Not all pennies are created equal; since the US cent coin first appeared in 1793, the metal used in it has gone from pure copper to mostly zinc, and steel was important for one year of production. The density depends on when the penny was made. Fairly new pennies have a density of 7.15 grams per cubic centimeter (g/cc), though very old ones can be as high as 9.0 g/cc.
TL;DR (Too Long; Didn't Read)
Density can range from 7.15 g/cc for a new penny versus 9.0 g/cc for a very old one.
Density and Pennies
Density is a measure of how much mass or weight an object has divided by the volume it takes up. For example, container of water weighs 1,000 grams, and it takes up 1,000 cc. Dividing 1,000 by 1,000 gives the water’s density, 1 g/cc.
Finding the density of one penny is not easy, since you have to measure its thickness. However, a 5-centimeter stack of pennies can make this easier. Measure the diameter of one penny with a ruler, multiply by 1/2, square the result, multiply by pi to find the surface area, then multiply again by 5 centimeters to get the volume. Next, weigh the stack on an accurate scale. Divide the weight in grams by the volume to get the density. Note that you'll probably have a mix of pennies in your stack, some more dense than others; your calculated density is the average for all of them.
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Making Cents: Density of Metals
Though copper has historically had the greatest use in pennies, zinc, nickel, tin, and iron have also gone into their manufacture. Of these metals, zinc has the lowest density, at 7.1 g/cc. Tin is a close second at 7.3 g/cc. Iron’s density falls roughly in the middle of the pack at 7.9 g/cc. Nickel is the second-densest at 8.9 g/cc. And copper is the densest of these metals at 9.0 g/cc.
Copper, Brass and Bronze
Pennies made before 1837 are pure copper, a metal whose density is 9.0 g per cc. After that year, the Mint experimented with a few different alloys, including brass and bronze, adding tin, nickel and zinc in various percentages. For example, from 1864 to 1962 the penny’s makeup was 95 percent copper and 5 percent tin and zinc, for a total density of 8.9 g/cc. One reason for creating these alloys is that copper is a fairly soft metal; mixing in other metals makes the penny more durable so the engravings take longer to wear out in circulation.
WWII - Steel Penny
In 1943, the US government faced a shortage of copper due to the Second World War. Copper was needed in the manufacture of guns, planes and ships, both as electrical wiring and to make alloys such as brass and bronze. Because of the great need for copper in other areas, the US Mint switched to steel, a cheaper, more plentiful metal. Steel is mostly iron with a small percentage of carbon and other metals mixed in. The density of steel pennies is close to that of iron, about 7.9 g/cc.
Copper over Zinc
In the 1970s the price of copper went up due to US and international demand. The value of the metal in a penny became greater than one cent - a big problem, since metal scavengers might be tempted to melt pennies into scrap to sell for a profit. In 1982, the US government solved the problem by making pennies mostly of zinc, a cheaper metal, with a thin coating of copper to make it look like a penny. Zinc’s lower density means these pennies are lighter, though not as light as pure zinc. Pennies are 97.6 percent zinc and 2.4 percent copper, giving them a density of 7.15 g/cc - the lowest of any US penny.