Twice during the last few years, somewhat similar incidents were reported by the news media. An Air Canada jet liner ran out of fuel in mid air. Fortunately, the captain was one of the very few who had the necessary skill to land the plane on zero power. The problem was attributed to a mix up between pounds and kilograms of fuel.

Later, NASA crashed a space probe that was designed to land on Mars. The cost of the probe was some hundreds of millions of dollars, not counting the salaries of the highly trained technical people who had to track and guide it through its months long journey from Earth. The cause was said to be a mix up between feet and metres in the rate of descent.

Failure to convert correctly from one system of units to another can have serious consequences, enough so that some attention to a conversion procedure is justifiable. The process of converting measurements in one system of units into another system is sometimes called scaling. There are likely numerous methods of performing conversions. The one that will be described here, however, is straightforward and virtually foolproof. The explanation will be clearest if an example is worked.

Your neighbor has come to you with a problem. He has bought some fertilizer for his lawn The directions on the bag say to apply it at a rate of 2 kg per 100 m ². He is in difficulty because he...