Fireworks Safety Distance Calculator

Why fireworks need a calculated clear zone: an introduction

A firework that looks graceful from a lawn chair is a controlled explosion at close range, and the fallout from that burst — sparks, unspent stars, cardboard casing, and the occasional dud — rains down in a circle around the launch point. The whole point of a safety distance is to make sure spectators are standing outside that circle when it happens. This tool takes the size of your largest shell and returns the radius that should stay clear of people, cars, tents, and anything else you would rather not have showered with hot debris.

The number comes from the code planners lean on across the United States. NFPA 1123, Code for Fireworks Display, sets the minimum radius of the display site at 70 feet for every inch of internal mortar diameter of the largest aerial shell being fired (section 5.1.3.1 of the code). A 5-inch mortar therefore wants at least 350 feet of open ground; an 8-inch shell wants 560. Bigger shells fly higher and break wider, so the buffer scales right along with the diameter. Non-bursting devices are treated more leniently: comets and mines that do not break into a spreading cluster of stars use half the figure, 35 feet per inch of mortar diameter.

This page is an educational estimator. Display fireworks are not a do-it-yourself activity. In the United States they are fired under a permit from the authority having jurisdiction by a licensed or certified pyrotechnic operator, with insurance, a site inspection, and a fallout plan that a fire official has actually looked at. Nothing here replaces any of that. It also does not apply to the consumer fireworks sold from a roadside stand — those live under the Consumer Product Safety Commission's rules in 16 CFR Parts 1500 and 1507 and under NFPA 1124, they carry their own much shorter printed distances, and conflating the two sets of rules in either direction is a mistake.

It helps to know where the 70 ft/in figure comes from. A shell is not just a light show; it is a lifting charge that throws a sphere of pyrotechnic stars hundreds of feet into the air, and a burst charge that scatters those stars outward at the top of the climb. Most of the show is designed to burn out before it falls back down, but not all of it does. Duds that fail to ignite, half-spent stars, cardboard and clay casing fragments, and the timber-and-paper wadding from the mortar all descend somewhere. The spectator radius is sized so that this cone of fallout lands short of the crowd even when a shell breaks a little low or a gust nudges the debris sideways. The larger the shell, the more energetic the lift and the wider that fallout footprint, which is exactly why the buffer grows one-for-one with diameter rather than staying fixed.

How to use the fireworks safety distance calculator

Enter the diameter of the largest shell you plan to fire — if a show mixes 3-inch and 6-inch shells, the 6-inch number governs the whole site. Pick inches or centimeters to match how the shell is labeled; European and metric-packaged product is often listed in centimeters, and the calculator divides by 2.54 to convert before it does anything else. The device-type menu then sets the clearance factor for you: 70 ft/in for aerial shells, 35 ft/in for non-bursting comets and mines, or a custom value when a permit, drought condition, or fire official specifies something wider. Editing the factor by hand automatically switches the menu to the custom setting, so the two controls never disagree.

The wind field adds a downwind planning margin. Leave it at zero and you get the bare code radius. Raise it and the tool reports a second, larger distance for the downwind arc only — the upwind side never shrinks below the baseline, because a tailwind does not make the rule less strict. The result reports the radius in both feet and meters, the area of the circular clear zone (in square feet, square meters, and acres, so you can sanity-check it against the field you actually have), and an estimated burst altitude at about 100 feet per inch of diameter. That altitude figure is a quick way to check for overhead problems — power lines, tall trees, a stadium roof — and to position photographers who want the break centered in frame. Below the result, a size-by-size table recalculates every common shell diameter with your chosen factor, and the site diagram draws the clear zone to scale next to a football field so the acreage stops being abstract.

Reading the radius as an area matters more than it first appears. Distance grows in a straight line with shell size, but the clear zone it defines grows with the square of that distance. Doubling from a 4-inch shell to an 8-inch shell doubles the radius, yet it quadruples the ground you have to keep empty. A backyard that comfortably handles a small consumer cake can fall an order of magnitude short of what a single large professional mortar demands, and the acreage figure makes that jump obvious before anyone lights a fuse. If the number lands close to the edge of your field, treat that as a signal to size down rather than to argue with the geometry.

The radius formula and the numbers behind it

Everything is driven by the shell diameter in inches. If you entered centimeters, the tool converts first, then multiplies by the clearance factor to get the radius:

where D is the shell diameter in inches and f is the clearance factor — 70 ft/in for aerial shells under NFPA 1123, 35 ft/in for non-bursting comets and mines, or whatever larger value your permit names. The clear-zone area is just the area of that circle, and the burst altitude uses the 100-ft-per-inch rule of thumb:

The downwind figure is deliberately separate from the code minimum. This tool stretches the fallout footprint on the downwind side only, by two percent of the baseline radius for each mile per hour of sustained wind, and it never returns anything smaller than the baseline:

That two-percent-per-mph coefficient is an illustrative planning heuristic chosen for this tool, not a figure published in NFPA 1123. The code does not set a numeric wind multiplier and does not name a single wind-speed cut-off; it requires the operator and the authority having jurisdiction to judge wind speed, wind direction relative to spectators, lightning, and precipitation together and to stop the display when conditions become hazardous. Treat the downwind number as a prompt to think about drift, and treat your permit and your operator's judgment as the authority.

