Tree Diameter Calculator | Circumference to Diameter

Tree Diameter Calculator

A tree diameter calculator finds the diameter of a tree trunk from its circumference. It applies a basic rule of circles: divide the distance around a circle by pi (π) to get the distance straight across it.

Foresters, arborists, and gardeners measure trunk circumference with a tape measure because it is easy to wrap around a tree. Diameter is harder to measure directly, so it is usually calculated instead.

How to calculate tree diameter from circumference

Formula

For any circle, circumference and diameter are linked by a fixed ratio:

C = π × D

• C is the circumference, the distance around the trunk.
• D is the diameter, the distance across the trunk through its center.
• π (pi) is a constant close to 3.14159.

To get diameter from a measured circumference, rearrange the formula:

D = C ÷ π

The calculator also works in reverse. Enter a diameter and it multiplies by π to get the circumference: C = D × π.

Example

A trunk has a circumference of 94.25 cm.

D = 94.25 ÷ π ≈ 94.25 ÷ 3.14159 ≈ 30.00 cm

The trunk is about 30 cm across.

Units

The calculator accepts centimeters, meters, inches, or feet. It does not convert between units. The diameter it returns is in the same unit as the circumference entered, so pick one unit and use it for the whole measurement.

How to measure a tree's circumference

Foresters measure circumference at a fixed height on the trunk, called breast height, so measurements can be compared between trees and over time.

1. Use a flexible tape measure.
2. Measure at breast height: 1.37 m (4.5 ft) above the ground in the United States and Canada, or 1.3 m in most other countries.
3. Wrap the tape around the trunk, level and snug, not twisted.
4. Read the circumference where the tape crosses its zero mark.
5. For odd trunks: measure above any bulge or root flare, measure on the uphill side of a leaning tree, and measure each stem separately if the tree forks below breast height.

A circumference taken this way, once converted to diameter, is called diameter at breast height, or DBH. DBH is the standard tree-size measurement used in forestry.

What tree diameter is used for

• Forestry: estimating the volume of wood in a tree and tracking growth over years.
• Arboriculture: judging a tree's health and stability, and setting doses for trunk treatments that scale with size.
• Ecology: estimating how much carbon a tree stores, since a wider trunk generally holds more carbon.
• City planning: many tree-protection rules require a permit to remove a tree above a set diameter.

Worked examples

Each diameter is the circumference divided by π, rounded to two decimal places.

Frequently asked questions

Why measure circumference at breast height?

A fixed height, usually 1.3 to 1.37 m above the ground, avoids the flare and root swelling near the base of a trunk. It lets people compare measurements between different trees, or the same tree over time.

Is the circumference-to-diameter formula accurate for real trees?

It assumes the trunk is a perfect circle, which most trunks are not exactly. Slightly oval or irregular trunks add a small error. For most forestry, gardening, and school uses, the error is small enough to ignore.

Does the calculator convert between units, such as inches to centimeters?

No. It only divides or multiplies the number entered by π. The result comes out in the same unit as the input. Enter centimeters to get centimeters; enter inches to get inches.

What does DBH mean?

DBH stands for diameter at breast height. It is the standard way foresters record tree diameter, measured 1.37 m above the ground in the US and Canada, and 1.3 m in most other countries.

How can I measure circumference without a tape measure?

Wrap a piece of string or non-stretch cord around the trunk, mark the point where it meets itself, then measure that length of string with a ruler.

Does bark count toward the diameter?

Standard forestry practice measures over the bark. This is called diameter outside bark. Diameter inside bark, which excludes the bark layer, is used only for specialized timber calculations.