I just remember that an inch is 2.54 cm, exactly (unless it's a "survey inch"), and there are 12 inches in a foot, and 3 feet in a yard ... but then I get stuck

I just remember that an inch is 2.54 cm, exactly (unless it's a "survey inch"), and there are 12 inches in a foot, and 3 feet in a yard ... but then I get stuck remembering how many yards there are in a chain and how many chains in a furlong.