My 740 Cents Worth

A couple of days few months ago my father-in-law mentioned that he has been rolling pennies since 1997 and he has about one hundred dollars worth squirreled away. He is aware that pre-1982 pennies are mostly copper whereas post-1982 pennies are mostly zinc, so he asked me “At what point will the pennies I roll be the mostly zinc kind?”

“Huh. That is a really good question. I don't know. Why?” I asked.

“Well, with copper prices being so high I was wondering if it would be worthwhile to melt them down just for the bullion and I don't want to sort the coins.”

Fair enough. Even if it only took me three seconds to grab, orient, read, and sort a penny I doubt that I would want to spend the next eight hours scrutinizing tiny numbers on hunks of metal on the off chance I was going to get something other than self-induced nearsightedness and blue-green fingertips for my troubles.

I told him I would get back to him with an answer. This post is that answer.

So what determines the distribution of dates of the coins in a coin roll, your pocket, your soda-goo-filled drink holder in your car, your change jar, and your kid's piggy bank?

Assuming that the pennies have been sufficiently shuffled through the hands of the multitudes before you get your grubby little paws on them there are three main forces at work:

1) The quantity of pennies released each year by the Mint.
2) The length of time that has passed since their release.
3) Hoarders and collectors.

The number of coins put out by the Mint varies widely from year-to-year. The list below shows the number of pennies made each year since 1959 (the year they switched from wheat reverse to Lincoln Memorial reverse).

1959 1,340,731,500
1960 2,167,289,000
1961 2,506,611,700
1962 2,399,193,140
1963 2,528,130,400
1964 6,447,646,500
1965 1,497,224,900
1966 2,188,147,783
1967 3,048,667,100
1968 4,852,420,571
1969 5,684,117,200
1970 5,480,313,904
1971 5,355,669,059
1972 5,975,265,508
1973 7,594,998,883
1974 8,876,665,183
1975 9,956,751,442
1976 8,895,884,881
1977 8,663,992,300
1978 9,838,838,400
1979 10,157,872,254
1980 12,554,803,660
1981 12,864,985,677
1982 16,725,504,368
1983 14,219,554,428
1984 13,720,317,906
1985 10,935,829,813
1986 8,934,262,191
1987 9,561,856,445
1988 11,346,550,443
1989 12,607,002,111
1990 11,774,659,533
1991 9,324,386,076
1992 9,097,578,300
1993 12,111,355,571
1994 13,632,615,000
1995 13,540,000,000
1996 13,123,260,000
1997 9,199,355,000
1998 10,257,508,500
1999 11,597,665,000
2000 14,277,420,000
2001 10,334,590,000
2002 7,288,855,000
2003 6,848,000,000
2004 6,836,000,000
2005 7,700,050,500
2006 8,234,000,000
2007 7,401,200,000
2008 5,408,400,000
2009 2,354,000,000
2010 4,010,830,000
2011 5,481,686,000

I include this list here because the Google refused to give me the answer in the form I needed however nicely I asked and I wanted to give others a chance to find the info easier. All data is from the 2011 Official Red Book: A Guidebook of United States Coins. Please note 1) any errors are my own – this was a lot of tedious data entry and I may have messed up in places 2) the numbers represent the sums of all non-proof-set types from all mints for that year 3) the 2011 number is just an estimate based on the average of the previous five years.

In case you were wondering, 439 billion Lincoln pennies have been minted since 1909. Twice as many pennies as there are stars in our galaxy... Wow.

Over the past five years I have rescued three wheat pennies from my change. One is from 1921 and the other two are from 1947. One time my mother found an Indian Head penny in her change but that was about about 25 years ago...

“And once I sawr a blimp.” Jeebus, SnowUrchin... Focus, wouldja?

The point is that when the Mint decided to change up the look of the penny in 1959 people started hoarding wheat pennies like crazy (many people still do this) so there is a statistically significant lack of pre-1959 coppers in our change today. It is reasonable to assume that there are no wheat pennies in my father-in-law's coin rolls.

