Estimate How Far Away

Here is a clever method to estimate how far away something is:

Now ... estimate how far it moved sideways (you could imagine the length of a car or something).

Multiply that by 10 and you have an estimate of how far away.


thumb distance


Here your thumb seems to jump about half a car length.

Half a car length is about 2.5 meters.

Times 10: the car is about 25 meters away.

How it Works

thumb distance is 10x eye distance


The distance from your eyes to your thumb is about 10 times the distance between your eyes

thumb distance far


And so the distance to the far object is also about 10 times the width your thumb seems to move at the far object.


thumb distance similar angles

This works because the triangles are similar,
and so the relative lengths are the same.

Learn the Size of Things

To be useful you need to know how long, wide or tall things are!

(Note: to use this method for height, tilt your head and thumb 90° to the side.)

Your Turn

Go outside and stand on a high spot where you can see lots of things (roads, building, etc).

Write down what you see and estimate how far away:

Object "Thumb move"
×10 =
How Far Away

Bonus Activity: Get a map, mark where you stood and where each object is, and work out how far away they really are. How good were you?