How to make a large number incomprehensible - or comprehensible

On August 15th Faith Pincus posted about Rhetorical Techniques: Sample Analogy in her Speech Advice blog. She discussed an article in TheWeek that was an extract from Charles Fishman’s recent book The Big Thirst. Her paraphrase was that:

“The United States uses 5.7 billion gallons of water every day, to flush the toilet.  5.7 billion gallons of water every day.  That's a hard number to get your mind around, so what does it mean? It means that the United States flushes the same amount of water every day that the UK and Canada, combined, use for all of their daily household water needs.”

She thought that comparison was great. I think it’s mediocre at best. First, it compares one incomprehensible large number with another. Both are large abstractions without an obvious meaning. Second, the comparison is more than slightly bogus since the population of the UK plus Canada isn’t nearly as large as the U.S. population.

A longer direct quote from the article is that:

“For Americans at home, flushing the toilet is the main way we use water. We use more water flushing toilets than bathing or cooking or washing our dishes or our clothes. The typical American flushes the toilet five times a day at home, and uses 18.5 gallons of water, just for that. What that means is that every day, Americans flush 5.7 billion gallons of clean drinking water down the toilet. And that’s just at home.

It’s impossible to get your brain around that number, of course—5.7 billion gallons of water a day. But here’s a way of thinking about it. It’s more water than all the homes in the United Kingdom and Canada use each day for all their needs—we flush more water down the toilet than 95 million Brits and Canadians use.”

When we divide 5.7 billion by 18.5 we get 308 million, which is about the U.S. population. Our population is over three times that 95 million, so you’d expect that we’d waste more water than them. Also, multiplying anything by 308 million makes it ridiculously large.

Is there another well-known large number we can compare with 5.7 billion gallons per day? Yes - it’s the amount of water going over Niagara Falls. During the summer tourist season that is 100,000 cubic feet per second, which converts to 64.6 billion gallons per day. So, flushing our toilets uses about 9% of what goes over the falls. If you’ve been there or seen it on TV 9% of Niagara Falls is pretty easy to imagine.

Stating the number per person as 18.5 gallons a day from five toilet flushes helps keep it comprehensible. We’re actually getting somewhere in learning to conserve water, but still have room to improve. That’s 3.7 gallons per flush. The toilets currently sold use just 1.6 gallons per flush. Back in the sixties toilets used 5.5 gallons per flush, and in the eighties we got down to 3.5 gallons per flush. We’ve clearly still got a lot of the older ones out there wasting water.