It's often useful to know how large something is. But most people don't carry rulers or tape measures around with them, and most people are also bad at guesstimating lengths by looking at them (or, at least, they think they're bad enough that they don't try). People have tried to create apps with rulers, but they all suffer from the problem that they can't measure beyond the small length of the screen. There's one thing you can never leave home without, though, no matter how hard you try, and that's your body. And once we're finished growing, most of our body parts stay at pretty much the same size.
People have measured by comparing the lengths of things to themselves for millennia, but nowadays if you go up to people on the street and ask them how long a fathom or cubit is, they'll probably give you blank stares. Fortunately, it's actually really easy to estimate distances using your body: just learn a few measurements and then add and subtract them for any distance you need.
Below is a table of the measurements I remember. The Accepted Estimates are surprisingly accurate in most cases (although I don't have one offhand for all these measures), but if you're going to go to the trouble of remembering values, I figure you might as well do it right and measure yourself first. I use inches because I end up using them nearly exclusively in the United States unless I'm being precise or scientific or going for an exact value on something that was originally measured in meters, and in all those cases I'll have a measuring tool. But if you prefer metric, this technique will work just as well with any system of measurement.
My values are included only for demonstration purposes and will be inaccurate for you, of course. (+ indicates the true value is slightly higher and - indicates it is slightly lower.)
|Unit||Description||My Measurement||Accepted Estimate|
|Fathom||distance between tips of middle fingers with arms fully outstretched||71- inches||100% of height|
|Cubit||distance between end of elbow and tip of middle finger||18+ inches (17 from inside of elbow)||18 inches|
|Foot||distance between heel and tip of big toe||11 inches (12+ with shoes)||15% of height|
|Span||distance between outer tips of outstretched thumb and pinky (#4 on this diagram)||8¾ inches||½ cubit, 9 inches|
|Small Span||distance between outer tips of outstretched index finger and pinky||6 inches (right hand)|
|Hand||distance from side to side across hand, including thumb held flat against hand (#2 on this diagram)||4 inches||4 inches|
|Two Fingers||distance between outside edge of index and middle fingers when spread apart||4+ inches|
|Ring Finger||length of palm side of ring finger||3 inches|
|Thumb||distance from tip of thumb to first knuckle||1 inch (to first line on knuckle)||1 inch|
|Pinky||width of pinky finger at nail||½ inch|
You don't need all these measurements to do a good job estimating. For most short distances, knowing the cubit, span, hand, and thumb values will probably be enough. However, the more you know, the easier it is to adapt to values that fall into inconvenient places. For instance, I added the short span to my list because it's otherwise awkward to measure half a foot—my choices would be using a hand and two inches (nasty) or a span minus a ring finger (requires the item being measured to be mostly covered up). As another example, the two-fingers measurement is less accurate than the hand because I can stretch my fingers apart to a lesser or greater degree, but it's a lot easier to measure the length of an object on a table that way than by holding it up to my hand.
If you have a ruler and a tape measure, most of these values should be pretty easy to measure for yourself, with the exception of the fathom—it's difficult to measure a distance without having room to move your arms in, and you have to stretch them all the way out. It's probably easiest if you can grab someone else to help you, but you can do it by yourself too if you need to. Find a wall or long vertical space you can write on temporarily (this can be the hardest part!) and grab a pencil and tape measure, and mark or have someone else mark both ends of your fingers against the wall (don't forget to get as flat up against the wall as you can—the error adds up with a five-to-six-foot measurement), then use the tape measure to measure between them. You can make this even easier if you can put one fingertip up against a shelf, doorframe, or perpendicular wall, so that you only have to make one mark.
For the most part, your left and right hand and foot measurements will probably be basically the same, but you should still check both sides and note any differences, or just use one side. I have found two noticeable differences for myself: the line on the knuckle of my right thumb that marks one inch almost exactly isn't present at all on my left hand, and my small span is a full inch longer on my left hand (I suspect it has to do with stretching to play the violin over years).
I said your measurements should stay basically the same above, but as we age, our bodies do change, so if a few years have passed, it's probably a good idea to crack out your tape measure again and make sure the numbers are still where you thought they were.
Once you've measured and memorized a few values, grab a ruler and practice estimating and then measuring the size of some objects. You'll probably be surprised how accurate you can be. This works especially well because many items are measured in neat multiples of inches; for instance, a standard photograph isn't going to be 4.25 x 5.9 inches, so if it's about the width of your hand and halfway between that and a span in length, you can safely assume it's a 4x6 photograph.
Also don't forget that you have your height to work with (which nearly everybody already knows unless they're currently growing rapidly). If you're measuring vertically and an item is nearly your height, you can stick your hand on top of your head and measure from there.
(To be continued)