How to Estimate Distances (by Using Your Thumb and Eyes)

Co-authored by wikiHow Staff

Last Updated: December 15, 2022

Hikers, photographers, hunters, golfers, home astronomers, and nature lovers, if you have ever wondered how far away you are from a given object, estimating that distance may be easier than you realized. The length of your arm is roughly ten times the distance between your eyes. With this fact in mind, you can measure the approximate distance between yourself and almost any object.

Part 1

Part 1 of 2:

Aligning Your Body

1
Place yourself at a good vantage point. To estimate distance, you need a clear line of sight. Try to stand in a high place, like atop a hill or a few floors up in a building. If this isn’t possible, try to find a spot where no other objects are in between you and the object you are focusing on.
2
Estimate the horizontal length of the object. Here are some standard lengths that may help you:
- Most average-sized cars are between 12 and 15 feet long.
- Mailboxes are usually around 1.5 feet long.
- Mature tree trunks are typically between 1 and 2 feet wide.
- The standard length of a trailer pulled by a semi-truck is 50 to 65 feet long.
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3
Hold your right arm out in front of you. Aim your arm toward the object, and try to keep it as straight as possible. Point your thumb up.^{[1]
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4
Close your right eye. Keep standing as still as possible.
5
Align the left edge of your right thumb with the right edge of the object. Try to keep your head and arm perfectly still.^{[2]
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Part 2

Part 2 of 2:

Calculating the Distance

1
Switch eyes. Keeping perfectly still, close your left eye and look through the right eye. Your thumb will appear to have “jumped” to the left. This “jumping” phenomenon is called “parallax.”
2
Estimate how far the left edge of your thumb jumped. Did your thumb jump 3 times the length of the object? Half the length? Keep this number in mind. For example:^{[3]
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- If your thumb jumped 3 times the length of a building that you believe is 100-feet-long, then it jumped 300 feet.
- If your thumb jumped half the length of a 14-foot-long car, then it jumped 7 feet.
3
Multiply that number (the jump) by 10. This will give you the approximate distance you're looking for. For example:^{[4]
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- If your thumb jumped 3 times the length of a 100-foot building, or about 300 feet (91 m), then you would multiply 300 by 10 to conclude that the building is about 3,000 feet (914 m) away from you.
- If your thumb moved half the length of a 14-foot-long car, or about 7 feet (2 m), then you would multiply 7 by 10 to conclude that the car is about 70 feet (21 m) away from you.

Community Q&A

Question

How might astronauts use parallax?

Community Answer

To find out how far away planets and stars are, or find the angle between two objects and find the distance between them.