The formula, and the part of it people skip
Height equals eye height plus horizontal distance times the tangent of the angle to the top. Stand 100 feet from the trunk, read 30 degrees up to the highest twig, and with your eye at 5.5 feet the tree is 5.5 + 100 x tan(30), which is 5.5 + 57.74, so 63.2 feet. Nothing about that is difficult.
The word doing the work is horizontal. Tangent is a ratio of vertical rise to horizontal run, and the number you feed it has to be the run. If you stretched the tape uphill to the trunk, or paced up a bank, the figure you wrote down is the hypotenuse, not the run, and it is longer than the run. On a 10 degree slope a 100 foot tape is 98.5 feet of horizontal distance. That is a small error, about nine tenths of a foot on a 63 foot tree, but it is a systematic one that always goes the same way, and it grows fast: on 20 degrees the same tape is only 94 feet horizontal. The slope field on this calculator does that correction. If you do not know the slope, walk the tape level and let one end hang, or measure from a spot on the same contour as the base.
One angle or two
The single-angle version adds your eye height to the rise. That is only correct when the base of the tree is at the same elevation as your feet. Stand uphill and you are looking down at the base, so your eye is more than your eye height above it; stand downhill and the reverse.
The two-angle version fixes this without you having to know anything about the ground. Read the angle up to the top, then the angle down to the base, and the height is the distance times the sum of the two tangents. Your eye height drops out of the arithmetic entirely, which is the point. If you are below the base — looking uphill at both the top and the bottom of the tree — the two angles are on the same side and you subtract rather than add, which the form handles through the above-or-below setting.
| Situation | Method that works | What goes wrong otherwise |
|---|---|---|
| Level ground, clear sightline | One angle plus eye height | Nothing much |
| You are uphill of the tree | Two angles | Eye height understates how far above the base you are |
| You are downhill of the tree | Two angles, base above eye | Adding both tangents doubles part of the slope |
| Dense understorey, base hidden | One angle, from a known level spot | You cannot see the base to read the second angle |
| No instrument at all | Stick method | Nothing, but the result is coarse |
The stick method, and why it works
Cut or pick a straight stick the same length as the distance from your eye to your closed fist with your arm held straight out. Hold it upright at arm's length so the bottom of the stick appears to sit on the base of the tree. Walk backwards or forwards until the top of the stick appears to touch the highest point. Stop. The distance from you to the trunk is the height of the tree.
It works because you have built two similar triangles: stick length over arm length equals tree height over your distance. Make the first ratio exactly one and the second ratio has to be one too. It needs no instrument and no arithmetic, and it is accurate to roughly ten percent in the hands of someone careful on flat ground, which is usually plenty for working out whether a tree will reach a shed. It falls apart on slopes, in wind, and when you cannot see the base and the top from the same standing position.
The highest point is rarely above the trunk
This is the error that separates a rough figure from a good one. On a mature broadleaf tree the tallest twig is often several feet horizontally away from the stem, out over one side of the crown. Reading the top angle to that twig while reading the base angle to the trunk mixes two different triangles and the answer is wrong, sometimes by a lot on a wide-crowned or leaning tree.
The correction that professional measurers use is to find the point on the ground directly beneath the highest twig — sight straight down from it, or have someone stand where you direct them — and measure horizontal distance to that point rather than to the trunk. On a strongly leaning tree the effect is larger still, because the whole crown is displaced. Standing so that the lean runs across your view rather than toward or away from you removes most of it.
Once you have a height, the questions it usually feeds are how much wood is in the stem, which the tree volume and log yield calculator covers, and how far the crown will eventually reach, which is the planting question in the tree spacing calculator. If the tree already exists and something is about to be built near it, the root protection zone calculator is the one that matters. A measured height is not, on its own, information about whether a tree can reach anything in a way that should worry you.
Where this calculator stops
It measures and it sizes. It does not tell you whether a tree is safe or hazardous, and no number on this page should be read as saying so. Tree risk assessment is a professional discipline with its own training and its own vocabulary, and it depends on defects, species, targets and site history that a form cannot see.
The other line is the work itself. Felling a tree kills people every year, and it kills experienced people, not only beginners. So does anything done off the ground. A tree under tension does not behave the way it looks — a leaning stem, a hung-up top, a bent sapling or a limb pinned by another one holds stored energy that releases in a direction you did not choose. A chainsaw injury an hour from a road is a categorically different problem from the same cut in a shop with a phone and a driveway. Nothing on this site gives felling, notching, limbing or rigging technique, and none of it is a substitute for training.
Any tree near a structure, a fence line, a road or a power line belongs to a qualified arborist. Utility clearance work in particular is restricted to trained line-clearance crews in most places, and the reason is that electricity crosses a gap to a conductive object such as a wet branch or an aluminium ladder without being touched.
Questions people ask
How accurate is a phone inclinometer for tree height?
Better than most people expect and worse than a real clinometer. The sensor itself is usually good to a fraction of a degree; the problem is holding the phone steady while sighting along its edge at a twig sixty feet up. Take three readings, discard any that disagree with the other two, and expect the result to be within about five percent on a clear sightline. The larger error is almost always the distance rather than the angle — at a 30 degree sight, five feet of distance error moves the answer nearly three feet, while one degree of angle error moves it about two. Pace carefully, or better, use a tape.
Why does my two-angle reading disagree with the one-angle reading?
Because one of them is being applied on ground it does not suit. The one-angle method assumes the base of the tree is level with your feet, and adds your eye height on that assumption. If you are standing even slightly uphill, your eye is higher above the base than your eye height, and the single-angle answer comes out short. The two-angle method makes no assumption about the ground at all, so where they disagree, the two-angle number is generally the one to keep. If they disagree wildly, check that you are entering the base angle as a magnitude and that the above-or-below setting matches where you are actually standing.
Can I measure a tree I cannot get a clear line of sight to?
Not reliably, and this is the honest limit of the tangent method. You need to see the base or a known point on the stem, see the highest point, and know the horizontal distance between yourself and the point beneath that highest point. In dense woodland one or more of those is usually missing. Foresters in that situation use a laser rangefinder with a built-in inclinometer that shoots to the top directly, which removes the pacing entirely. Without one, find the least obstructed direction, accept a coarser figure, and treat it as a bracket rather than a measurement.
Does the calculator tell me if the tree can reach my house?
No, and it is worth being blunt about why. A height and a distance would let you draw a circle, but whether a tree falls, where it goes if it does, and whether any of that is likely are questions about defects, root condition, species, soil, exposure and history. That is tree risk assessment, it is a professional discipline with its own qualifications, and a form has none of the inputs. Use the height as a fact about the tree. If the question behind it is a worry about a structure, that is a call to a qualified arborist rather than a calculation.
What is the difference between crown spread and canopy diameter?
They are usually the same idea under different names, but the convention for a recorded measurement is specific: pace the widest width of the crown as projected on the ground, then the narrowest width taken at right angles to it, and average the two. A single width overstates a lopsided crown badly. The average spread is what champion-tree registers use alongside height and trunk circumference, and it is also the number you want when planning where a young tree will eventually reach, which the tree spacing calculator uses directly.