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I was thinking to make comparative drawings on a paper and stick the paper on the wall.
Do you have other ideas?
I have studied math, but used for many years cm/m/km, and now I need to learn the new measurement system.

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I think your best bet will just be to learn the numbers. If you think about it you really are only memorizing three different numbers. the centimeters in an inch (2.54). the kilometers in a mile (~1.61), and the feet in a meter (3.28). Looking at an American ruler should help you with the centimeters in inches. Looking at a measuring stick with metric and standard measurements should help with feet in a meter. and the last one if you can remember that a kg weighs more than a lb, just remember that the relationship is opposite for length. That's how I remember miles/km anyway because I usually forget which one is bigger.

9 digits to memorize each with a decimal after the first digit. shouldn't be as hard as you think. Anything worth while takes practice.

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These things work by association. We learn from kids what a six-footer or a 1,80cm tall person means by associating mental images--we don't have to think about it. For someone not that familiar with the specific measure system he just won't have a good picture of the number in his head. For miles, you need to associate specific distances. When you think '100km' you usually get a grip of it's meaning by associating some familiar car trip that takes that much. You need to find yourself those for miles as well. And inches is the same, but for small measures of everyday things.

So my suggestion is not to just memorize the straight conversion (1 inch is 2,54cm), but the multiples--12 inches is a foot which is about 30cm. And 3 inches, and 5 inches, etc. Tie to each a real-world object for your brain to associate with. And I'd make each of these a flashcard, to get down to actual memorization--hit google for 'SuperMemo'-style software. Here on productivity SE there should also be some questions regarding SuperMemo.

So each flashcard could follow this format:

Question: [real world object/distance] in [miles/feet/inches]?

(optional hint: measure number in metric system)

Answer: measure number in miles/feet/inches

And then you could have another set of cards swapping the answer for the question (i.e. say the real world thing that has measure X).

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try to stick with conversion that are nearly round:

  • 2 inches are more or less equal to 5cm
  • 3 feet are more or less equal to 1 meter
  • 5 miles are more or less equal to 8 km

and then have a bit of mental arithmetic.

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