Field Strength & Power Density Calculator
Convert an electric field strength in the far field to power density in free space, and express the field in dBµV/m. Enter the field strength E in volts per metre to get the power density in W/m² and mW/cm² plus the field level in dBµV/m.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the electric field strength E in volts per metre (V/m).
- Read the power density S in W/m² and in mW/cm² (the unit common in RF-exposure work).
- Read the field strength in dBµV/m for logging against measurement or emission limits.
How it works
For a plane wave in free space, power density S = E² / Z₀ where Z₀ ≈ 377 Ω is the impedance of free space, giving S in W/m². Since 1 W/m² = 0.1 mW/cm², dividing by 10 gives mW/cm². The field in decibels is dBµV/m = 20 × log₁₀(E × 10⁶), decibels relative to one microvolt per metre.
Worked example
Worked example. At E = 1 V/m: S = 1² / 377 = 0.0026525 W/m² = 0.00026525 mW/cm², and 20 × log₁₀(1 × 10⁶) = 120 dBµV/m.
Common mistakes
- Applying S = E²/377 in the near field of an antenna, where E and H are not related by 377 Ω and the formula does not hold.
- Mixing up W/m² and mW/cm² — they differ by a factor of 10, not 1000.
- Treating the result as a compliance decision instead of assessing against the applicable ARPANSA / ICNIRP / FCC exposure limits.
Frequently asked questions
Why 377 ohms?
377 Ω (more precisely 120π ≈ 376.73 Ω) is the intrinsic impedance of free space — the ratio of the electric to the magnetic field in a plane wave. It links E and power density the way resistance links voltage and power in a circuit.
Is this valid close to an antenna?
No. The S = E²/377 relationship is a far-field (plane-wave) result. In the reactive near field the E and H fields are not simply proportional, so use full near-field methods there.
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