Where the extra volume comes from
Grind a 24 inch stump 12 inches below grade with no flare and you have removed about 3.1 cubic feet of solid wood. That is a small figure and it feels like the answer. It is not the answer, because you now have chips rather than wood, and loose chips carry air between them. At a two-fold expansion the pile is 6.3 cubic feet, near a quarter of a cubic yard, and that is before the grinder starts picking up soil below grade.
Expansion is a range rather than a constant, which is why it is an input here rather than baked in. A coarse cutting wheel throwing large chips fluffs more than a fine one; wet or stringy wood packs differently from dry brittle wood; and how much the operator lets the pile get walked on changes it again. Somewhere between one and a half and two and a half times solid volume covers most of what people actually see. If you have ground stumps before, use what you remember rather than the default.
The soil percentage is the other half of the surprise. Below grade the wheel does not distinguish between root and dirt, and a substantial share of what comes off the machine is mineral soil and small stones. That fraction is why grindings are a poor mulch — more on that below — and it is why the pile is heavier than a wood volume suggests.
The flare is most of the work
People measure the cut face because it is a flat circle they can put a tape across. The grinder does not care about the cut face; it has to sweep everything the root plate occupies down to the depth you asked for. On a mature tree the base at ground level can be six inches to a foot wider than the stem, sometimes far more on a buttressed species, and the area goes up with the square of the diameter. Going from a 24 inch stump to a 30 inch working circle is a 56 percent increase in the area being ground, and the volume rises with it.
| Depth ground | What it is enough for | What it is not enough for |
|---|---|---|
| 4 to 6 in | Laying turf, spreading soil and mowing over it | Planting, digging, anything structural |
| 8 to 12 in | Planting a shrub or a small replacement tree nearby | Planting in the same hole, footings, paving |
| 12 to 18 in | Bed preparation, most landscaping work | Foundations, a slab, a post in the same spot |
| Below 18 in | Specialist work, usually with excavation as well | Nothing a grinder alone reliably clears |
Note what none of these depths achieve: removing the roots. A grinder takes the stump and the flare. The lateral roots stay in the ground and rot in place over years. If the reason for the removal is that something has to be built or dug where the tree was, grinding is the wrong tool and excavation is the right one, which is a volume question the excavation volume calculator handles.
Backfilling with grindings, and why the ground sinks
The convenient move is to push the grindings straight back into the hole. It works, with one caveat that everybody discovers the following spring: the fill settles. Chips consolidate under their own weight and then the wood decomposes, and the hole reappears as a saucer in the lawn. Mounding the backfill twenty to thirty percent proud of grade is the standard answer, along with expecting to top it up once.
The second caveat is nitrogen. Fresh wood chips decomposing in contact with soil pull nitrogen out of it, and anything planted directly into a bed of grindings tends to go yellow and sulk. Grindings are usable as a surface mulch on paths and around established plantings where that matters less; they are a poor growing medium and a poor bed amendment. If you want mulch depth and coverage figures for the surplus, the mulch calculator converts volume to area, and if the plan is to compost the pile instead, the compost amendment calculator covers the carbon-to-nitrogen side of it. To buy soil to fill the hole properly instead, the topsoil calculator is the one.
What decides whether this is a job you take on
A stump grinder is a machine with an exposed carbide wheel spinning fast enough to throw a stone through a window, and the debris field extends much further than people expect in every direction, including toward the operator. Hidden objects in and around old stumps are routine — rocks, fence wire, buried concrete, old nails, and utility services that were laid close to a tree line years ago. Striking a service is not a machine problem, it is an emergency, and getting utilities located before any ground disturbance is not optional in most places.
One more thing that catches people out: in a great many places you need a permit to remove a tree, including a tree on land you own, and in some places to prune one over a certain size. Protected species, heritage listings, conservation overlays, subdivision covenants and street-tree rules all exist and none of them are consistent between jurisdictions. This page cannot tell you which apply to you and does not try. Ask your local authority before the saw comes out, because the penalties are usually assessed on the tree rather than on the intent.
Trees on or near a boundary are a legal question before they are a horticultural one. Who owns a trunk that straddles the line, what you may do to branches or roots that cross it, and what you owe a neighbour if your cutting kills their tree are all answered by local rules and, sometimes, by where the line actually runs rather than where the fence does. Cutting a neighbour's roots or branches has consequences that can run well past the cost of the tree. If the line matters, get a survey and read the local rules before anyone starts.
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 many cubic yards of grindings will a stump make?
Take the working diameter including the root flare in feet, square it, multiply by 0.785, multiply by the total depth in feet including anything above grade, and multiply the result by your expansion factor. Divide by 27 for cubic yards. A 30 inch working circle ground 12 inches deep is 4.9 cubic feet of wood, which at two-fold expansion is a shade under 10 cubic feet, or about 0.36 cubic yards, plus whatever soil the wheel picks up. A row of six such stumps is over two cubic yards, which is a trailer rather than a barrow.
Do I have to grind the roots as well as the stump?
A grinder does not remove roots and no depth setting changes that. It removes the stump and the flare, and the lateral roots stay in the soil to decay over a period of years. That is usually fine for turf and for most planting, since the roots break down and the ground gradually recovers. It is not fine if something has to go in that spot: a footing, a post hole, a slab, a trench or a paved surface all hit roots that grinding never touched. In that case the removal is an excavation job, priced and planned differently.
Can I plant a new tree in the same hole?
Not well, and not straight away. A ground stump leaves a pocket of decaying wood and grindings that is short of nitrogen, holds water badly, and settles unevenly under a new root ball. The common recommendation is to plant a few feet to one side rather than in the same spot, or to excavate the grindings out, replace with topsoil, and let it settle for a season first. If the site is fixed and there is no room to move, that is a conversation with someone who can look at it rather than a rule.
Are stump grindings good mulch?
They are acceptable on paths and around established woody plants, and poor for beds. Two reasons. First, decomposing fresh wood in contact with soil ties up nitrogen at the interface, so anything shallow-rooted planted into it struggles. Second, grindings from below grade carry a meaningful fraction of soil, grit and small stones, which makes them heavier, less uniform and more likely to introduce whatever was in that soil. They are also a possible route for pathogens if the tree came out because it was diseased, which is a reason not to spread them around related species.
Why does the ground sink where the stump was?
Because the fill is doing two things over time and both reduce its volume. Loose grindings consolidate mechanically under rain and foot traffic within the first months, and then the wood itself decomposes over years until it is no longer there at all. The depression that shows up is normal rather than a sign that anything was done badly. Mounding the backfill above grade by a quarter of its depth absorbs most of the first stage, and expecting to add soil once after the first winter absorbs the rest. Filling with topsoil instead of grindings avoids most of the second stage at the cost of buying soil.