Want to know the height of a person or building?Using simple trigonometric principles, you can closely estimate the height of objects with an easy-to-make hypsometer (hyps means height in Greek).

1. Make it.

Tape the straw along the top of the card. Punch a small hole in the right side of the card, 10cm from the bottom. Straighten the paper clip, bend one end into a small hook, and hang it in the hole so it swings freely. Next, write numbers 0 to 12, 1cm apart, from right to left across the bottom of the card, starting with 0 directly beneath the hanging paper clip, proceeding leftward to 12. Here’s a tip: If you use a thick plastic bag instead of the card, your hypsometer can be rolled up and carried in your pocket.

2. Test it.

Standing 10' away, look through the straw at the top of a friend’s head. When you tilt your head (and the hypsometer), the paper clip will move leftward at an angle as gravity keeps it pointing down at the ground. In Figure 2, it points to number 1.

By measuring your distance from an object (in centimeters) and the angle of tilt indicated by your hypsometer, you can calculate your friend’s height.

3. Calculate it.

You’ll need 2 more numbers for your calculation: the height of the card (10cm), and the height of your eye line, measured from the ground. You can also measure from the top of your head, then subtract this number from your height. For instance, if you’re 5'8" tall and your eyes are 4" from the top of your head, your eye line is 5'4", or 164cm, off the ground.

Metric measurements are much easier to calculate with a hypsometer, because metric is a base 10 system. Use a metric ruler, or write equivalents on your hypsometer as shown in Figure 3.

Multiply the distance to your friend (10' or 305cm) by the angle number indicated by the paper clip (1), then divide by the card height (10); in this case, you get 30.5. Add this to your eye line height (164cm), for a total of 194.5cm. That’s about 6'3" tall.

Now test your hypsometer with objects of unknown height, such as doors, walls, and buildings, to calibrate it and sharpen your measuring skills.