Free-Space Path Loss & Link Budget Calculator

Radio signal weakens as it spreads out — the free-space path loss between two isotropic antennas separated by distance d at frequency f follows the Friis equation FSPL = (4π·d·f / c)², or in decibels FSPL(dB) = 20·log₁₀(d) + 20·log₁₀(f) + 32.44 with d in km and f in MHz (it grows 20 dB per decade of distance and 20 dB per decade of frequency). A link budget adds it all up: P_r = P_{t + G_{t + G_{r − FSPL − L_{other in dBm/dB, giving the power that actually reaches the receiver, and the fade margin is the gap between that and the receiver's sensitivity. Enter frequency, distance, transmit power, antenna gains, and losses to get the path loss, received power, EIRP, wavelength, and fade margin. Everything runs locally in your browser.}}}}

Path

Frequency

Freq unit

Distance

Distance unit

Radio

Tx power P_t

Tx power unit

Tx antenna gain G_t (dBi)

Rx antenna gain G_r (dBi)

Other losses (dB)

Rx sensitivity (dBm)

Result

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The math. As a wave radiates outward it thins over the surface of an expanding sphere, so power density falls with the inverse square of distance; combined with the ratio of the antenna aperture to the wavelength this gives the Friis relation P_r/P_t = G_t·G_r·(λ/(4πd))². Taking 10·log₁₀ turns the products into sums and the square into a factor of 20, yielding FSPL(dB) = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c) — the constant is 32.44 when d is in km and f in MHz (−27.55 when d is in meters and f in Hz). Doubling distance or frequency adds ~6 dB of loss; a decade of either adds 20 dB. The link budget stacks this against everything else: P_r(dBm) = P_t(dBm) + G_t + G_r − FSPL − L_other, and the EIRP (equivalent isotropically radiated power) is P_t + G_t. The fade margin P_r − sensitivity should be positive — typically 10–30 dB — to absorb multipath fading, rain attenuation, and body loss that the free-space model ignores. This is the ideal free-space case; real links also suffer ground reflection, diffraction, atmospheric absorption (notably oxygen and water-vapour lines, and rain above ~10 GHz), and building penetration. Pairs with the Decibel, VSWR, Wavelength, and Reactance & Impedance tools. Everything runs locally — nothing leaves your browser.