How tall would one million pennies be in centimeters. And how do you know

One US-penny is 19 mm in diameter.

One British-penny is 20.3 in diameter.

We can make an accurate measurement of the diameters, or research on the internet.

1 milion US_pennies are

1,000,000*19 mm = 19,000,000 mm = 1,900,000 cm tall, since 1cm=10mm

1 milion British_pennies are

1,000,000*20.3 mm = 20,300,000 mm = 2,030,000 cm tall, since 1cm=10mm