Actually it isn't hard to get used to. Think in terms of guns and weed. An ounce, is 28 grams, you can divide that easily. A kilo is 2.2 pounds. And a half an inch, or .50 cal. is 12.7 mm. That means an inch is 25.4mm, so just divide or multiply by 25.4, depending on which way you need it to go.
Or not. Whatever, but you should learn the gun caliber conversions, and the weed weight conversions.