Estimate modeled RF power density, distance to an FCC table value, or ideal straight-wire magnetic field. These screening models do not establish a safe distance or regulatory compliance.

Assumes 1 W/mยฒ at 1 m in a far-field inverse-square model; no real source is specified.

EMF Distance Formula

The following formula is used to calculate the EMF Distance using a normalized inverse-square model for RF power density. 

D = dแตฃef ยท SQRT (Iแตฃef / I)
  • Where D is the distance from the source (same units as dref)
  • I is the RF power density at distance D (e.g., W/mยฒ)
  • dref is a reference distance (commonly 1 m)
  • Iref is the power density at dref (in this pageโ€™s normalized modes, Iref=1 W/mยฒ at dref=1 m, so D = SQRT(1/I) in meters when I is in W/mยฒ)

To calculate distance under this model, multiply the reference distance by the square root of the reference intensity divided by the intensity at the unknown distance.

How to Calculate EMF Distance?

The following example problems outline how to calculate EMF Distance.

Example Problem #1:

  1. First, determine the (normalized) power density.
    • The power density is given as 2.5 W/mยฒ.
  2. Finally, calculate the distance using the equation above with dref=1 m and Iref=1 W/mยฒ: 

D = d_ref ยท SQRT (I_ref / I)

The values given above are inserted into the equation below and the solution is calculated:

D = 1 ยท SQRT (1/2.5) = 0.632 (m)

FAQ

What is EMF intensity?

โ€œEMFโ€ is often used informally to mean electromagnetic fields (even though โ€œEMFโ€ can also mean electromotive force in volts). The โ€œintensityโ€ of electromagnetic exposure depends on what quantity you are measuring. Common measures include:

  • RF power density (far-field): W/mยฒ (used in this pageโ€™s normalized and RF transmitter modes).
  • Electric field strength: V/m.
  • Magnetic flux density: T or ยตT (used in this pageโ€™s ideal straight-wire modes).

These are different physical quantities and are not interchangeable without additional information and assumptions.

Why is calculating EMF distance important?

Distance is one of the simplest ways to reduce exposure because many sources decrease with distance. For RF sources in the far field, power density often decreases approximately with the inverse square of distance. The RF distance mode uses the whole-body MPE table in 47 CFR 1.1310 as a screening criterion for 30โ€“100000 MHz. It assumes continuous power, with maximum averaging windows of 30 minutes for general-population exposure and 6 minutes for controlled exposure. Portable-device SAR, localized limits, near-field effects and other sources require separate assessment. A model output does not certify compliance or safety.

Can the EMF distance formula be used for any type of electromagnetic radiation?

No. The inverse-square distance relationship is an approximation that is most appropriate for a point-like source in free space in the far field. It can be inaccurate in the near field, indoors (reflections/absorption), for directional antennas, and for low-frequency magnetic-field sources (which often require different models). It is also not intended for ionizing radiation (such as x-rays), which uses different safety and attenuation models.