What formula does this calculator use?

Power density S = P·G/(4πd²) and E = √(Z₀·S), with Z₀ = 376.730313 Ω. That is practically the familiar E = √(30·P·G)/d, where P·G is the EIRP in watts and d is in metres. H = E/Z₀. These hold only in the far field of the antenna, in free space.

How do I calculate RF power density at a distance?

Multiply transmitter power by linear antenna gain to get EIRP, then divide by the area of a sphere: S = EIRP/(4πd²). Example: 100 W into a 10 dBi antenna is 1,000 W EIRP; at 10 m, S = 1000/(4π·100) = 0.796 W/m² = 0.0796 mW/cm², and E = √(376.73 × 0.796) = 17.3 V/m.

What are the FCC RF exposure (MPE) limits?

Under 47 CFR 1.1310 the general-population limit is 0.2 mW/cm² (27.5 V/m) from 30 to 300 MHz, f/1500 mW/cm² from 300 to 1,500 MHz and 1.0 mW/cm² from 1.5 to 100 GHz, averaged over 30 minutes. Occupational limits are five times higher in power density (1.0, f/300 and 5 mW/cm²) with 6-minute averaging. Below 30 MHz the limits are set in field strength (614 V/m, then 824/f or 1842/f V/m).

What are the ICNIRP 2020 reference levels?

ICNIRP 2020 Table 5 (whole body, 30-minute average) gives 2 W/m² (27.7 V/m) for the general public from 30 to 400 MHz, f/200 W/m² from 400 to 2,000 MHz and 10 W/m² from 2 to 300 GHz; occupational levels are 10 W/m², f/40 and 50 W/m². Between 0.1 and 30 MHz only E and H levels apply (300/f^0.7 V/m public, 660/f^0.7 V/m occupational).

Why include a ground-reflection factor of 1.6?

FCC OET Bulletin 65 recommends assuming that a ground reflection can add in phase with the direct wave, raising the field by up to 1.6 times (2.56 times the power density) for predictions near ground level. It makes the estimate conservative; it is not a measured value.

What distance is required for the far-field formula to be valid?

The far field (Fraunhofer region) begins at roughly 2D²/λ, where D is the largest antenna dimension and λ the wavelength, and the distance should also be several wavelengths. For a 2 m panel at 900 MHz (λ = 0.333 m) that is 24 m. Closer in, the field does not fall off as 1/d and these equations can under- or over-estimate.

How is dBµV/m calculated from V/m?

dBµV/m = 20·log10(E in V/m × 10⁶). For example 1 V/m = 120 dBµV/m and 3 V/m = 129.5 dBµV/m. Magnetic field in dBµA/m uses the same form with A/m.

Does a result under the limit prove RF-exposure compliance?

No. This is a far-field, free-space screening estimate. A compliance evaluation must also account for the antenna pattern, near-field conditions, duty cycle and averaging time, all transmitters at the site, and the rules of the applicable jurisdiction. Use it to decide whether a formal evaluation is needed.

RF Field Strength Calculator

Calculate ideal far-field EIRP, E-field (V/m, dBµV/m), H-field (A/m) and power density, then see the percentage of FCC and ICNIRP 2020 limits and the far-field distance at which each limit is met. A screening estimate, not a compliance report.

Transmitter Parameters

Transmitter Power

Antenna Gain (dBi)

Distance from Antenna

Frequency

Largest antenna dimension D (m, optional)

Include ground reflection (OET-65: field × 1.6, power density × 2.56)

Quick Presets

Enter transmitter parameters and click Calculate

Electric Field Strength

—

V/m (volts per meter)

Power Density

—

W/m²

Field Strength

—

dBuV/m

EIRP

—

Power Density

—

mW/cm²

Magnetic Field

—

A/m

Magnetic Field

—

dBµA/m

Calculation Details

Exposure screening at —

Limit	Limit S	% of limit	Distance to meet
FCC general population	—	—	—
FCC occupational	—	—	—
ICNIRP 2020 general public	—	—	—
ICNIRP 2020 occupational	—	—	—

Limit S is the plane-wave-equivalent power density in W/m² (field-strength limits below 30 MHz are converted with S = E²/Z₀). Distance is the far-field point where the limit is reached on the main beam, with the reflection option applied.

Far-field check

—

Distance for a target field strength

V/m

—

Ideal far-field free-space screening estimate. A result under a limit does not prove RF-exposure compliance: antenna pattern, near field, duty cycle, averaging time, other transmitters and local rules all matter.

Formula Reference

E (V/m) ≈ sqrt(30 × P_EIRP) / d

P_EIRP (W) = P_tx (W) × 10^G_dBi/10

S (W/m²) = P_EIRP / (4πd²)

E (V/m) = sqrt(376.730313 × S)

E (dBuV/m) = 20 × log10(E × 10⁶)

H (A/m) = E / 376.730313

H (dBµA/m) = 20 × log10(H × 10⁶)

d_limit (m) = sqrt(k × P_EIRP / (4π × S_limit)), k = 1 or 2.56

Far field: d > 2D²/λ and d > λ, λ = 299.792458 / f_MHz

RF exposure limits: FCC MPE and ICNIRP 2020

f in MHz. FCC 47 CFR 1.1310 Table 1 (power density in mW/cm²; 1 mW/cm² = 10 W/m²) and ICNIRP 2020 Table 5 whole-body reference levels (W/m²). * plane-wave equivalent.

