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I did this with my 3-year-old and we spent half the walk finding leaves for his dinosaurs to eat - you’d be amazed at how picky his dinos can be! - Hannah

How big is that tree? Figure it out with this activity!

You will learn how to measure the Diameter at Breast Height (DBH) of a tree and how to calculate the height of a tree.

The simple measurement activities described below are a great practical application of geometry skills!

DBH is a common tree measurement used by foresters and arborists. DBH measurements help estimate volume, biomass, and carbon storage of trees. Foresters also utilize specialized measurement tools to calculate how many logs and boards they can harvest from a tree. You most likely aren’t looking to harvest the wood from a tree around your home, but calculating the DBH and the height of the tree can help you understand the environmental benefits that a tree provides. 

It’s fun to know the height of a giant tree. In 1940, The American Forestry Association launched a campaign to locate the largest specimens of American trees. This competition is now the National Register of Champion Trees. Check out a complete list of Nebraska State Champion Trees here. Omaha has nine State Champion Trees, and Nebraska has one National Champion Tree, an Eastern Cottonwood located in Gage County.

  • Tape measure
  • Yarn or twine
  • Marker
  • Pencil
  • Calculator
  • Two brightly colored flags, handkerchiefs, or marking sticks

Measure Diameter at Breast Height (DBH)

1. If your tree has one main trunk and is taller than 4.5 feet, use your tape measure to measure 4.5 feet (54 inches) from the base of the tree. Keep your finger on this spot.

  • If your tree has two main trunks and splits below 4.5 feet, measure just the larger of the two trunks.
  • If your tree has a large scar, canker, whorl, or bulge at 4.5 feet, measure at the skinniest point above or below the abnormality.
  • If your tree is small or has multiple trunks, measure the largest of the trunks just below where it begins to develop main branches.
  • Check out this image for how to measure other atypical tree trunks.

2. Put the end of the yarn under your finger on the spot that is 54 inches from the bottom of the tree. 

3. Wrap the yarn around the trunk of the tree, keeping your finger on the end of the yarn.

4. Use the marker to mark the size of the circumference (the distance around).

5. Take the yarn off of the tree. Use the tape measure to measure the length of the yarn from the initial point to the spot you marked.

  • Round up to the nearest inch, and write down the total as your tree’s circumference.

6. To determine the diameter of a circle (or a tree trunk), simply divide the circumference by pi (3.14). This is the diameter of your tree, or your DBH. 

  • For example, if your tree has a 15-inch circumference, you would figure 15 / 3.14 = 4.8. Your tree has a 4.8-inch diameter.

Measure Tree Height

You will need two people to do this. Assign each person a role: person one and person two.

  1. Person one stands a short distance away from the tree and holds up a pencil with their arm stretched out straight, facing the tree.
  2. Person one walks backwards (be mindful of where you’re walking) until the pencil appears to be the same height as the tree.
  3. Person two puts a flag, handkerchief, or some other bright marker on the ground at the base of the tree (this marker should be where the first person sees the bottom of the pencil).
  4. Person one keeps the bottom of the pencil so it looks like it is at the base of the tree and turns the pencil 90 degrees, so it is parallel to the ground.
  5. Then person two walks away from the tree to where person one sees the tip of the pencil (person one needs to direct person two where to go). Person two places a second marker at this location.
  6. Measure the distance between the two markers - this will tell you the approximate height of the tree.

Asking guiding questions helps your kids understand the real-world, practical application of mathematics. Pose questions like:

  • Before doing this activity, ask your kids to consider how they could measure the height of a tree - what methods can they come up with? 
  • Why would knowing a tree’s height matter?
  • Why would knowing a tree’s DBH matter?
  • Why would knowing a tree species be important for this activity? (Check back next week for a tree ID activity!)

We would love to see where this lesson led you! Share your photos and videos with us by tagging #natureinyourneighborhood and #keepomahabeautiful.

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