Here's a quick way to convert between some common units.
It's not very precise in some cases, but sometimes the ability to have a quick answer is more important than precision.

• Meters to Yards: Add 10%
  
• Yards to Meters: Take off 10%
  
• Meters to Feet: Multiply by 3
  
• Feet to Meters: Divide by 3
  
• Square Meters to Square Feet: Multiply by 9
  
• Square Feet to Square Meters: Divide by 9
  
• Pounds to Kilograms: Divide by 2, then take 10% off the result.
  
• Kilograms to Pounds: Multiply by 2, then add 10% to the result.
  
• Milliliters to US fluid Ounce: Divide by 30
  
• US fluid Ounce to Milliliters: Multiply by 30