How to learn the metric system
If you can understand and use pounds, inches, gallons, or Fahrenheit, you can learn metric. The metric system is not hard to learnmost people just try to learn it the hard way.
Rule #1: Don't try to do conversions between metric and non-metric.
Just forget about the old units. If I weigh myself and find that I am 65 kilograms, then now I have an idea of what 65 kg means. I don't need to know what that is in pounds - in fact, thinking about the pounds equivalent will just make things more confusing. Next, I can weigh my one-year-old daughter and find that she is 9 kg. I also know that a full 0.5 liter water bottle weighs half a kilogram. Now I have some reference points to build a mental framework for what a kilogram means. It doesn't mean a certain number of pounds; it means weight that I can relate to my own experience.
Rule #2: Metric does not mean "math problems". But if you do need to do some arithmetic involving measurements, there's no better way.
Do you have to do math problems when you buy a gallon of milk or look at a thermometer that shows Fahrenheit? Of course not. Then why would you need to know lots of math to buy a two-liter bottle of Sprite or look at a Celcius thermometer? And for situations where you do need to use math, using metric units in math is inherently easier than using colloquial units (for example, see the section on area measurements).
Rule #3: Be prepared for some mental discomfort.
If you are not familiar with using a particular unit of measure, it can seem like gibberish. Relating that a mental picture can involve some effort and feeling uncertain. Keep in mind that most people aren't experts in the non-metric units either. For example, suppose you can see a street sign. You may not be able to say if it is 100 meters away or 150, but how accurate would you be if you tried to estimate it in feet?
Learning metric really only requires a few reference points and a way to use them in everyday life. To help me use metric units, I have a Celsius thermometer, a kilogram scale, a liter water bottle, several meter sticks, and a metric odometer and speedometer on my bike. The first three are available at Walmart, the meter sticks are available online from Office Depot, and you can get a bike computer at any bike shop. (If you are ever in Falls Church, VA, I'll give you a meter stick.) When you get a thermometer or scale, get a digital one because these will let you to show only the metric units. It is much easier to think in the new units if you don't have the old ones around.
To understand Celsius temperatures just remember:
That is really all you need to know most of the time, except perhaps the boiling point of water (100 °C) and body temperature (37 °C).
To understand body weight in kilograms, it is best to get a scale (or change the setting on yours if you can). These can be some reference points:
If you get a scale, you can weigh yourself with and without a backpack full of books or other items to find out how much the backpack weighs. This can give you an idea of how heavy 5 kg or 7 kg is, for example.
To understand a person's height in centimeters, it is best to tape a meter stick to the wall so you and others can measure the height directly. These can be some reference points:
One very simple thing you can do to begin using millimeters is to change your word processor (and other software on your computer) to use millimeters instead of inches. After a few weeks of using 20 mm margins or 25 mm margins, it feels completely natural and more intuitive than inches. To change the settings in:
I recently got a kitchen food scale that shows weights up to 2 kg. Now I can know that I ate 100 g of pasta (before cooking) last night!
One thing you can do right now to help you understand grams is to look at the package for a loaf of bread. The serving size will probably be one slice, with a weight in grams given. If you get out a slice and see how heavy it feels, you have a reference point right there in your kitchen.
When you go to the grocery store, instead of looking at the ounces or fluid ounces on the label, pay attention to the milliliters (mL), liters (L), grams (g), and kilograms (kg). If you have a water bottle that is either 500 mL or 1 L, you can just look at the size of the bottle an get an idea of how much those quantities are. The two liter soft-drink bottle can also serve as a reference point. If you have a box of cereal that weighs 425 g, you just see how heavy the box feels to get an idea of how much 425 g is.
Compare the elevation of your town to these values.
Most of these measurements are just FYII wouldn't expect the average citizen to know many of these. An exception would be the Earth-Sun distance or the Earth's circumference (notice the round value).