{
  "id": "quantities:power-density",
  "collection": "quantities",
  "slug": "power-density",
  "name": "Power density",
  "summary": "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.",
  "status": "verified",
  "lastVerified": "2026-08-12",
  "data": {
    "aliases": [
      "S",
      "power flux density",
      "µW/m²",
      "mW/cm²"
    ],
    "quantityId": "power-density",
    "symbol": "S",
    "siUnit": "W/m²",
    "units": [
      {
        "symbol": "W/m²",
        "name": "watt per square metre",
        "toBase": 1
      },
      {
        "symbol": "mW/m²",
        "name": "milliwatt per square metre",
        "toBase": 0.001
      },
      {
        "symbol": "µW/m²",
        "name": "microwatt per square metre",
        "toBase": 0.000001
      },
      {
        "symbol": "mW/cm²",
        "name": "milliwatt per square centimetre",
        "toBase": 10
      },
      {
        "symbol": "µW/cm²",
        "name": "microwatt per square centimetre",
        "toBase": 0.01
      }
    ],
    "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.",
    "conversions": [
      {
        "to": "electric-field-strength",
        "formula": "E (V/m) = √(S (W/m²) × 377)",
        "note": "Far-field relation. 1 V/m ≈ 2653 µW/m²; 6 V/m ≈ 95,500 µW/m²; 61 V/m ≈ 10 W/m²."
      },
      {
        "to": "power-density",
        "formula": "S = EIRP / (4πd²)",
        "note": "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": [
      "limits:fcc-mpe-public-1500-100000",
      "limits:sbm-2024-rf-no-anomaly"
    ]
  },
  "sources": [
    {
      "id": "nist-si",
      "title": "The International System of Units (SI), NIST Special Publication 330",
      "publisher": "National Institute of Standards and Technology",
      "url": "https://www.nist.gov/pml/special-publication-330",
      "kind": "government",
      "retrieved": "2026-08-12"
    },
    {
      "id": "itu-fs-relations",
      "title": "Free-space path loss and power density relations (ITU-R P.525)",
      "publisher": "International Telecommunication Union",
      "url": "https://www.itu.int/rec/R-REC-P.525/en",
      "kind": "standard",
      "retrieved": "2026-08-12"
    },
    {
      "id": "fcc-1310",
      "title": "47 CFR § 1.1310 — Radiofrequency radiation exposure limits",
      "publisher": "U.S. Government Publishing Office (eCFR)",
      "url": "https://www.ecfr.gov/current/title-47/section-1.1310",
      "kind": "regulation",
      "retrieved": "2026-08-12"
    },
    {
      "id": "icnirp-2020",
      "title": "Guidelines for Limiting Exposure to Electromagnetic Fields (100 kHz to 300 GHz)",
      "publisher": "International Commission on Non-Ionizing Radiation Protection",
      "url": "https://www.icnirp.org/en/publications/article/rf-guidelines-2020.html",
      "year": 2020,
      "kind": "guideline",
      "retrieved": "2026-08-12"
    }
  ],
  "links": {
    "self": "https://emfbase.com/api/quantities/power-density",
    "html": "https://emfbase.com/quantities/power-density",
    "markdown": "https://emfbase.com/quantities/power-density.md",
    "collection": "https://emfbase.com/api/quantities"
  }
}