How long does a coin stay in circulation? According to this site, “most modern coins that circulate last about 25 years”. That answer is 100% accurate while at the same time 100% useless for our purposes. What is “most”? What is “about 25 years”? What happens to them? Do these hunks of metal just decay away?

No, they don't decay away... not really. They get lost. Or turned into jewelry. Or thrown into wells. Or crushed by railroad cars (awesome, btw) or by those 51-cent souvenir maker dealies. Or hoarded and rolled and placed in a box in the closet. We can assume that people take pennies “out of play” in some form or fashion all the time independent of the date of the coin.

Hey... If that is a valid assumption then we can actually assume they decayed away and we can figure out what percentage of coins of a certain year are still in circulation by using a formula that is typically used to determine the amount of radioactive material left in a sample after some time has passed.

Here N0 is the original number of coins, T is the amount of time that has passed, and t ½ is the half-life of a penny. “The half-life of a penny,” you say. What have you done here besides turning a barely interesting question about coins into a nearly unreadable geeky one about coins?

Half-life is just a term that means the amount of time that needs to pass before the sample have been reduced to ½ its original size. If I have a stash of pennies laying around I should be able to figure out this alleged half-life by looking at the dates on a small number of them.

I searched my son's piggy bank (calm down – he was napping and he got them back anyway), rifled through the junk drawers, and checked the cars and came up with 73 pennies. Probably not enough for a good sample but that has never stopped me in the past so let's see what we got.

Of the 72 American pennies (there was one Canadian coin in the lot) two were from 1982. They started making the zinc-core pennies in October of that year so it was a coin-toss to determine what their composition was... literally. The newer pennies sound different than the older ones when dropped – I had one of each. Check it out for yourself. I'll wait.

Those of you who actually did this little experiment will notice the pre-82s have a distinctive “ring” when they fell, while the post-82s have a duller, almost-clunky sound. Go ahead and try as many times as you need to hear the difference or until you hear “Hey! What the hell are you doing in there?!” coming from the next room, whatever comes first.

All of the coins were collected during this year (2011). I sorted them by date and discovered that 50% of them were from 1995 to 2011, and the others were from 1959 to 1994. I weighted my radioactive decay formula with the quantity of pennies minted and adjusted my guess at a half-life until I got the same 50/50 date as I found in my little pile of pennies. This gave me the half-life of a penny of about 23 years... Pretty close to the 25 year number from the source above. Cool.

You know... I had a lot more to say on this, but I have been circling the runway here for over a month. Let's land the plane already and present the data cuz I am really tired of having this post in my “to do” list. If anyone out there wants to chat more about it, shoot me an email or post a question in the comments section. Here is the graph:

Here you can see that my father-in-law needed to start collecting pennies in 1990 (not 1997) or so to have at least some rolls with 50% copper. Also, even not including the cost of operating a smelter, the price for committing a Class 5 Felony for melting down coins in bulk is $10,000 and/or five-years in a federal prison...

Is it worth it? Not for him. When you try to ask “the Google” what the current price of copper is you, again, don't get a straight answer. Instead you get the feeling like you were just dropped into a busy marketplace in Marrakesh: “Sir! Yes, you sir! Come here my friend! You look for copper, I give you best price, no problem! Come, come! My office is just down this narrow, badly-lit alleyway...”

Go to WolframAlpha, instead, if you want to find the price of metals. $3.28/pound as of today, FWIW.

I have graphs showing the number of non-wheat pennies per person in the USA peaked in 2001 at 848, but now we are down to 740 per person – roughly the same as what we had in 1990. Wait... don't go... I have more charts... Didn't you want to hear about the effect the 1943 copper shortage had on the copper distribution per capita in terms of pennies, or how recessions may be able to be predicted by the pattern of penny production in the USA... No?

'Kay, then... Bye...