EMF Exposure Calculator

Introduction to far-field radiofrequency exposure assessment

This calculator answers one narrowly defined question: given a radio transmitter of known power and antenna gain, how much radiofrequency energy passes through a square metre of space at a stated distance, and how does that compare with the published exposure limits? The quantity it produces is incident power density, written $S_{\mathrm{inc}}$, expressed in watts per square metre. From that single number the plane-wave electric field strength, magnetic field strength and magnetic flux density follow directly, and all four can be checked against the reference levels in the ICNIRP 2020 radiofrequency guidelines and the maximum permissible exposure tables the United States Federal Communications Commission publishes in 47 CFR 1.1310.

The scope matters as much as the arithmetic. The calculation covers non-ionising radiofrequency fields from roughly 100 kHz to 300 GHz, which is where heating is the established mechanism of interaction and where the exposure limits are written in terms of specific absorption rate and absorbed power density. It does not cover the 50 Hz or 60 Hz magnetic fields around power lines and household wiring, which are governed by a separate document, the ICNIRP 2010 low-frequency guidelines, and by a different mechanism, the induction of electric fields in nerve and muscle tissue. It does not cover ionising radiation at all. A reference table further down gives the power-frequency numbers so you can see the difference in magnitude, but the calculator itself is a radiofrequency tool.

Two honest caveats belong at the top rather than buried in a footnote. First, this is a screening estimate for a single dominant source in free space; it is the same first-pass calculation an engineer runs before deciding whether a site needs a full assessment, not a substitute for a calibrated broadband survey. Second, being below a reference level is a statement about compliance with a standard, not a universal declaration of safety, and being above one is a signal to measure and mitigate rather than a prediction of harm. The limits themselves already carry reduction factors of 10 for workers and 50 for the general public relative to the thermal thresholds the standards bodies identified.

How to use each input on this exposure form

Work through the fields in order. Transmitter power is the average power actually delivered to the antenna, after feedline loss, not the peak envelope power of a modulated signal and not the power rating on the amplifier. You can enter it in watts, kilowatts, milliwatts or dBm; the calculator converts dBm with $P={10}^{(\mathrm{dBm}-30)/10}$ watts.

Antenna gain is the gain in the direction you care about, which for a screening calculation is normally the main-beam gain because that is the worst case. Enter it as dBi, as dBd, or as a plain numeric ratio. Gain in dBd is referred to a half-wave dipole and is converted internally by adding 2.15 dB, equivalently multiplying the numeric gain by 1.64, exactly as FCC OET Bulletin 65 instructs when converting effective radiated power to equivalent isotropically radiated power. Distance is measured from the centre of radiation of the antenna to the point where a person would stand, in metres or feet. Frequency selects which reference level applies, and it is the input that most calculators of this type ignore; because the limits vary by more than an order of magnitude across the spectrum, ignoring it produces a compliance percentage that is simply wrong.

Three optional inputs sharpen the estimate. Largest antenna dimension lets the page test whether your distance is genuinely in the far field, using the aperture criteria in OET Bulletin 65. Ground reflection lets you add the enhancement that occurs when the direct wave and a reflected wave combine at a surface: choose free space for no reflection, the EPA model for a 1.6-fold field enhancement, or the 100 percent reflection worst case for a doubling of field strength. Transmit duty factor is the fraction of the averaging window during which the source actually transmits, which is how intermittent sources such as a repeater or a radar are time-averaged. Finally, choose the exposure tier that matches the people involved. Press Calculate exposure, or drag the what-if distance slider to sweep the result across a range of distances and watch the chart update.

Formula set: power density, field strength and free-space impedance

Radiated power spreads over the surface of an expanding sphere of area $4\pi r^2$. An antenna with numeric gain $G$ concentrates that power, so the on-axis far-field power density is the equation given as Equation 3 in FCC OET Bulletin 65:

The numerator is the equivalent isotropically radiated power, so the same relation is often written with EIRP in place of the product:

Gain quoted in decibels is converted to a numeric ratio before it enters the equation:

In a plane wave the electric and magnetic fields are orthogonal, in phase, and tied together by the characteristic impedance of free space $Z_0$. ICNIRP gives the relation as Equation 19 of its 2020 guidelines and the FCC gives the same relation in Section 1 of OET Bulletin 65:

Rearranged for the two field quantities the calculator reports, with $Z_0$ taken as 376.730 ohms and rounded to 377 ohms in both standards:

