The range of an electromagnetic pulse depends entirely on its source. A small, non-nuclear EMP device might affect electronics within a few hundred meters. A nuclear weapon detonated at high altitude can produce a pulse covering thousands of kilometers, potentially blanketing an entire continent. Natural solar events can affect the whole planet at once.
High-Altitude Nuclear EMP: The Largest Range
The most powerful type of EMP comes from a nuclear weapon detonated high above the Earth’s surface, known as a high-altitude electromagnetic pulse (HEMP). The range scales directly with detonation altitude because the pulse travels in a line of sight from the blast point to the Earth’s surface. The higher the detonation, the more of the curved Earth it can “see,” and the larger the area it blankets.
At 30 kilometers altitude, a nuclear detonation produces a pulse with roughly a 600-kilometer radius. That’s enough to cover an area the size of Washington, Oregon, and Idaho combined. At 400 kilometers altitude, the radius expands to about 2,200 kilometers. A single detonation at 300 kilometers or more over the central United States could expose the entire continental U.S. to substantial EMP fields, according to Department of Defense assessments.
This relationship follows basic geometry. The pulse radiates outward from the detonation point until it hits the horizon created by Earth’s curvature. You can approximate the line-of-sight distance to the horizon using a simple formula: distance in kilometers equals roughly 3.57 times the square root of altitude in meters. At 400 kilometers (400,000 meters), that works out to about 2,257 kilometers, which lines up closely with the military estimates.
How Field Strength Drops With Distance
Range isn’t just about whether the pulse reaches a location. It’s about whether it arrives with enough energy to damage anything. Electric field strength follows an inverse square relationship with distance, meaning that doubling the distance from the source cuts the field strength to one quarter. A pulse that fries circuits at 100 kilometers may only cause brief glitches at 500 kilometers.
How much field strength is needed to actually break electronics? Testing by the Department of Energy found that simple semiconductor components began failing at field strengths around 256 volts per meter, with about a 19% failure rate. At 460 volts per meter, the failure rate climbed to roughly 69%. Modern microchips, which pack far more transistors into smaller spaces, are generally more vulnerable than the components tested in those older experiments. The practical takeaway: electronics closer to the detonation point face near-certain damage, while those at the outer edges of the pulse’s footprint may survive, depending on how well they’re shielded and how sensitive their components are.
Non-Nuclear EMP Devices
Non-nuclear EMP weapons, sometimes called high-power microwave (HPM) devices, use powerful batteries or chemical reactions to generate a concentrated electromagnetic burst. Their range is dramatically smaller than a nuclear HEMP. Most tactical devices are effective from a few meters to a few hundred meters, with some larger military systems potentially reaching a kilometer or two under ideal conditions.
The tradeoff is precision versus coverage. These devices can target a specific building, vehicle, or piece of equipment without affecting an entire region. They’re also far more practical to build and deploy, which is why they get more attention in discussions about infrastructure security. But their limited range means they’re a localized threat, not a continental one.
Solar Storms: Planetary-Scale Events
The natural equivalent of an EMP comes from severe space weather. When the sun ejects a massive burst of charged particles toward Earth, it can create what’s called a geomagnetic disturbance (GMD). Unlike a nuclear EMP, which is a single fast pulse, a solar storm produces slower, longer-duration fluctuations in Earth’s magnetic field. These fluctuations induce currents in long conductors like power lines and pipelines.
The range of a severe solar storm is effectively global. Space weather bulletins describe effects on a planetary scale, though the intensity varies by location. Higher geomagnetic latitudes (closer to the poles) experience stronger and more frequent effects. During intense storms, however, the affected zone expands toward the equator, putting more populated regions at risk. The 1989 Quebec blackout, caused by a geomagnetic storm, knocked out power for nine hours across the province. A Carrington-level event, the kind of extreme storm that occurs roughly once every century or two, could potentially disrupt power grids across multiple continents simultaneously.
Comparing EMP Types by Range
- Non-nuclear EMP devices: Meters to roughly 1 kilometer. Useful for targeted disruption of specific electronics or facilities.
- Nuclear HEMP at 30 km altitude: Approximately 600 km radius. Covers a multi-state region.
- Nuclear HEMP at 300-400 km altitude: Approximately 2,200 km radius. A single burst can cover the entire continental United States.
- Severe solar storm: Planetary scale, with strongest effects at higher latitudes. Duration can last hours to days rather than the microseconds of a nuclear pulse.
Why Range Numbers Vary So Much
When you see wildly different range figures for EMPs, it’s usually because people are conflating different types of events or different definitions of “range.” A pulse can technically reach a location without carrying enough energy to damage anything there. The functional range, where electronics actually fail, is always smaller than the theoretical footprint.
Terrain, shielding, and the vulnerability of specific equipment all play a role. A device inside a metal enclosure may survive a pulse that destroys an unprotected device right next to it. Long cables and power lines act as antennas, collecting the pulse energy and funneling it into connected equipment, which is why power grids are especially vulnerable even at the outer edges of an EMP’s reach. Military facilities designed to survive an EMP use shielded enclosures, surge protectors at every cable entry point, and strict limits on exposed wiring, with cables restricted to no more than 200 meters and kept below 7 meters in height to reduce energy pickup.
The bottom line: range depends on the source. A homemade device is a room-scale threat. A nuclear weapon detonated in the upper atmosphere is a continent-scale threat. A major solar storm is a planet-scale threat. The physics are the same in each case, just applied at vastly different energy levels.