EIRP Calculator
Compute Effective Isotropic Radiated Power from TX power (dBm or Watts), antenna gain and cable/connector losses.
What EIRP is
EIRP (Effective Isotropic Radiated Power) describes the power that would need to radiate from a theoretical isotropic antenna (one that radiates equally in every direction) to match the signal strength actually produced by a real transmitter, feeder and antenna in a given direction. It is the standard figure used in coverage planning, interference analysis and regulatory compliance.
The EIRP formula
EIRP(dBm) = TX power(dBm) - cable/connector losses(dB) + antenna gain(dBi). Because every term is already in decibels (a logarithmic, ratio-based unit), the calculation is a simple sum, no multiplication needed.
dBm and Watts
Radio TX power is sometimes specified in Watts instead of dBm. The conversion is dBm = 10 × log10(power in mW), so 1 W (1000 mW) is 30 dBm, 100 W is 50 dBm. This calculator accepts either unit and converts automatically before applying the EIRP formula.
Why losses matter
Feeder cable, jumpers, connectors and combiners between the radio and the antenna all dissipate a small amount of power as heat. On a long cable run those losses can reach several dB, enough to noticeably change coverage predictions or push a site over a regulatory EIRP limit if ignored.
Worked example
A 43 dBm (20 W) transmitter feeding an 18 dBi antenna through 3 dB of cable and connector loss: 43 - 3 + 18 = 58 dBm EIRP, roughly 631 Watts EIRP.
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Frequently Asked Questions
What is EIRP?
EIRP (Effective Isotropic Radiated Power) is the power an isotropic antenna would need to radiate to produce the same signal strength in a given direction as the actual transmitter and antenna combination. It is the standard figure regulators and link-budget calculations use to describe radiated power.
What is the EIRP formula?
EIRP(dBm) = TX power(dBm) minus cable and connector losses(dB) plus antenna gain(dBi). All three terms are logarithmic (dB), so they simply add or subtract.
How do I convert Watts to dBm?
dBm = 10 x log10(power in milliwatts). For example 1 Watt (1000 mW) equals 30 dBm. This calculator accepts TX power in either dBm or Watts and converts automatically.
Why do cable and connector losses matter?
Every meter of feeder cable and every connector or jumper between the radio and the antenna dissipates some power as heat. Ignoring these losses overestimates the real radiated power, sometimes by several dB, enough to matter for coverage planning or regulatory compliance.
Is there a legal EIRP limit?
Yes, most regulators cap EIRP per band and license type, and limits vary by country and service. Always check the applicable regulatory limit for your specific band and license, this calculator only computes the value, it does not know your local rules.
Need the full report?
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