The wavelength that decides where the far field begins is

ICNIRP places the boundary between the reactive near field and the radiative near field at $\lambda /2\pi$, and OET Bulletin 65 notes that for most antennas the outer edge of the reactive near field is commonly taken as half a wavelength from the antenna surface. For an aperture antenna of largest dimension $D$, the same bulletin gives the extent of the Fresnel near field and the onset of the far field as

Neither criterion alone is sufficient, because they describe different antennas. The aperture expression dominates for a dish or a panel, where $D$ is many wavelengths; the half-wavelength-from-the-surface rule dominates for an electrically small radiator such as an HF whip, where $D$ is a fraction of a wavelength and the aperture expression would place the far field absurdly close. The calculator therefore takes the far-field onset as the largest of the three, treating the antenna dimension you supply as the extent of the radiating structure:

If you leave the antenna dimension blank the calculator falls back to half a wavelength and says so, because without knowing how big the radiator is the onset of the far field cannot be established. Reflections are handled by a multiplier on power density. Assuming total reflection at a ground plane or rooftop doubles the field and therefore quadruples power density, which OET Bulletin 65 writes as Equation 6; the EPA model the bulletin cites assumes a more realistic 1.6-fold field increase, giving a factor of 2.56 in Equation 7:

Because the limits are time-averaged rather than instantaneous, a source that transmits only part of the averaging window is averaged down by its duty factor $d$, the fraction of the 6-minute or 30-minute window during which it radiates:

Inverting the power-density equation gives the minimum compliance distance, the radius at which the time-averaged density falls to the reference level $S_{\lim }$:

When several sources of different frequencies illuminate the same point, ICNIRP requires that the exposure ratios add rather than each being checked in isolation:

Finally, the unit conversion that causes more errors than any other in this field. One milliwatt per square centimetre is ten watts per square metre, because a square metre contains ten thousand square centimetres and a watt contains a thousand milliwatts:

Worked example: a 50 W VHF repeater seen from 20 metres

A volunteer emergency network proposes a 150 MHz repeater on a short mast. The transmitter delivers 50 W to a 6 dBi collinear antenna whose longest dimension is about 1 metre, and the nearest first-floor window is 20 metres away on the main beam. Free space is assumed, the duty factor is left at 100 percent for a worst case, and the tier is general public.

Numeric gain is ${10}^{6/10}=3.981$, so the EIRP is 50 x 3.981 = 199.05 W. The spherical surface at 20 metres has area 4π x 400 = 5026.5 m², so the incident power density is 199.05 / 5026.5 = 0.0396 W/m², which is 0.00396 mW/cm² or 3.96 μW/cm². The electric field is the square root of 0.0396 x 376.73, which is 3.86 V/m; the magnetic field is 3.86 / 376.73 = 0.01025 A/m; the magnetic flux density is 1.2566 x 10^-6 x 0.01025 = 1.29 x 10^-8 T, that is 0.0129 μT or 0.129 mG.

The wavelength is 299792458 / 150000000 = 2.00 m. The reactive near field ends around 2.00 / 2π = 0.32 m; for a 1 m antenna the aperture criterion gives 0.6 x 1 / 2.00 = 0.30 m and the half-wavelength-from-the-surface criterion gives 1.00 + 1 = 2.00 m, so the far field begins at about 2.0 metres and 20 metres is comfortably inside it, which is what makes the formula valid here. At 150 MHz the ICNIRP 2020 general public whole-body reference level is 2 W/m² and the FCC general population limit is 0.2 mW/cm², which is the same 2 W/m². The result is therefore 1.98 percent of both, and 0.40 percent of the 10 W/m² occupational reference level. Rearranging for the compliance distance, the general public level would be reached at the square root of 199.05 / (4π x 2) = 2.81 metres on the main beam, which is well inside the mast structure. That is the kind of specific, checkable statement a planning committee can act on.

Reading the result and comparing it with the published limits

The result panel reports the raw physical quantities first and the compliance ratios second, because those are two different kinds of claim. The physical quantities are the output of a model; the compliance ratios depend on which standard and which tier you selected. A ratio of 20 percent means the time-averaged density at that point is one fifth of the reference level for the tier you chose. Ratios below 100 percent indicate compliance with the reference level; ratios above it do not automatically mean the basic restriction is exceeded, because reference levels are deliberately conservative proxies, but they do mean the screening calculation has failed and a measurement or a full assessment against the basic restrictions is required.

