Anyone who reads Diary of Numbers regularly knows I’m far from a grammar nazi. Despite my best efforts, my grammar is typically atrocious. There is, however, one thing I’m very particular on. Even though both of the following are equally correct, I much prefer the latter:
(1) “Dorothy was afraid of lions, tigers and bears.”
(2) “Dorothy was afraid of lions, tigers, and bears.”
Recently, something struck me: since both usages are perfectly correct, I’ve been wasting printer ink all my life. How many cartridges of ink do you waste in your lifetime by always including the extra comma?
This is a tricky problem because the amount of printer ink a person uses varies considerably depending on his/her career, hobbies, etc. If you’re a novelist, you’ll probably be printing a lot more commas than a rodeo clown. Moreover, the problem’s a little wishy-washy because of how I originally phrased it1. A better way to phrase the question would have been, “How many cartridges of ink would you waste in your lifetime if everyone included the extra comma?”
Some days I print 100 pages, other days I print none. On average, I probably print about one full page per day2. Lists appear about once every 10 pages3. This means I’d be printing one superfluous comma every 10 days. At this rate, I’d produce 2900 needless commas over an 80-year lifespan.
According to HP’s website, the average HP LaserJet Q2612A Black Print Cartridge yields 2000 pages worth of ink. A single comma requires about one-fourth the ink of a letter and according to one of my MS-Word documents, there are about 2000 letters per page. This means one page worth of ink is equivalent to about 8000 commas. From this and the info above, we can estimate the number of printer cartridges one will waste in a lifetime,
# ink cartridges = (# ink cartridges per page) · (# pages per comma) · (# of commas)
= (1 ink cartridge per 2000 pages) · (1 page per 8000 commas) · (2900 commas)
= 0.00018 ink cartridges.
Over the course of your lifetime, you will waste 1/5,000th of an ink cartridge by including the extra comma. Even if everyone in America included the extra comma, we’d waste only about 680 printer cartridges each year.
After posting this problem, a good friend, who had previously worked for Yale University Press as a copy editor, told me that Yale never included the extra comma to save ink!
 Sorry about that.
 This is more than printing one page every ten days and less than printing 10 pages per day, which seems like reasonably good bounds for and order of magnitude estimate.
 Again, this estimate is reasonably good if you consider upper and lower bounds. A list certainly does not appear on every page but you’ll probably see more than one on a hundred pages of text.