Calculate EMF power density, safe RF distance, or magnetic flux density from distance, transmitter settings, frequency, and current.

EMF Distance Calculator

Inverse-Square
RF Power
Magnetic Field

Enter exactly one value to calculate the other

This tab uses a normalized inverse-square model (I โˆ 1/dยฒ) for quick back-of-the-envelope estimates. It assumes a reference of Iref = 1 W/mยฒ at dref = 1 m (so I = 1/dยฒ in W/mยฒ).

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 tab, 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 Inverse-Square and RF Power tabs).
  • Electric field strength: V/m.
  • Magnetic flux density: T or ยตT (used in this pageโ€™s Magnetic Field tab).

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. In workplace and consumer settings, distance estimates are also used when evaluating compliance with regulatory exposure limits (such as FCC MPE for RF).

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.