The following conversions are not 100% accurate, but they work well enough in most cases. If, for example, you are driving in Italy, you may not want to plug in your laptop to use the calculator to determine how fast you're really going (particularly at the speed at which Italian traffic moves). So do it this way:
Kilometers to miles - Divide by 8 and multiply by 5. Or divide miles by 5 and multiply by 8 to get kilometers.
Liters to U.S. gallons - Roughly 4 liters to a gallon, or 4 cups to a liter.
Kilos to U.S. pounds - Multiply by 2 and add 10% of that. For pounds into kilos, divide by 2, then subtract 10% of that.
Meters to feet - Multiply by 3 and add 10%. For feet to meters, divide by 3, then subtract 10%. (A yard is 90 centimeters and there are 100 centimeters in a meter.)
The rule of two and a half - Hectares and centimeters can be multipled by 5 and then divided by 2 to get acres or inches. To turn acres or inches into hectares or centimeters, divide by 5 and then multiply by 2. Think of a hectare as 2-1/2 football fields. The first joint of most adult thumbs is somewhere between two and three centimeters in length.