Standard	Frequency (MHz)	E (V/m)	H (A/m)	Power density	Averaging
FCC general population	0.3–1.34	614	1.63	100* mW/cm²	30 min
FCC general population	1.34–30	824/f	2.19/f	180/f²* mW/cm²	30 min
FCC general population	30–300	27.5	0.073	0.2 mW/cm²	30 min
FCC general population	300–1,500	—	—	f/1500 mW/cm²	30 min
FCC general population	1,500–100,000	—	—	1.0 mW/cm²	30 min
FCC occupational	0.3–3.0	614	1.63	100* mW/cm²	6 min
FCC occupational	3.0–30	1842/f	4.89/f	900/f²* mW/cm²	6 min
FCC occupational	30–300	61.4	0.163	1.0 mW/cm²	6 min
FCC occupational	300–1,500	—	—	f/300 mW/cm²	6 min
FCC occupational	1,500–100,000	—	—	5 mW/cm²	6 min
ICNIRP 2020 general public	0.1–30	300/f^0.7	2.2/f	—	30 min
ICNIRP 2020 general public	>30–400	27.7	0.073	2 W/m²	30 min
ICNIRP 2020 general public	>400–2,000	1.375·f^0.5	0.0037·f^0.5	f/200 W/m²	30 min
ICNIRP 2020 general public	>2,000–300,000	—	—	10 W/m²	30 min
ICNIRP 2020 occupational	0.1–30	660/f^0.7	4.9/f	—	30 min
ICNIRP 2020 occupational	>30–400	61	0.16	10 W/m²	30 min
ICNIRP 2020 occupational	>400–2,000	3·f^0.5	0.008·f^0.5	f/40 W/m²	30 min
ICNIRP 2020 occupational	>2,000–300,000	—	—	50 W/m²	30 min

Worked example: 100 W, 10 dBi, 900 MHz, 10 m

EIRP = 100 W × 10^10/10 = 1,000 W.
S = 1000 / (4π × 10²) = 0.796 W/m² = 0.0796 mW/cm².
E = √(376.73 × 0.796) = 17.3 V/m (√(30 × 1000)/10 = 17.3 V/m); H = 0.046 A/m.
FCC general-population limit at 900 MHz = 900/1500 = 0.6 mW/cm² (6 W/m²), so the point is at 13% of the limit.
The limit is met beyond d = √(1000 / (4π × 6)) = 3.64 m, or 5.83 m with the 2.56 ground-reflection factor.

Method, sources and privacy

The E/H relationship assumes a plane wave in ideal free space and is intended for far-field engineering estimates. It does not model antenna near fields, reflections, terrain, duty cycle, simultaneous transmitters or regulatory exposure averaging.

Privacy: parameters and results remain in this browser tab; this calculator sends no RF inputs to a calculation service.

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Summary

This RF field strength calculator applies the far-field free-space equations S = P·G/(4πd²) and E = √(30·P·G)/d to compute power density (W/m², mW/cm²), electric field (V/m and dBµV/m) and magnetic field (A/m) at a distance from a transmitting antenna. Add the frequency to compare the result with the FCC 47 CFR 1.1310 MPE limits and the ICNIRP 2020 whole-body reference levels, see the minimum far-field distance for each limit, solve for the distance at a target field strength, and check whether the point is in the antenna far field. An optional OET-65 ground-reflection factor (1.6 in field, 2.56 in power density) gives a conservative estimate.

How it works

Enter transmitter power (W, mW, kW or dBm) and antenna gain in dBi; EIRP = P × 10^(G/10).
Enter the distance and the frequency. Optionally add the antenna's largest dimension D for the far-field check.
Power density S = EIRP/(4πd²); E = √(Z₀·S) with Z₀ = 376.730313 Ω (≈ √(30·EIRP)/d); H = E/Z₀.
Tick the ground-reflection option to multiply E by 1.6 and S by 2.56, as FCC OET Bulletin 65 does for conservative predictions.
The exposure table divides S by each frequency-dependent limit and solves d = √(k·EIRP/(4π·S_limit)) for the far-field distance where the limit is reached.
The far-field check compares d with the Fraunhofer distance 2D²/λ and with one wavelength; closer than that, these equations are not reliable.

Use cases

Screen an antenna installation against FCC general-population and occupational MPE limits before a formal evaluation.
Compare the same transmitter against ICNIRP 2020 reference levels used outside the US.
Find the distance at which a transmitter produces a target field strength such as 3 V/m or 10 V/m for EMC immunity planning.
Convert a calculated V/m result to dBµV/m or A/m.
Check whether a measurement or calculation point is in the antenna far field.
Cross-check EIRP, inverse-square and link-budget calculations.

Frequently Asked Questions

Last updated: 2026-10-09 · Reviewed by Nham Vu