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You don't need any other equipment to use this method, and you should get an accurate estimate of how tall the tree is. You will need to do multiplication and division problems, but no other math. If you want to avoid doing any math at all, you can use a tree height calculator online, such as this one, and enter the measurements you find using this method. Use a tape measure or yardstick (meter ruler) to measure your height while standing straight. Do this while wearing the shoes you will be wearing to perform this method. Since you'll need a piece of paper anyway, write your height down so you don't forget the exact number.  You'll need a single number, such as your height in inches, not a combination of feet and inches. If you're not sure how to convert into one number, you can use the height of a yardstick or meter ruler standing vertical instead (3 feet or 1 meter). Use the ruler's height and the length of the ruler's shadow anytime you are asked to use If you are in a wheelchair or cannot stand straight for another reason, measure your height in whatever position you'll be in when you go outside to measure the tree. Try to find a spot where your shadow falls along flat ground so you can get an accurate measurement. For best results, do this method on a bright, sunny day. If the sky is overcast, it may be difficult to precisely measure the shadows. Use a tape measure or yardstick (meter ruler) to measure the distance from your heels to the tip of your shadow. If you don’t have someone to assist you, you can mark the end of the shadow by tossing a rock onto it while you’re standing. Or better yet, place the rock anywhere on the ground, and then position yourself so the tip of your shadow is at the rock; then measure from where you're standing to the rock. Write down and label each measurement you make right after you make it, to avoid confusing them with each other. Use your measuring tape to determine the length of the tree’s shadow from the base of the tree to the tip of the shadow.  This works best if the ground all along the shadow is fairly level; if the tree is on a slope, for example, your measurement won’t be as accurate. Do this immediately after you've measured your shadow, since the sun's movement will cause the shadow length to change. If the tree's shadow is on a slope, there might be a different time of day when the shadow avoids the slope either by being shorter or by pointing in the other direction. Most trees grow straight upwards, so the highest tip of the tree should be above the exact middle of the tree. To get the total length of its shadow, you should add 1/2 the diameter of the tree trunk to your shadow measurement. This is because the highest tip is actually causing a longer shadow than you measured; some of it is just falling over the top of the tree trunk where you can't see it. Measure the width of the trunk with a long ruler or straight tape measure, then divide by 2 to get 1/2 the tree's width. If you're having trouble seeing how wide the trunk is, draw a tight square around the base of the trunk and measure one side of that square. You should now have three numbers written down: your height, your shadow's length, and the length of the tree's shadow (including 1/2 its trunk width). Shadow lengths are proportional to the height of the object. In other words, (your shadow's length) divided by (your height) will always equal (the tree's shadow's length) divided by (the tree's height). We can use this equation to find out the tree's height:  Multiply the length of the tree’s shadow by your height. If you are 5 feet (1.5 meters) tall, and the tree's shadow is 100 feet (30.48 meters) long, multiple them together: 5 x 100 = 500 (or for the meter measurements, 1.5 x 30.48 = 45.72). Divide the answer by the length of your shadow. Using the example above, if your shadow is 8 feet (2.4 meters) long, divide your answer by that number. 500 / 8 = 62.5 feet (or in metric, 45.72 / 2.4 = 19.05 meters). If you're having trouble with the math, find a tree height calculator online such as this one.

Summary:
Use this method if you only have a tape measure or ruler. Measure your height. Stand on sunny, flat ground near the tree. Measure the length of your shadow. Measure the length of the tree’s shadow. Add 1/2 the tree's width to the length of the tree's shadow. Calculate the tree’s height using the numbers you wrote down.