# 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.

- Canonical URL: https://emfbase.com/quantities/power-density
- Collection: Quantities & units
- Record id: `quantities:power-density`
- Status: verified
- Last verified: 2026-08-12
- JSON: https://emfbase.com/api/quantities/power-density
- Also known as: S, power flux density, µW/m², mW/cm²

## Key values

- **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](https://emfbase.com/limits/fcc-public-power-density-above-1-5-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.
- [Building Biology SBM-2024 RF band: no anomaly](https://emfbase.com/limits/building-biology-rf-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.

## Sources

- `nist-si` National Institute of Standards and Technology. *The International System of Units (SI), NIST Special Publication 330*. https://www.nist.gov/pml/special-publication-330 — government, link checked 2026-08-12
- `itu-fs-relations` International Telecommunication Union. *Free-space path loss and power density relations (ITU-R P.525)*. https://www.itu.int/rec/R-REC-P.525/en — standard, link checked 2026-08-12
- `fcc-1310` U.S. Government Publishing Office (eCFR). *47 CFR § 1.1310 — Radiofrequency radiation exposure limits*. https://www.ecfr.gov/current/title-47/section-1.1310 — regulation, link checked 2026-08-12
- `icnirp-2020` International Commission on Non-Ionizing Radiation Protection (2020). *Guidelines for Limiting Exposure to Electromagnetic Fields (100 kHz to 300 GHz)*. https://www.icnirp.org/en/publications/article/rf-guidelines-2020.html — guideline, link checked 2026-08-12

Data licence: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Dataset version 2026.08.1. Retrieved from EMFBase, https://emfbase.com.

This record is reference data, not medical or legal advice. Regulatory limits, scientific guidelines and precautionary values are labelled separately and answer different questions.