Specific Absorption Rate: SAR Testing for Devices

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Specific absorption rate (SAR) measures how fast body tissue absorbs radio energy from a transmitter, in watts per kilogram. Under FCC rules, a device used within 20 cm of the body must meet SAR limits such as 1.6 W/kg over 1 gram and needs an evaluation unless it qualifies for an exemption in 47 CFR 1.1307(b)(3).

A watch, pendant or handheld with a radio may need a SAR evaluation before its FCC grant, on top of the radio emission tests. Whether it does depends on three things you choose during design: how close the antenna sits to the body, how much power the radio sends, and how often it sends. Every rule below links to eCFR or fcc.gov, read in November 2026.

This is an engineering summary of public FCC texts. It is not legal advice, and an FCC-recognized lab and your certification body make the final call on your device.

What is specific absorption rate?

The FCC describes SAR as a measure of the rate of radio frequency energy absorption by the body from the source being measured (FCC consumer guide, as of November 2026). The limits are stated in watts per kilogram (W/kg). Under 47 CFR 1.1310(a), SAR is the measure used for human exposure from 100 kHz to 6 GHz. Above 6 GHz, portable devices are evaluated against the maximum permissible exposure (MPE) field limits instead (47 CFR 2.1093(d)(1), as of November 2026).

SAR applies to portable devices. Section 2.1093(b) defines a portable device as one designed to be used so that its radiating structure is within 20 centimeters of the user's body. A device normally kept 20 cm or more from the user and nearby people is a mobile device under 47 CFR 2.1091, and it is assessed with the MPE field limits (as of November 2026). A desk speaker may count as mobile, but only if real use keeps that distance.

For a short take on SAR in an AI companion product, see AI companion compliance for teens and older adults.

What is the FCC SAR limit for head, body and extremities?

The limits are in 47 CFR 1.1310. Consumer devices use the general population column, and 2.1093(d)(6) says no advisory or manual instruction changes that (as of November 2026).

Source: 47 CFR 1.1310(b) and (c), eCFR up to date as of November 4, 2026, read November 6, 2026.

A wrist-worn device meets two numbers. The case against the wrist is an extremity, judged against 4 W/kg over 10 grams. Raised to the mouth to talk, it is judged against 1.6 W/kg over 1 gram. That 1.6 W/kg is also the public exposure limit the FCC SAR page gives for cell phones (as of November 2026).

When does a device need SAR testing, and when is it exempt?

Under 47 CFR 2.1093(c)(1), a portable device with one RF source needs evaluation when its available maximum time-averaged power is above 1 mW, above the ERP in Table 1 to 1.1307(b)(3)(i)(C), or above the P_th formula, whichever is greater (as of November 2026). The exemption routes are in 47 CFR 1.1307(b)(3):

Source: 47 CFR 1.1307(b)(3) and 47 CFR 2.1093(c), as of November 2026. The P_th formula is printed as an image in the rule; read it on eCFR rather than from a copy.

Three conditions are easy to miss.

• An exemption is narrow. Section 1.1307(b)(2) says it covers only the routine evaluation. The device still needs equipment authorization and must still comply with the limits.
• You still file something. The FCC KDB 447498 page says that, during the current transition period, every certification application must include an RF exposure exhibit, also when it claims exemption from testing (KDB 447498, as of November 2026).
• Power is time-averaged and includes antenna gain. The P_th route uses the time-averaged power or the ERP, whichever is greater, so a high-gain antenna can push a source over the line.

A quick example. Nordic lists +8 dBm transmit power for the nRF52840 Bluetooth SoC (Nordic product page, as of November 2026). Sent continuously, +8 dBm is about 6.3 mW at the chip output, above the 1 mW route. The time-averaged figure depends on how often the radio sends, and ERP adds the antenna gain, so the lab confirms which route applies.

How do antenna placement, power and duty cycle change SAR?

Separation distance

Section 2.1093(c)(1) defines d as the minimum distance from any part of the antenna or radiating structure to the user's body (as of November 2026). In a wearable, the PCB position, the case wall and the strap or clip set that distance, and moving an antenna after the mold is cut often means changing the tool. For the antenna itself, see the guide on PCB antennas for small devices.

Output power

Section 2.1093(d)(1) says maximum time-averaged power levels must generally be used. The FCC consumer guide says phones are tested at their highest power in every band (as of November 2026). A power cap you want credit for must be built into the product, and the lab and your certification body decide whether it counts.

Duty factor

Consumer devices cannot use the occupational time-averaging rules. Section 2.1093(d)(4) allows source-based time averaging, based on an inherent property of the RF source such as a TDMA scheme, over no more than 30 minutes (as of November 2026). Section 1.1307(b)(2) defines source-based time averaging as an average based on an inherent property or duty cycle of the device. Agree the duty factor with the lab before you count on it.

