⚡ RF Power Density Calculator

RF power density describes how concentrated electromagnetic energy is at a point: the power passing through a unit area, usually quoted in mW/cm², W/m² or µW/cm². For a transmitter radiating into free space the power spreads over a sphere, so the density at distance r from an isotropic radiator is S = EIRP / (4πr²), where EIRP is the effective isotropic radiated power. Real antennas concentrate the energy, so the EIRP already includes the antenna gain.

The same formula is used in three very different places: EMF safety assessment (is this exposure under the ICNIRP or FCC limit?), link planning (what signal reaches the receiver?), and radar (the echo power density at the target). The calculator takes EIRP (or TX power and gain) plus a distance, and returns the density in the units EMF reports prefer, alongside the relevant exposure-limit comparison.

The calculator first builds EIRP from TX power and antenna gain (if you only have EIRP, enter it directly). It then applies S = EIRP/(4πr²) and converts the result between mW/cm², W/m² and µW/cm² (1 W/m² = 100 µW/cm²). The optional frequency band selection reports the corresponding ICNIRP public and occupational limit for a quick sanity check.

The derived safe exclusion distance (distance at which density drops below the public limit) is the most actionable output for installers and site managers — it directly answers "how far should people stay?" Pair the density result with the Friis path loss calculator for a complete picture of a link.

1. Enter EIRP in dBm, or TX power and antenna gain separately.
2. Enter the distance from the antenna in metres or feet.
3. Optional: pick the frequency band to compare with the ICNIRP limit.
4. Click Calculate to see the density in mW/cm², W/m² and µW/cm².
5. Use the safe exclusion distance output for site planning.

What is the difference between power density and field strength?

Power density S (W/m²) and field strength E (V/m) describe the same wave: in free space S = E²/377. Power density is the natural unit for plane waves and reports, while E/H are used for near-field and compliance meters. The calculator focuses on the far-field density used in most standards.

How far is the far field for my antenna?

Roughly 2D²/λ, where D is the antenna aperture and λ the wavelength. For a 60 cm dish at 5 GHz that is about 1.2 m; for a tall base-station panel at 900 MHz it can be 5–15 m. Closer than that, the simple inverse-square law overestimates or underestimates the true density.

Does the calculator sum multiple transmitters?

No. It evaluates one source at a time. For a multi-antenna site, compute each source separately and add the densities (they are powers, so they add linearly). The safe-distance output then refers to the dominant source.

Why do cellular antennas need exclusion zones on rooftops?

Base-station panels can deliver hundreds of watts EIRP in a narrow beam. At 5–10 m in front of the panel the density can approach or exceed the public limit, so operators fence off the beam area even though the density a few metres to the side is trivial.

Are WiFi routers a risk according to these numbers?

A typical 100 mW EIRP router gives about 0.8 µW/cm² at 1 m — hundreds of times below the ICNIRP public limit of 200–450 µW/cm² in the 2.4 GHz band. The calculator will show you this comparison directly.

What units do I need for a compliance report?

Most regulators accept W/m² or mW/cm². ICNIRP 2020 uses W/m², the FCC historically used mW/cm². The calculator outputs both plus µW/cm², and the limit comparison table uses W/m².

Always use EIRP (which includes antenna gain) when computing exposure near a directional antenna — using TX power alone underestimates the density in the main beam by the full gain figure. Remember that 1 W/m² = 100 µW/cm², a conversion people get wrong constantly. The far-field approximation S = EIRP/4πr² is only valid beyond roughly 2D²/λ from the antenna; inside the near field the density is not described by this simple law. For a roof with multiple antennas, sum the contributions from each source at the point of interest. Occupancy rules: the public limit is the binding one near walkways and windows; the occupational limit applies only where trained staff are restricted from entry.