1.2 (1.6% error)

Meters to Feet Multiply by 3, add 10 % of new number

2 m. = 6.56 ft. (2 x 3.28)

1. 2 x 3 = 6 (8.5% error)

2. 10% of 6 = .6
3. 6 + .6 = 6.6 (1.6% error)

Miles to kilometers Multiply by 3, divide by 2, [add 10% of miles]

2 mi. = 3.33 km (2 x 1.67)

1. 2 x 3 = 6
2. 6 / 2 = 3 (9% error)

3. 3 + 10% of 3 = 3.3km (1% error)

or add ½ of miles to itself, [add 10% of miles]
1. 2 + 1 = 3 (9% error)

2. 10% of 3 = .3
3. 3 + .3 = 3.3 (1% error)

5 mi. = 8.333... km (5 x 1.666...)
1. 5 + 2.5 = 7.5 = 7½
2. 10% of 7.5 = .75 = 3/4 = ½ + ¼
3. 7½ + ½ + ¼ = 8 ¼ (1% error)

Kilometers to Miles Divide by 2, add 10% of km.

2 km = 1.2 mi. (2 x 0.6)

or divide by 2, [add 10% of kilometers]
1. 2 / 2 = 1 (20% error)
2. 10% of 2 = .2
3. 1 + .2 = 1.2 mi. (very small error)

To convert Fahrenheit (ºF) to Celsius (Centigrade) (ºC)

Subtract 30, divide result by 2, [add 10%, subtract 1]

90ºF = 32.22ºC ((90 - 32) x 5 / 9)

1. 90 - 30 = 60
2. 60 / 2 = 30ºC (7.3% error)

3. 30 + 10% of 30 (3) = 33ºC (2.5% error)

4. 33 - 1 = 32ºC (0.7% error)

Below 10ºF or for more precise temperatures:
Subtract 32, divide by 2, add 10% of remainder

-10ºF = -23.33ºC ((-10 - 32) x 5 / 9)

1. -10ºF - 32 = -42
2. - 42 / 2 = -21 (10% error)
3. 10% of -21 = -2.1
4. -21 + (-2.1) = -23.1 (1% error)

or stick with the old formula
1. -10ºF - 30 = - 40
2. -40 / 2 = -20 (14% error)
3. 10% of -20 = -2
4. -20 + (-2) = -22 (5.7% error)

Celsius to Fahrenheit
ºC times 2, add 30, [-10% of ºC]

20ºC = 68ºF ((20ºC x 9 / 5) + 32)

1. 20 x 2 = 40
2. 40 + 30 = 70 (2.9% error)

3. 10% of 20ºC = 2
4. 70 - 2 = 68ºF (very small error)

Below 10ºF or for more precise temperatures:
Multiply by 2, add 10% of product, add 32,

-20ºC = -4ºF ((-20 x 9 / 5) + 32

1. -20ºC x 2 = -40
2. 10% of 40 = 4
3. -40 + 4 = -36
4. -36 + 32 = -4ºF (very small error)

Alternate Ditty:
Water freezes at 32,
So change ºF and ºC with 30 and 2.
Inches and centimeters, relate twice each day,
Kilos are twice what pounds do weigh,
While Miles and klicks (kilometers), are buddy's 2.
But feet and meters, it's easy to see,
Differ by what is almost three,
Square miles and klicks make no sound,
Because they are over 3 times bound.
Gallons and liters are so sore,
That they differ by almost four.
Square ft and meters consider each other a sin;
Because they differ by almost 10.
But Quarts and liters are such good friends,
That they only differ by half of 10 -- percent that is.
And 10 percent is how small the stuff
There is between good enough and rough.
And you'll never forget how good it is when
All your conversions are with the power of 10.

Beginnings and or endings for kids, groups:

1. We're cultured citizens of our world wide city,
Knowing how metrics fit, being very witty.

2. We're savvy citizens of our world wide city,
(with one of the following)

a. Wealthy from metrics, we'll be sitting pretty.

b. The wealthy all know metric's nitty-gritty.

c. Metric money managers know the nitty-gritty.

d. Metric managers, sitting pretty.

3. We're world wide warriors, ready to fight,
Cause we can change measures to metric all night.

*feel free to correct, amend, extend, pass around.