Distance is what you are told, area is what you do
Every wildfire preparedness conversation is conducted in feet. A number of feet from the structure, a second number further out, sometimes a third. That is how the rules are written and it is a sensible way to write them, but it is a terrible way to plan a weekend, because feet are not what you clear. You clear square feet, and the relationship between the two is not linear.
The area inside a ring at distance d around a rectangular footprint is the footprint itself, plus the perimeter multiplied by d, plus pi times d squared. That last term is the corners, and it is the term that runs away from you. Take the placeholder distances in the box, which are not a recommendation and not a requirement, purely as arithmetic. Around a 48 by 32 foot house, the ground inside the middle one is roughly 7,600 square feet once the footprint is taken out. Move the boundary out to the third and the figure is roughly 47,400 square feet. The distance went up by a factor of 3.3 and the ground went up by a factor of 6.2.
That is the single most useful thing this page does. It stops you agreeing to a project whose size you have never calculated.
The band against the wall is the small one
Run the default placeholder figures and the innermost band comes out at under 900 square feet, which is under two percent of everything inside the outer boundary. It is a strip you can walk in a couple of minutes. Meanwhile that band is where wind-carried embers accumulate, because an ember that lands on open ground keeps moving and an ember that reaches a wall, an inside corner, a deck-to-wall junction or the base of the siding stops there and stays.
This page does not tell you what to do about that, and it will not tell you what belongs in that band or how far out it should run, because those figures come from the authority that adopted them for your area. What the arithmetic can honestly show you is the trade: the innermost band is a rounding error in area and hours against the whole project. Whatever your fire authority asks you to do in it costs you almost nothing compared with the outer ring.
Debris is a volume problem, and it is bigger than the pile looks
| Step | What changes | Why it catches people out |
|---|---|---|
| Standing vegetation | Cleared area multiplied by average height | Sounds enormous and is mostly air |
| Cut and piled | A single-digit percentage of that | Still fills a driveway. The default here is 8 percent and it is a guess until you cut a measured patch |
| Chipped | Roughly a third of the loose pile | Chipping is the difference between four trips and twelve |
| Hauled | Loads of whatever you can borrow | Trailer volume, not weight, is usually the binding constraint on green material |
The bulking percentage is the weakest number on this page and it is deliberately an input. Cut ten by ten feet of your own brush, pile it, measure the pile, and you will have a figure for your own property that beats any published average. If the material is going out as firewood rather than chips, the firewood cord calculator handles the stacked volume side, and log volume covers anything with a diameter worth measuring.
Rings do not respect property lines
A ring at 100 feet around a house on a quarter-acre lot is mostly not on that lot. It crosses into neighbouring land, over a road, into a right of way, or onto ground held by somebody else entirely. Cutting vegetation you do not own is a legal question and occasionally an expensive one, and it is why the share-you-control field exists. Set it honestly. A plan that assumes access you do not have is not a plan.
Trees are their own subject and the rules around them are separate again — many jurisdictions protect specific species or sizes, and root damage from clearing equipment kills trees slowly enough that nobody connects the two. The root protection zone calculator is the place to work out the ground around a trunk that machinery should stay off, and stump grinding volume deals with what is left after removal.
What this page will not do
It will not tell you what any zone distance should be, whether slope changes it, what counts as cleared, what may stay, what spacing applies between plants or crowns, or whether your property meets anything. Those figures differ by state, by county, by fire district and by insurer, they get revised, and several of them are conditions of an insurance policy rather than of a code. Get them from your fire authority or state forestry agency, get the insurance version from your insurer in writing, and take structural questions to the building department. The related work on the house itself — screening openings, hardening the roof line — is covered by the vent screening takeoff, and there is background reading in the home fire safety guide.
Questions people ask
How far should defensible space extend from my house?
This page will not tell you, and it is worth understanding why. Zone distances are set by state law in some places, by county or fire district ordinance in others, and by an insurance policy in others again, and the same property can be subject to more than one of them at once. They also get revised. The distances in the input box are placeholders labelled as such, put there so the arithmetic has something to chew on. Get the real figures from your fire authority or state forestry agency, get the insurance version from your insurer in writing, and put those numbers in.
Why does the area grow so much faster than the distance?
Because a ring gets longer as it moves outward. The area added by pushing a boundary from d to d plus one foot is roughly the perimeter at d, and the perimeter keeps growing. Formally, the ground inside a ring at distance d around a rectangle is the footprint plus the perimeter times d plus pi d squared, and the squared term dominates once d gets large relative to the building. Around an ordinary house, and taking the placeholder distances in the input box purely as arithmetic, moving from the middle boundary to the outer one is roughly six times the ground for roughly three times the distance.
How accurate is the debris volume?
The area and the standing volume are geometry and they are exact for the shape you entered. Everything downstream of them rests on the bulking percentage, which is how much of that standing volume ends up as an actual pile, and that number varies with species, season, moisture and how you cut. The default of eight percent is a working figure for mixed brush and it is an input for exactly that reason. Clear a measured ten by ten patch, pile it, measure the pile, and substitute your own number. That single measurement is worth more than any published average.
Does the calculator account for slope?
Only through a multiplier you supply. Some authorities extend zone distances on sloping ground, using factors that depend on the steepness and on which side of the house the slope falls away. Others handle it differently or not at all. Since this page cannot know which applies to you, the slope field is a plain multiplier applied to every distance you entered, and it sits at 1 until you are given a figure. It also does not adjust the area for the fact that sloping ground has more surface than its plan view, which for moderate slopes is a small effect next to the uncertainty in everything else.
Can I clear vegetation on my neighbour property to make my zones work?
Not on the strength of a calculation. Cutting vegetation you do not own is a legal matter, and in many places so is cutting protected species or trees above a certain size on your own land. The share-you-control field is there to make the constraint visible rather than to work around it: set it to what you actually control, and the cleared area drops accordingly. If your zones genuinely need ground you do not own, that is a conversation with the neighbour, with the road authority or with your fire authority, and possibly with a lawyer, before it is a conversation with a chainsaw.