Tree Height Calculator (clinometer angles & distance)

Understanding the tree height calculator — concept and principle

You can measure a tall tree without climbing it by standing a known distance away and reading two angles with a clinometer or a phone app. This calculator uses the tangent method that foresters use and also shows how sensitive the answer is to a 1° error in the angle to the top.

20 × tan 50° above the eye plus 20 × tan 4° below it gives 25.2 m.

How it is calculated

Two-angle method: H = d × (tan αtop − tan αbase)

Eye-height method (level ground): H = d × tan αtop + eye height

Sensitivity = d × (tan(αtop + 1°) − tan αtop)

d is the horizontal distance to the trunk in metres.
αtop is the angle up to the treetop; αbase is the angle to the base of the trunk, negative when you look down.
Height is also shown in feet (÷ 0.3048).

The principle

Looking up at the treetop forms a right triangle with the horizontal line from your eye to the trunk. The height above your eye level is the opposite side, so it equals d × tan(angle). The part of the trunk below your eye level forms a second triangle when you look down at the base; its height is −d × tan(base angle). Adding the two pieces gives the total height and works on slopes, as long as d is the horizontal distance. On flat ground you can simply add your eye height instead of sighting the base.

Worked example

Default inputs: two-angle method, d = 20 m, top angle 50°, base angle −4°.

Above eye level: 20 × tan 50° = 20 × 1.1918 = 23.84 m.
Below eye level: −20 × tan(−4°) = 20 × 0.0699 = 1.4 m.
Height = 23.84 + 1.40 = 25.23 m, shown as 25.2 m = 82.8 ft.
Sensitivity: 20 × (tan 51° − tan 50°) = 20 × 0.0432 = 0.86 m per degree.

Each point is the calculator re-run with only “Horizontal distance to the trunk (m)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Horizontal distance to the trunk (m)	Tree height (m)
5	6.3
10	12.6
15	18.9
20	25.2
25	31.5
30	37.9
40	50.5

How to read the result

The sensitivity tells you how much a small reading error matters. It grows quickly as the top angle gets steeper, so stand back far enough that the angle to the top is roughly 45° or less where possible. Measuring from two different positions and averaging improves accuracy. Make sure you are sighting the true top, not a branch nearer to you; on broad-crowned trees the highest point may be hidden.

Common mistakes

Using the sloping distance instead of the horizontal distance on hills.
Forgetting the sign of the base angle: looking down is negative.
Sighting a near branch instead of the true top of the crown.
Ignoring lean; a tree leaning toward you appears taller.

Keep in mind: The method assumes the top is directly above the base. Leaning trees and hidden tops give errors; for records or safety work use a laser hypsometer and trained staff.

Frequently asked questions

What is a clinometer?

A device that measures the angle of slope or elevation; many phone apps do the same job.

How far away should I stand?

About as far as the tree is tall gives a top angle near 45°, a good balance of accuracy and visibility.

Can I use my shadow instead?

Yes, the shadow method compares a tree's shadow with a stick's shadow, but it needs sun and flat ground.

Why measure the base angle instead of using eye height?

It works on slopes, where your eye height above the tree's base is unknown.

Sources: BC BigTree website, UBC Faculty of Forestry, "Height measurements" (tangent method)

Checked 9 Nov 2026. BC BigTree website, UBC Faculty of Forestry & Environmental Stewardship, “Height measurements” (tangent method h = tan A × d + eye height; add or subtract the angle to the base when it is below or above eye level; measure horizontal distance on slopes).

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