SAR (Specific Absorption Rate)

The Specific Absorption Rate (SAR) measures the rate at which radio frequency (RF) energy is absorbed by human tissue when exposed to electromagnetic fields from wireless devices. It is expressed in watts per kilogram (W/kg) and serves as a key safety metric for evaluating electromagnetic exposure.

Need to verify SAR compliance for your wireless device? Learn more about accredited testing procedures and exposure limits on our SAR testing services page.

Regulatory Limits and Compliance

SAR values are used to demonstrate compliance with human-exposure limits. The applicable limit depends on the market, frequency range, exposure scenario, and body region. For localized exposure of the general public, key reference values include:

• European Union: 2.0 W/kg for the head and trunk, averaged over 10 g of contiguous tissue

• United States (FCC): 1.6 W/kg peak spatial-average SAR, averaged over 1 g of tissue

Because the averaging mass and test rules differ, SAR values determined under different regulatory systems should not be compared directly.

Calculation of SAR

SAR is calculated using the formula:

SAR = σE² / ρ

where:

• σ = tissue conductivity (S/m)

• E = electric field strength (V/m)

• ρ = tissue density (kg/m³)

Practical Assessment

SAR is evaluated in defined operating modes, positions, distances, and power configurations. The reported value is therefore a compliance result for the tested configuration, not a measure of a user's typical exposure during everyday operation.

Application in Product Certification

Portable wireless devices used close to the body, such as mobile phones, tablets, wearables, and some IoT devices, may require a SAR assessment. Depending on the product, frequency, output power, and separation distance, compliance may be demonstrated by measurement, calculation, or an exemption procedure under the applicable rules.