Tree Diameter Calculator

What do DBH and basal area mean?

DBH stands for diameter at breast height: how wide the trunk is at about chest height. Any circle is a little more than 3 times as far around as it is across, so the calculator divides your tape reading by pi (about 3.14) to get the width.

Basal area is the size of the flat circle you would see if the tree were sawn across at chest height, in square feet (ft²) or square metres (m²). Foresters add it up across a woodland to judge how much wood the trees hold.

For example, a trunk 60 inches around is about 19 inches across, and its cut face would cover about 2 square feet. The calculator works this out for you.

Where to measure DBH

US

4.5 ft above the ground

Britain and most other countries

1.3 m above the ground

The US height is the rule in the field guide the US Forest Service uses for its national tree survey, the Forest Inventory and Analysis (FIA) National Core Field Guide. In Britain the 1.3 m rule is in the Forestry Commission's tree measuring handbook. The two heights are only 7 cm (3 in) apart, so on most trunks the reading barely changes. The FIA guide covers the awkward cases:

• Flat ground: wrap the tape level around the trunk 4.5 ft above the ground.
• On a slope: stand on the uphill side of the tree and measure the 4.5 ft up from there.
• Leaning tree: measure the 4.5 ft along the trunk on the side it leans toward, following the trunk rather than going straight up.
• Trunk splits below 4.5 ft: treat each stem as its own tree and measure each one.
• Bump or swelling at 4.5 ft: measure just above it, where the trunk is back to its usual shape.

Diameter tape or ordinary tape

Foresters often carry a diameter tape. Its special markings show how wide the trunk is as soon as you wrap it around. An ordinary tape measure or a piece of string works just as well here: measure around, type the number, and the calculator works out the width for you.