Power density
Radiofrequency energy flow per unit area — µW/m² on consumer RF meters, mW/cm² in FCC rules, W/m² in ICNIRP guidelines. 1 mW/cm² = 10 W/m² = 10,000,000 µW/m².
Last verified 12 August 2026.
Also searched as: S, power flux density, µW/m², mW/cm²
- Symbol
- S
- SI unit
- W/m²
- Units in use
- W/m², mW/m², µW/m², mW/cm², µW/cm²
What it measures
Radiofrequency exposure in the far field. The single most confused quantity in EMF discussion, because regulators, guideline bodies and consumer meters each prefer a different unit and differ by factors of 10,000 or more.
Units
Multiplier to the SI unit (W/m²).
- W/m² (watt per square metre)
- 1 W/m² = 1 W/m²
- mW/m² (milliwatt per square metre)
- 1 mW/m² = 0.001 W/m²
- µW/m² (microwatt per square metre)
- 1 µW/m² = 1 × 10^-6 W/m²
- mW/cm² (milliwatt per square centimetre)
- 1 mW/cm² = 10 W/m²
- µW/cm² (microwatt per square centimetre)
- 1 µW/cm² = 0.01 W/m²
Relationships to other quantities
- E (V/m) = √(S (W/m²) × 377)
- Far-field relation. 1 V/m ≈ 2653 µW/m²; 6 V/m ≈ 95,500 µW/m²; 61 V/m ≈ 10 W/m².
- S = EIRP / (4πd²)
- Free-space power density at distance d from an isotropic radiator of a given equivalent isotropically radiated power. Ignores reflections, absorption and antenna pattern, so it is an upper-bound estimate on boresight and an over-estimate elsewhere.
Related records
- FCC general population MPE, 1.5–100 GHz
1.0 mW/cm² power density averaged over 30 minutes — the band that covers most cellular, Wi-Fi and 5G frequencies in the United States.
Exposure limits
- Building Biology SBM-2024 RF band: no anomaly
Below 0.1 µW/m² peak per RF source in a sleeping area is graded 'no anomaly' — a precautionary target eight orders of magnitude below the FCC limit.
Exposure limits
Sources
- nist-si The International System of Units (SI), NIST Special Publication 330 — National Institute of Standards and Technology. government, link checked 2026-08-12.
- itu-fs-relations Free-space path loss and power density relations (ITU-R P.525) — International Telecommunication Union. standard, link checked 2026-08-12.
- fcc-1310 47 CFR § 1.1310 — Radiofrequency radiation exposure limits — U.S. Government Publishing Office (eCFR). regulation, link checked 2026-08-12.
- icnirp-2020 Guidelines for Limiting Exposure to Electromagnetic Fields (100 kHz to 300 GHz) — International Commission on Non-Ionizing Radiation Protection, 2020. guideline, link checked 2026-08-12.
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