The table below reproduces the whole-body reference levels for exposure averaged over 30 minutes from Table 5 of the ICNIRP 2020 guidelines alongside the FCC maximum permissible exposure values from 47 CFR 1.1310 as printed in OET Bulletin 65. Note that below 30 MHz ICNIRP gives no incident power density reference level at all, because near that part of the spectrum the electric and magnetic fields are not reliably coupled and each must be checked separately; the FCC values in that range are labelled plane-wave equivalent and are supplied for reference only.

The last row is the ICNIRP 2010 low-frequency guideline and is included only for scale: 200 μT is 2 gauss, and a typical living room a few metres from household wiring sits in the range of a few hundredths of a microtesla. It is not produced by this calculator, which is a radiofrequency tool, and mixing the two families of limits is a category error rather than a conservative choice.

Typical sources, and what the numbers usually look like

Some calibration helps. A domestic Wi-Fi access point radiating 0.1 W into a 3 dBi antenna produces about 0.016 W/m² at one metre, roughly 0.16 percent of the 10 W/m² ICNIRP general public level that applies at 2.4 GHz. A cellular macro sector radiating 20 W into a 17 dBi panel produces about 0.088 W/m² at 30 metres on the main beam at 900 MHz, about 2 percent of the 4.5 W/m² reference level at that frequency. A 10 kW effective radiated power FM transmitter at 100 MHz produces roughly 0.033 W/m² at 200 metres, under 2 percent of the 2 W/m² level. A point-to-point microwave dish is the interesting exception: 0.25 W into a 38 dBi antenna at 18 GHz gives about 0.05 W/m² at 50 metres, but the same dish produces very high on-axis densities within a few metres of the feed, which is exactly why such links are mounted where nobody can stand in the beam.

Handsets are the case this calculator deliberately does not cover. A phone against the head is in the reactive near field, where incident power density has no useful meaning; compliance there is assessed as specific absorption rate averaged over 10 g of tissue, restricted by ICNIRP to 2 W/kg for the general public head and torso and by the FCC to 1.6 W/kg averaged over 1 g. If your question is about a phone, the number you want is its published SAR, not anything on this page.

Limitations, assumptions and where this model breaks

The model assumes a single source radiating into free space with a smooth main-beam gain, and it inherits every limitation of that assumption. It does not model multipath, so a metal roof, a parapet or a nearby facade can create standing-wave hotspots the calculation will miss even when the reflection multiplier is enabled. It does not model the antenna pattern, so a point off the main beam receives far less than the calculator predicts, sometimes by 20 dB or more, which means the result is conservative for most real observers and correct only for someone standing directly in the beam. It does not model absorption or shielding by buildings, vegetation or the human body itself.

The far-field assumption is the most important limitation. Inside the near field the equation over-predicts, which OET Bulletin 65 explicitly endorses as a worst-case screening use, but the number returned there should be treated as a bound rather than an estimate, and ICNIRP is clear that within the reactive near field the incident power density cannot be used to demonstrate compliance at all: the basic restrictions must be assessed instead. The calculator flags the zone rather than hiding the number, so you can see when this applies.

Time-averaging is a second source of error in both directions. The duty factor input assumes the source is either fully on or fully off within the averaging window, which is a reasonable model for a repeater or a rotating radar but a poor one for an adaptive base station whose power tracks traffic. Where the exposure varies during the window, the correct procedure is to average the power density itself, not to average the transmitter power. Third, the calculator treats one source at a time; where several transmitters share a site, the summation rule shown above must be applied across all of them, and a site that passes for each source individually can still fail collectively.

Finally, a note on what the limits represent. Both ICNIRP 2020 and the FCC rules are grounded in thermal effects, with reduction factors applied to the thresholds identified in their literature reviews. IARC classified radiofrequency electromagnetic fields as Group 2B, possibly carcinogenic to humans, in Monographs Volume 102 in 2013, on the basis of limited evidence for glioma among heavy mobile phone users, and classified extremely low frequency magnetic fields as Group 2B in Volume 80 in 2002. Group 2B denotes limited evidence rather than established causation. A calculator can tell you the field strength; it cannot settle that literature, and a low percentage on this page should be read as compliance with a conservative engineering limit, no more and no less.