How does SAR testing work for wearables and handhelds?

The FCC consumer guide describes the method for phones. The lab fills standardized head and body models with tissue-simulating liquid, runs the device at its highest power in every band in set positions, and a robotic probe measures the electric field on a grid. Only the highest SAR value per band goes into the authorization (FCC consumer guide, as of November 2026).

The FCC measurement procedures page lists IEEE Std 1528-2013, KDB 865664 (SAR from 100 MHz to 6 GHz), KDB 248227 (Wi-Fi), KDB 941225 (3G and LTE) and KDB 447498 for RF exposure (FCC measurement procedures, as of November 2026). IEEE 1528 is a paid standard, and we quote nothing from it.

What the guidance says about positions

• Wrist-worn transmitters. FCC slides from the April 2012 TCB workshop say voice on a wrist watch is typically in speaker mode, held next to the mouth. 1-gram SAR applies at the mouth, tested with the front of the device 10 mm from a flat phantom and the bands strapped. 10-gram SAR applies at the wrist, tested with the back of the device touching the flat phantom and the bands unstrapped. The slides say SAR test exclusion applies to body and extremity, and that the procedures may not apply to wrist-worn devices with other use conditions (FCC TCB workshop slides, as of November 2026). They predate the current KDB 447498 attachments, so ask the lab which procedure it will follow.
• Body exposure for portable devices. Interim guidance posted April 2, 2025, written with handsets, tablets, laptops and similar rectangular devices in mind, says body exposure for 2.1093 portable devices must be shown at no more than 5 mm, or up to 25 mm when an on-body holder is shown in the grant exhibits. The FCC presents it as an interim alternative to parts of KDB 447498, 616217 and 648474 on body testing at zero mm, so ask the lab which approach it will apply (KDB 447498, as of November 2026).
• Which guidance version. D04 v01 and D01 v06 can both be used during the transition period that began August 13, 2022, but an application must follow one of them entirely.

Which design choices should you make before the SAR lab?

Start from how the product is worn or held. The rows are engineering reasoning from the rules above, not accounts of a specific project.

Two cases need extra care. A device with several radios, such as LTE, Wi-Fi and Bluetooth, is judged on the sum of fractions in 1.1307(b)(3)(ii)(B) when they transmit in the same averaging period. And a pre-certified module's grant may not cover a body-worn host, so read what a pre-certified LTE module covers first.

The order that follows from the rules: fix the wear positions and the minimum antenna distance on the enclosure drawing, run the exemption arithmetic with the lab using real power and gain figures, then build test samples with the final enclosure and firmware power settings.

What should you ask your hardware team?

Bring these questions to the design review where the enclosure and antenna positions are set, and write down who answered each one.

• Is the product portable (within 20 cm of the body) in any use a buyer can expect, including holding it or wearing it?
• What is the minimum distance from every antenna to the body in each wear or hold position, measured on the real enclosure?
• What are the time-averaged power and ERP of each radio, and where did the antenna gain come from?
• Which exemption route in 47 CFR 1.1307(b)(3) do we expect to use, and has the lab agreed with the arithmetic?
• Which radios can transmit in the same time-averaging period, and what is the sum of their fractions?
• Does the duty factor we plan to use come from an inherent property of the transmission, and will the lab accept it?
• Does a pre-certified module grant cover our wear position, or will the host need its own RF exposure evaluation?

Gate 0 Feasibility plans the FCC and lab path for the device before the enclosure is drawn.

If you are planning a wearable or handheld with a radio and want the SAR question answered early, tell us about your device.

Sources. All pages read November 6, 2026. eCFR text was up to date as of November 4, 2026. Rules and KDB guidance change, so check the live pages before you rely on them.

• 47 CFR 2.1093, RF exposure evaluation for portable devices (eCFR)
• 47 CFR 2.1091, RF exposure evaluation for mobile devices (eCFR)
• 47 CFR 1.1307, including (b)(2) definitions and (b)(3) exemptions (eCFR)
• 47 CFR 1.1310, RF exposure limits (eCFR)
• FCC KDB 447498, Mobile and Portable Device RF Exposure Procedures, publication date April 2, 2025, with attachment D04 Interim General RF Exposure Guidance v01 (FCC KDB)
• FCC, Specific Absorption Rate (SAR) for Cell Phones: What It Means for You (fcc.gov)
• FCC, Specific Absorption Rate (SAR) for Cellular Telephones, updated August 22, 2019 (fcc.gov)
• FCC, Equipment Authorization Measurement Procedures (fcc.gov)
• FCC OET, RF Exposure Procedures, TCB workshop slides, April 2012 (fcc.gov)
• Nordic Semiconductor, nRF52840 product page (nordicsemi.com)