Sources checked: the 70 ft-per-inch display-site radius and the 35 ft-per-inch figure for non-bursting comets and mines are from NFPA 1123, Code for Fireworks Display (sections 5.1.3.1 and 5.1.3.2), which most U.S. jurisdictions adopt for permitted public displays — see the NFPA 1123 standard development page and the current NFPA 1123 edition listing; NFPA offers free read-only access to its codes. Consumer fireworks are outside that code's scope and sit under 16 CFR Part 1507 (CPSC, fireworks devices) and NFPA 1124. The ~100 ft-per-inch burst altitude is an industry rule of thumb rather than a code requirement; display-operator training material from bodies such as the American Pyrotechnics Association quotes it with the caveat that real break heights vary. Unit conversions: 1 in = 2.54 cm, 1 ft = 0.3048 m, 1 acre = 43,560 ft². Last reviewed August 2026.

A worked example: an 8-inch shell on a 500-foot field

Say a town festival wants to fire 8-inch aerial shells and uses the standard 70 ft/in factor. The radius works out to 8 × 70 = 560 feet, enclosing a clear zone of π × 560² ≈ 985,000 ft² — about 22.6 acres — with bursts near 800 feet up. If the only field available spans 500 feet, that 8-inch plan simply does not fit. Drop to 6-inch shells and the radius falls to 420 feet, which leaves room to set a secondary rope line at 450 feet as a buffer.

Now add weather. Suppose the forecast is 12 mph out of the west with the crowd standing on the west rope line, upwind. Feeding 12 mph into the calculator stretches the eastern arc to 560 × 1.24 ≈ 694 feet while the western arc that faces the crowd stays at the 560-foot code minimum. That is the whole shape of the problem in one number: the crowd side is governed by the code, the downwind side is governed by drift, and a site that only just satisfies the code with still air can stop working the moment the fallout area has to grow eastward into a parking lot or a stand of dry brush. Punching a few shell sizes and wind speeds into the calculator is the fastest way to see which product your site can actually support before you order anything.

Limitations and assumptions of the 70 ft per inch rule

This figure is a planning minimum for outdoor aerial shells fired vertically from properly sized mortars in an open field with no overhead obstructions. Knowing the limitations keeps the number honest — it is not a guarantee, and it does not cover every situation:

• Strong wind carries fallout well past the calculated radius. Keep the fallout area downwind of the crowd, widen the downwind arc, and be ready to postpone; many permits and operators impose their own wind ceiling even though the code names no single number.
• Mortars that are angled rather than fired straight up push the fallout footprint in the direction of the tilt, and NFPA 1123 requires the separation distance to grow accordingly on that side.
• Dense urban sites, very large shells, and complex choreographed shows routinely need more separation than the bare rule gives, and the authority having jurisdiction can require it.
• The tool is not meant for indoor fireworks, close-proximity stage pyrotechnics, or confined spaces — those fall under NFPA 1126 and a different set of distances entirely.
• Consumer-grade fountains, ground spinners, and roman candles behave nothing like aerial mortars; CPSC rules and the manufacturer's printed minimum distances govern instead of anything on this page.

One subtlety worth spelling out: the 70 ft/in figure describes the minimum radius of the whole display site, measured from the mortars, and the nearest spectator sits on or outside that boundary. Fire officials frequently treat the area that must be cleared of vehicles, structures, and unrelated people separately, and can set it larger for the biggest shells. When a factor other than the default appears in your permit, enter it directly so the calculator reflects the requirement you are actually held to rather than the generic baseline. The clear-zone circle should also be oriented, not just sized: keep the prevailing wind blowing the fallout away from the crowd, parking, and any dry brush, and leave yourself extra room on the downwind arc even when the raw radius looks satisfied on the upwind side.

The assumptions baked into the burst-height estimate deserve the same caution. The 100 ft/in figure describes a typical spherical shell fired from a properly buried, correctly sized mortar; canister shells, low-break effects, and tilted mortars all behave differently, and quoted rules of thumb range from roughly 70 to 100 feet per inch. Where a permit, manufacturer instruction, or fire official specifies a larger distance, that requirement takes precedence over anything here. These distances are approximations for planning discussions only — you remain responsible for complying with local law, for holding the required permit, and for engaging a licensed pyrotechnic operator before any display. Use the tool at your own risk.

Fireworks distance questions planners ask