Explosions are measured primarily by the pressure they produce, expressed in pounds per square inch (psi) or kilopascals (kPa). The two core metrics are peak overpressure, the maximum pressure above normal atmospheric levels, and impulse, which captures how that pressure accumulates over time. For larger blasts, especially nuclear detonations, yield is expressed as a TNT equivalent, translating the total energy released into the weight of TNT that would produce the same effect. These measurements, combined with scaling laws and specialized sensors, give scientists, engineers, and emergency planners a complete picture of an explosion’s destructive potential.
Peak Overpressure and Impulse
When an explosive detonates, it sends a shock wave outward that briefly raises the air pressure well above its normal level of about 14.7 psi. The highest pressure recorded during that event is the peak overpressure. In military blast testing, for example, sensors placed at specific distances from a charge routinely capture peak overpressures in the range of 70 to 88 kPa (roughly 10 to 13 psi), depending on orientation relative to the blast.
Peak overpressure alone doesn’t tell the whole story, though. A very brief spike of high pressure may do less damage than a slightly lower pressure sustained over a longer period. That’s where impulse comes in. Impulse measures the total pressure change delivered over time, expressed in units like kPa·ms (kilopascals times milliseconds). In controlled blast tests, impulse values tend to be more consistent than peak pressure readings, often holding steady even when peak pressure varies with sensor angle. One military study found impulse values clustering tightly around 33.5 kPa·ms across an entire test series, even as peak pressure readings fluctuated by more than 17 kPa.
The Shape of a Blast Wave
Blast researchers describe the pressure profile at any given point using something called the Friedlander waveform. When a shock wave arrives at a location, pressure jumps almost instantly to its peak, then decays back to atmospheric pressure over a period called the positive phase duration. After that, pressure actually drops below atmospheric levels into a negative phase, creating a brief partial vacuum that can pull debris back toward the blast center.
The speed of that pressure decay, the duration of the positive phase, and the depth of the negative phase all influence how much damage the blast causes. Two explosions with identical peak overpressures can behave very differently if one has a much longer positive phase. Engineers use the Friedlander equation to model this curve mathematically, plugging in peak overpressure, decay time, and a waveform parameter to predict what the pressure history looks like at any distance from the charge.
TNT Equivalence
Because explosives vary enormously in their chemistry and power, the industry needed a common reference point. TNT became that standard. The energy density of TNT is about 1,160 calories per gram, and every other explosive is compared against it. If a material produces twice the blast effect of an equal weight of TNT, its TNT equivalence is 200 percent.
Determining that equivalence isn’t as simple as looking up a chemical formula. It’s measured experimentally through several standardized tests, each capturing a different aspect of explosive output. The air blast test compares peak pressure or impulse at matched distances. The plate dent test measures how deeply a charge dents a steel plate relative to TNT. The ballistic mortar test measures how high an explosion lifts a heavy pendulum compared to a 10-gram TNT charge. The sand crush test measures how much sand is pulverized. Each method can yield slightly different equivalence values for the same explosive, because each emphasizes a different physical effect.
Scaling Laws for Distance
One of the most useful tools in blast measurement is the Hopkinson-Cranz scaling law, which lets you predict how blast effects change with distance and charge size. The formula defines a “scaled distance” by dividing the actual distance from the charge by the cube root of the charge weight: Z = R / W^(1/3). If two charges of the same explosive but different sizes produce the same scaled distance value, they’ll generate the same overpressure at their respective distances.
This means a 1-kilogram charge at 10 meters produces roughly the same peak overpressure as an 8-kilogram charge at 20 meters, because the cube root of 8 is 2, and 20/2 equals 10/1. The relationship allows engineers to run small-scale tests and confidently predict what a much larger explosion would do, without actually detonating massive charges.
What Different Pressure Levels Actually Do
Overpressure numbers become meaningful when you connect them to real-world damage. NOAA and CDC data provide a detailed picture of what happens at each threshold:
- 0.15 psi: Typical threshold for glass failure.
- 0.5 to 1.0 psi: Windows shatter, some frame damage occurs.
- 1.0 psi: Houses partially demolished and rendered uninhabitable.
- 2.0 to 3.0 psi: Non-reinforced concrete or cinder block walls shatter.
- 3.0 psi: Steel-frame buildings distorted and pulled from foundations.
- 5.0 to 7.0 psi: Houses nearly completely destroyed.
- 10.0 psi: Probable total destruction of all buildings.
For the human body, the thresholds are sobering. At 1 psi, injuries from flying fragments begin. At 2 psi, flying glass and debris cause injuries. Eardrums start rupturing at around 5 psi in roughly 1 percent of people, with nearly universal rupture at 45 psi. Lung damage begins at about 15 psi. Fatalities become widespread at 5 psi due to secondary effects like debris, and at 35 to 45 psi, direct blast fatalities begin. By 55 to 65 psi, almost no one survives.
How Nuclear Yields Are Measured
Nuclear explosions operate on an entirely different scale and require distinct measurement methods. Yield is expressed in kilotons (kt) or megatons (Mt), where one kiloton was standardized during the Manhattan Project as the release of 10^12 calories of energy, roughly equivalent to 1,000 tons of TNT.
Three primary techniques are used to estimate nuclear yields. Seismic methods analyze the ground waves an underground detonation produces, much like measuring an earthquake. Stations more than 2,000 kilometers away record P-waves and surface waves, and their amplitudes are converted to yield estimates using calibrated formulas. Hydrodynamic methods measure the speed of the shock wave traveling through the surrounding rock, which requires instruments placed near the test site. Radiochemical methods analyze the nuclear byproducts of the explosion to back-calculate the energy released, though this requires both site access and potentially sensitive weapon design information.
For monitoring foreign nuclear tests, seismic methods dominate because they work at great distances without any access to the test site. Different wave types (P-waves traveling through the Earth’s interior, Rayleigh waves and Lg waves traveling along the surface) each offer advantages. Surface waves are less affected by temperature variations deep in the Earth’s mantle, making them more reliable for certain geological settings.
The Sensors That Capture It All
Measuring a blast wave requires sensors that respond in microseconds. The most common commercial blast sensors use piezoelectric materials, typically quartz crystals, tourmaline crystals, or a polymer film called PVDF. When the shock wave hits, these materials generate an electrical signal proportional to the pressure, fast enough to capture the nearly instantaneous rise of a blast front.
Newer research is pushing toward even faster piezoresistive sensors built on miniaturized silicon membranes, as well as optical sensors that use changes in light reflection or interference patterns to measure pressure. Optical designs have the advantage of being immune to the electromagnetic interference that explosions can generate. In laboratory settings, these sensors are connected to oscilloscopes sampling at 2 billion data points per second, with a time resolution of 0.5 nanoseconds, capturing the full pressure history of a blast in extraordinary detail.