{
    "name": "Free-Space Path Loss Calculator",
    "url": "https://calculators.geekmidst.com/calculators/free-space-path-loss-calculator",
    "markdown": "https://calculators.geekmidst.com/calculators/free-space-path-loss-calculator.md",
    "category": "IT, networking & security",
    "description": "Calculate free-space path loss in dB from distance and frequency, and the received signal in dBm from transmit power and antenna gains, for Wi-Fi and radio links.",
    "summary": "Even with no obstacles, a radio signal spreads out: free-space path loss = 20 log₁₀(4π d f ÷ c). Doubling the distance or the frequency adds about 6 dB.",
    "also_called": [
        "FSPL calculator",
        "Wi-Fi path loss calculator",
        "Link budget calculator"
    ],
    "applies_to": "Worldwide",
    "last_updated": "2026-10-29",
    "computation": "Results are computed in the visitor’s browser on the HTML page. This API describes the calculator; it does not compute results.",
    "inputs": [
        {
            "name": "distance",
            "label": "Distance",
            "type": "number",
            "default": "100"
        },
        {
            "name": "distance_unit",
            "label": "Distance unit",
            "type": "segmented",
            "default": "m",
            "options": {
                "m": "m",
                "km": "km",
                "ft": "ft",
                "mi": "miles"
            }
        },
        {
            "name": "frequency",
            "label": "Frequency",
            "type": "number",
            "default": "2.4"
        },
        {
            "name": "frequency_unit",
            "label": "Frequency unit",
            "type": "segmented",
            "default": "GHz",
            "options": {
                "MHz": "MHz",
                "GHz": "GHz"
            }
        },
        {
            "name": "tx_dbm",
            "label": "Transmit power",
            "type": "number",
            "default": "20",
            "unit": "dBm"
        },
        {
            "name": "tx_gain",
            "label": "Transmit antenna gain",
            "type": "number",
            "default": "2",
            "unit": "dBi"
        },
        {
            "name": "rx_gain",
            "label": "Receive antenna gain",
            "type": "number",
            "default": "2",
            "unit": "dBi"
        }
    ],
    "outputs": [
        {
            "key": "fspl_db",
            "label": "Free-space path loss",
            "format": "number"
        },
        {
            "key": "rx_dbm",
            "label": "Received signal",
            "format": "number"
        },
        {
            "key": "rx_mw",
            "label": "Received power",
            "format": "number"
        },
        {
            "key": "wavelength_cm",
            "label": "Wavelength",
            "format": "number"
        }
    ],
    "formula": {
        "text": "FSPL (dB) = 20 log₁₀(d) + 20 log₁₀(f) + 20 log₁₀(4π ÷ c) = 20 log₁₀(d km) + 20 log₁₀(f MHz) + 32.45;  Received = Tx power + Tx gain + Rx gain − FSPL",
        "variables": {
            "d": "distance in metres",
            "f": "frequency in hertz",
            "c": "299,792,458 m/s"
        }
    },
    "worked_example": {
        "inputs": {
            "distance": "100",
            "distance_unit": "m",
            "frequency": "2.4",
            "frequency_unit": "GHz",
            "tx_dbm": "20",
            "tx_gain": "2",
            "rx_gain": "2"
        },
        "steps": [
            "20 log₁₀(0.1 km) = −20; 20 log₁₀(2,400 MHz) = 67.60.",
            "FSPL = −20 + 67.60 + 32.45 = 80.05 dB.",
            "Received = 20 + 2 + 2 − 80.05 = −56.05 dBm."
        ],
        "result": "Over 100 m at 2.4 GHz, free-space loss is 80.05 dB, so a 20 dBm transmitter with 2 dBi antennas delivers about −56 dBm."
    },
    "interpretation": "Compare the received level with the receiver’s sensitivity for the data rate you want, and keep a fade margin of 10–20 dB for walls, bodies and multipath, which free space ignores.",
    "assumptions": [
        "Clear line of sight with the first Fresnel zone unobstructed, in the far field of both antennas."
    ],
    "limitations": [
        "Walls, floors, foliage, rain and reflections are not included; indoor losses are usually much higher than free space."
    ],
    "rules": null,
    "faqs": [
        {
            "question": "Why does 5 GHz reach less far than 2.4 GHz?",
            "answer": "Path loss rises 20 log₁₀(5 ÷ 2.4) ≈ 6.4 dB at the same distance with similar antennas, and walls absorb more at higher frequencies."
        },
        {
            "question": "What is dBm?",
            "answer": "Power relative to 1 milliwatt on a log scale: 0 dBm = 1 mW, 20 dBm = 100 mW, −60 dBm = one nanowatt (10⁻⁶ mW)."
        }
    ],
    "sources": [],
    "related": [
        {
            "name": "Wi-Fi Channel Capacity Calculator",
            "url": "https://calculators.geekmidst.com/calculators/wifi-channel-capacity-calculator"
        },
        {
            "name": "Network Latency (RTT) Calculator",
            "url": "https://calculators.geekmidst.com/calculators/network-latency-rtt-calculator"
        },
        {
            "name": "Conversion Calculator",
            "url": "https://calculators.geekmidst.com/calculators/conversion-calculator"
        }
    ],
    "disclaimer": null,
    "publisher": "GM¹X CalcVerse"
}