It's about one-and-a-half times as long as an Aspirin.
In other words, the length of an Aspirin is 0.67 times 21.21 milimeters.(for up & up aspirin, 325mg, regular strength tablets) (a.k.a. acetylsalicylic acid, a.k.a. ASA)
It's about two-fifths as tall as a Golf Tee.
In other words, 21.21 milimeters is 0.39 times the height of a Golf Tee, and the height of a Golf Tee is 2.6 times that amount.The most common golf tee size and the length preferred for iron and most wood clubs is 54 milimeters tall. The tee did not come into common use until it was promoted by professional golfers Walter Hagen and Joe Kirkwood, as part of a marketing effort by the patent-holder of the well-known, "Reddy Tee."
It's about twelve times as long as a Strand of Spaghetti (thickness).
In other words, the length of a Strand of Spaghetti (thickness) is 0.083 times 21.21 milimeters.(average; restaurant and supermarket applications) (based on FMA Omcan Italian Made TR50 Counter Top Pasta Machine Extruder 13317)
It's about seventeen times as long as a Grain of Sand.
In other words, the length of a Grain of Sand is 0.0588 times 21.21 milimeters.(medium grain)
It's about one-twentieth as tall as a Bowling Pin.
In other words, 21.21 milimeters is 0.0557 times the height of a Bowling Pin, and the height of a Bowling Pin is 18 times that amount.(USBC specifications; tenpin)
It's about one-thirtieth as long as a Woman's Footstep.
In other words, the length of a Woman's Footstep is 31 times 21.21 milimeters.(for healthy, middle-aged woman of average height; single step length)
It's about one-thirty-fifth as long as a Man's Footstep.
In other words, the length of a Man's Footstep is 37 times 21.21 milimeters.(for healthy, middle-aged man of average height; single step length)