How Much Does An Acre of Water Weigh?

My father-in-law just asked me an interesting question:

How much would an inch of water covering an acre weigh?

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It’s not such a difficult question. It seems like a perfect query for Wolfram Alpha.


WA doesn’t get the question at all. In fact, it seems to have classified an acre as an animal. Maybe I should have said hectare? Nope. It does no better.

Interestingly, Google makes solving this problem trivial, thanks to the useful “convert” command. I solved the problem in two steps. (It helps to know that a cubic centimeter of water weighs a gram – in fact, that’s the definition of a gram.)

Convert 1 inch x 1 acre to cubic centimeters -> 102 790 153 cubic centimeters

We already have the answer, 102,790,153 grams, but to put it into terms that are more human I Googled:

convert 102790153 grams to tons

and got the correct answer, 113.306748 short tons.

No matter that my father-in-law used a calculator, we both got the same answer, proving that Google is, in some cases, better at numeric analysis than Wolfram Alpha.

The real problem with WA is that it’s not easy to formulate a query that produces the results you’re looking for. Type ‘blood alcohol‘ and you’ll get fascinating results (thanks to Chris Heath for finding that, by the way), but not necessarily results you can expect. And, as it turns out, predictability is an important feature of any search engine, or computational knowledge engine for that matter. Before it’s useful you need to have some idea of what kind of answers you might get, and, for the moment, Wolfram Alpha’s results seem utterly random. I’m rooting for it, but it may be that it’s just too smart for The Rest Of Us.

(Thanks to Edward Coffey who points out that the query “1 acre * 1 inch * 1g / cubic centimetre” works on Wolfram Alpha. Quite well, in fact. But I don’t think that changes my point.)

8 Replies to “How Much Does An Acre of Water Weigh?”

  1. It's pretty straightforward in W|A, where you don't have to remember the weight of a cubic cm of water: an input of “1 acre x 1 inch” tells us that W|A knows the interesting unit “acre-inch”, abbreviated ac-in. The input “1 ac-in of water” tells us that that much water weighs 113 short tons (click on “More units” in the “Basic properties” pod).The two google queries you formulated work as you'd expect in W|A, where they give lots of extra context, too. I think it's a matter of learning what sorts of inputs W|A likes to work with, as we did with google – which was so many years ago that ginning up google inputs has become second nature.

  2. I just think of WA as an android…that doesn't quite understand English yet…he/she/it/whatever is learning…so be nice

  3. I think it's already been stated that WA gets to the same answer if you ask the same questions. Out of curiosity I did a google search for “How much would an inch of water covering an acre weigh?” and I got… well, this blog entry. In a couple forms but that's about it. I find that interesting.

  4. Well, how about you google the weight of one acre-foot of water (a standard unit of measurement) and divide by twelve? One google search, one calculation:google: weight acre foot waterIn the summary for the first search returned you can read “One acre-foot weighs 2719226 pounds (325851 gallons x 8.345 pounds)”.Divide 2719226 pounds by twelve, and you get 226,602 pounds or 113 short tons.

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