Tree Girth Calculator: Measure Trunk Size

This calculator works out a tree’s girth (trunk circumference) from its diameter, or its diameter from its girth, using the standard C = π × d relationship. You enter either the trunk diameter or the measured circumference, and you get the corresponding value in your chosen unit. The result is the same figure a forester or arborist would record when measuring tree size for growth records, heritage listings, or timber volume estimates.

Formula

The relationship between a circle’s circumference (girth) and its diameter is:

Girth (C) = π × diameter (d)

where:

• C = trunk circumference (girth), in centimetres or inches
• π ≈ 3.14159
• d = trunk diameter, in the same unit as C

To reverse it: d = C ÷ π. This assumes the trunk cross-section is a perfect circle, which is rarely true, but it is the accepted approximation used in forestry and arboriculture.

Worked Example

Using the default values from the calculator: a trunk diameter of 30 cm.

1. Start with the formula: Girth = π × diameter.
2. Plug in the diameter: Girth = 3.14159 × 30 cm.
3. Multiply: Girth = 94.2477 cm.
4. Round to 1 decimal place: 94.2 cm.

The calculator shows 94.2 cm as the girth, matching this step-by-step result. If you instead enter a girth of 94.2 cm, it divides by π to return a diameter of 30.0 cm.

Why Measure Tree Girth?

Tree girth is one of the most common measurements taken by foresters, ecologists, and tree enthusiasts. It is used for:

• Growth tracking: Measuring the same tree yearly shows how fast it is adding wood.
• Carbon storage estimates: Girth correlates with total tree biomass, which is used in carbon cycle studies.
• Heritage and champion tree registers: Many countries list notable trees by their girth at breast height.
• Timber volume: Girth (or diameter) feeds into formulas that estimate usable wood volume.
• Ecological surveys: Forest health and biodiversity assessments often record girth as a proxy for tree age and size.

Because girth is easier to measure in the field than diameter (you just wrap a tape around the trunk), it is the standard field measurement in many parts of the world.

How to Measure Girth Correctly

The most widely used standard is girth at breast height (GBH), measured at 1.3 metres (about 4.5 feet) above the ground on the uphill side of the tree. Here is the proper procedure:

1. Find the point 1.3 m up the trunk. On a slope, measure from the highest ground contact point.
2. Wrap a flexible tape measure (cloth or metal) around the trunk at that height, keeping it horizontal.
3. Pull the tape snug but not tight enough to compress bark.
4. Read the circumference. If the tree has a bulge, buttress, or branch at that height, measure slightly below or above the irregularity and note where you measured.

For trees with multiple stems, measure each stem separately and record them individually. For very large trees, a tape may not reach around; use a string and then measure the string with a ruler.

Common Mistakes and Misconceptions

A frequent mistake is confusing diameter with radius. The formula uses diameter, not radius. If you measure the radius (half the diameter), you must double it before using the calculator, otherwise your girth will be half of the true value.

Another error is measuring at the wrong height. Girth changes with height, so a measurement at 30 cm above ground will be larger than at 130 cm. Always record the height at which you measured.

People also assume the trunk is a perfect circle. In reality, trunks are often oval or irregular. The girth measurement itself is exact, but converting it to diameter assumes a circle, which introduces a small error. For most purposes this is acceptable, but for precise timber volume calculations, foresters often measure diameter at two perpendicular angles and average them.

Finally, do not compress the bark. On thick-barked species like cork oak, a tight tape can give a reading several centimetres smaller than the true girth. Use a consistent, gentle tension.

Units and Conversions

The calculator works in any unit you choose, but you must be consistent. If you enter the diameter in centimetres, the girth comes out in centimetres. If you enter inches, you get inches.

Common conversions:

• 1 inch = 2.54 cm
• 1 foot = 30.48 cm
• 1 metre = 100 cm

For example, a tree with a 50 cm diameter has a girth of about 157 cm (50 × 3.14159). In inches, that is roughly 61.8 inches (157 ÷ 2.54).

Limitations of the Formula

The C = π × d formula is exact for a perfect circle, but tree trunks are not perfect circles. The error is usually small (a few percent) and is generally ignored in field work. However, for trees with deep furrows, buttresses, or irregular growth, the girth measurement can overestimate the actual cross-sectional area.

Also, the formula gives you the circumference, not the tree’s age or health. Girth alone cannot tell you a tree’s age because growth rates vary by species, site, and climate. A 100 cm girth oak could be 50 years old on a good site and 150 years old on a poor one.

Frequently Asked Questions

What is the standard height for measuring tree girth?

The most common standard is 1.3 metres (4.5 feet) above ground level, often called “breast height.” This is used by most forestry and tree survey protocols worldwide. On sloping ground, measure from the uphill side of the tree to minimise errors from root flare.

Can I measure girth with a regular tape measure?

Yes, any flexible tape works. A cloth or fibreglass tape is best because it conforms to the trunk. A metal tape can also work but may be stiff. If the tree is too large for your tape, wrap a rope or string around it, mark the overlap, and then measure the string on a flat surface.

Why do some trees have a girth but no diameter?

Girth is the direct field measurement, so it is often recorded without converting to diameter. Diameter is a derived value that assumes a circular cross-section. Some databases list both, but girth is the raw data.

Does bark thickness affect the measurement?

Yes. The tape measures the outside of the bark, not the wood inside. For species with very thick bark, such as cork oak or redwood, the girth can be several centimetres larger than the wood girth. This is usually acceptable for general records, but for growth studies you may need to subtract bark thickness.

How do I measure a tree with multiple trunks?

Measure each trunk separately at breast height and record each girth. Do not combine them into one number. If you need a single value for a register, some programmes use the largest trunk, others sum the cross-sectional areas. Check the rules of the specific register you are submitting to.

What is the difference between girth and diameter?

Girth is the distance around the trunk (circumference). Diameter is the straight-line distance across the trunk through the centre. They are related by the formula C = π × d. Girth is easier to measure in the field, while diameter is often used in volume and growth equations.

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in d | Trunk diameter | cm | 30 | 0
in c | Trunk circumference (girth) | cm | 94.2 | 0
out girth | Girth (circumference) | cm | pi * d | 1
out diameter | Diameter | cm | c / pi | 1