Free Space Path Loss Calculator
Calculate free space path loss (FSPL) from distance and frequency. Includes received signal power estimation.
About this calculator
Free-space path loss (FSPL) quantifies how much a radio signal weakens simply from spreading out over distance in an unobstructed vacuum or air, with no reflections, absorption, or obstacles involved. This calculator uses the standard logarithmic form FSPL(dB) = 20·log10(distance in km) + 20·log10(frequency in MHz) + 32.44, which comes from the inverse-square law for radiated power expressed in decibels. Two things follow directly from the 20·log10 terms: doubling the distance costs about 6 dB of additional loss, and doubling the frequency costs the same 6 dB — so a 5.8 GHz link suffers noticeably more path loss than a 900 MHz link over the identical distance, all else equal.
From FSPL, the calculator derives received power by adding your transmit power and both antennas' gains and subtracting the path loss, then converts that dBm figure into watts for an intuitive absolute-power comparison, and separately reports the signal's wavelength (speed of light divided by frequency), which matters for antenna sizing and Fresnel zone calculations. Because this model assumes true free space, it systematically underestimates real-world loss for any link with terrain, foliage, buildings, or even Earth's curvature in the way — treat the result as a best-case floor, then compare it against your receiver's sensitivity (via a full link budget) to judge whether a real link has enough margin to survive fading and obstructions.
Inputs
Results
Free Space Path Loss
91.52 dB
Received Power
-61.52 dBm
How to Use This Calculator
- Enter the link frequency (MHz or GHz) and path distance.
- Enter transmit power and antenna gains for both ends of the link.
- Review free-space path loss (dB) and estimated received power in dBm and watts.
- Use received power vs. your receiver sensitivity to determine link margin.
How the result changes with Frequency
| Frequency | Free Space Path Loss | Received Power |
|---|---|---|
| 450 | 85.5 dB | -55.5 dBm |
| 675 | 89.03 dB | -59.03 dBm |
| 1,350 | 95.05 dB | -65.05 dBm |
| 2,250 | 99.48 dB | -69.48 dBm |
What each input means
- Frequency
- Signal frequency in megahertz
- Distance
- Distance between transmitter and receiver in kilometers
- Transmit Power
- Transmitter output power in dBm (30 dBm = 1 watt)
- Transmit Antenna Gain
- Transmit antenna gain in dBi (0 = isotropic)
- Receive Antenna Gain
- Receive antenna gain in dBi (0 = isotropic)
How this is calculated
Formula
FSPL (dB) = 20·log₁₀(d) + 20·log₁₀(f) + 32.44Worked example, using the default values
- Identify Input Parameters4 parametersFrequency = 900, Distance = 1, Transmit Power = 30, Transmit Antenna Gain = 0 = 5 input(s) provided
- Calculate Free Space Path LossFree Space Path Loss91.52 = 91.52
- Calculate Received PowerReceived Power-61.52 = -61.52
- Calculate Received PowerReceived Power = Number(receivedPowerWatts.toExponential(3))7.039e-10 = 7.039e-10
- Calculate WavelengthWavelength0.3331 = 0.3331
Engine last updated . Checked against 4 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does doubling distance cost the same 6 dB as doubling frequency?
Both distance and frequency enter the formula as 20·log10(x) terms, and log10(2) ≈ 0.301, so doubling either value adds 20 × 0.301 ≈ 6 dB regardless of which one changed. That's why a link at twice the distance loses the same additional signal as the identical link run at twice the frequency — the math treats them symmetrically even though the physical causes (inverse-square spreading versus shorter wavelength) are different.
Why is received power shown in both dBm and watts?
dBm is the logarithmic unit most RF engineering specs and receiver sensitivity ratings use, which is why the calculator computes it directly from EIRP-style addition and subtraction. Watts is included alongside it (via 10^(receivedPower/10)/1000) because the absolute power number in linear units is often more intuitive for comparing to everyday reference points, especially at very low received power levels where dBm values become large negative numbers.
What is wavelength used for besides being reported here?
Wavelength (speed of light divided by frequency) is a direct input to related calculations like Fresnel zone radius and antenna sizing, both of which scale with it. It's reported here mainly for convenience so you don't have to compute it separately when moving on to those calculators for the same frequency.
Why might my real-world signal loss be worse than what this calculator predicts?
This formula assumes true free space — no terrain, foliage, buildings, atmospheric absorption, or Earth curvature in the path — so it represents a best-case floor on loss, not a real-world prediction. Any obstruction or non-line-of-sight condition adds loss on top of this figure, which is why a full link budget calculation (adding a fade margin for real-world effects) is the better tool for judging whether an actual link will work.
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