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ERP and EIRP calculator

Two quantities that are constantly confused

EIRP is referred to an isotropic radiator, ERP to a half-wave dipole, and there are exactly 2.15 dB between them. Licence limits in different countries are written in one or the other, so mixing them up means either wasting the power you are allowed or exceeding it by half as much again.

50 W−2.3 dBantennaEIRPERPthe difference is exactly 2.15 dB
EIRP is referred to an isotropic radiator, ERP to a half-wave dipole
Transmitter power (W)
Antenna gain
Cable loss (dB)
Loss in connectors and adapters (dB)
Frequency for the effective area (MHz)
Gain
In dBi
In dBd
Radiated power
EIRP — referred to an isotropic radiator
ERP — referred to a dipole
Power at the antenna input

📐 The formulas​

G[dBi] = G[dBd] + 2.15

EIRP[dBm] = P_tx − L_cable − L_connectors + G[dBi]
ERP  = EIRP − 2.15 dB

Effective antenna area:  Ae = G · λ² / 4π

A check point:​

50 W (46.99 dBm), 2 dB of cable, a 9 dBd antenna (= 11.15 dBi) → EIRP = 56.14 dBm = 411 W, ERP = 53.99 dBm = 251 W.

The effective area of a dipole (2.15 dBi) at 145 MHz is 0.558 m².

Where this calculator stops:
  • Antenna gain is the free-space figure from the datasheet; over real ground it is different
  • Cable loss depends on frequency: the quoted “dB per 100 m” is given for one particular frequency
  • The result is the power in the direction of the main lobe; to the side and behind it is far lower
  • SWR is not accounted for: with a mismatch, part of the power never reaches the antenna

🎯 In practice​

  • Checking compliance with a licence whose limit is written in ERP or EIRP.
  • Comparing two stations: 10 W with a good antenna often beats 100 W with a poor one.
  • Input for the safe-distance calculation and for a link budget.
Practical advice:
  1. A gain quoted in plain “dB” is always a reason to ask whether it means dBi or dBd.
  2. Cable loss reduces radiated power and receive sensitivity by the same amount.
  3. Gain in the antenna is usually cheaper than gain in the transmitter.
  4. The safe-distance calculation needs EIRP specifically — take it from here.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «ERP and EIRP calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/erp (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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