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EIRP calculator

EIRP — equivalent isotropically radiated power — is the power an ideal isotropic antenna would have to radiate to match what your antenna puts into its main beam. Take the transmitter power in dBm, subtract everything between radio and antenna, then add the antenna gain in dBi. Regulators cap EIRP, not transmitter power.

dBm

= 100 mW

dB

dBd + 2.15 = dBi.

EIRP
27.0 dBm
= 501 mW
ERP
24.9 dBm
referenced to a half-wave dipole
Antenna gain used
8.00 dBi
signal chain · beam shape schematicTX20.0 dBm− 1.0 dB cable+ 8.00 dBiEIRP27.0 dBm= 501 mWEIRP against common 2.4 GHz limits20 dBm — EU 2.4 GHz limit (ETSI EN 300 328)36 dBm — US 2.4 GHz PtMP limit (FCC §15.247)010203040EIRP (dBm)

The top row is the signal chain: the radio’s output, minus what the feedline and connectors eat, plus the antenna gain, ends up as the EIRP radiated in the main beam. Below it, that EIRP sits on a 0–45 dBm scale next to the two most common 2.4 GHz limits; the green bar runs from 0 dBm to your result.

The limit markers are illustrative for common unlicensed bands. The applicable limit depends on band, region, service and antenna setup — check the current regulation text.

How it works

  1. 01

    Start from transmitter power in dBm

    EIRP is an absolute power, so the chain has to start in absolute units: 0 dBm = 1 mW and 30 dBm = 1 W, which makes a 100 mW radio 20 dBm. Convert watts first if the datasheet gives them, and keep dB (a ratio) and dBm (a power) apart — mixing them is the classic mistake.

  2. 02

    Subtract everything between radio and antenna

    Coax, jumpers, connectors, lightning arrestors and filters all take their share before the antenna sees the power. Budget roughly 0.1–0.3 dB per well-made connector and use the cable’s dB/100 m figure at your frequency: 10 m of LMR-400 costs about 2.2 dB at 2.4 GHz.

  3. 03

    Add the antenna gain in dBi

    Gain in dBi is referenced to an isotropic radiator; gain in dBd is referenced to a half-wave dipole, which itself has 2.15 dBi. So G(dBi) = G(dBd) + 2.15: a 14 dBd Yagi is a 16.15 dBi Yagi — one antenna, two reference points.

  4. 04

    Compare with the limit that applies to you

    Unlicensed bands are capped by radiated power, not by what the radio produces: 20 dBm EIRP in the EU 2.4 GHz band under ETSI EN 300 328, and 36 dBm EIRP for US point-to-multipoint under 47 CFR §15.247 (30 dBm conducted plus 6 dBi of antenna). Fixed point-to-point links in the US 2.4 GHz band get an exception, and other bands, regions and services have their own numbers.

Formulas

Equivalent isotropically radiated power
EIRP (dBm) = PTX − Lcable + Gant
  • EIRP — equivalent isotropically radiated power, dBm
  • PTX — transmitter output power, dBm
  • Lcable — feedline, connector and filter loss, dB
  • Gant — antenna gain, dBi
ERP and the dipole reference
ERP = EIRP − 2.15 dB ; G(dBi) = G(dBd) + 2.15
  • ERP — effective radiated power, referenced to a half-wave dipole, dBm
  • 2.15 dB — gain of a lossless half-wave dipole over isotropic

Worked example

A 20 dBm access point, 1 dB of coax, an 8 dBi panel
  1. EIRP = 20 − 1 + 8 = 27 dBm
  2. 27 dBm = 10(27 − 30)/10 W = 0.50 W
  3. ERP = 27 − 2.15 = 24.85 dBm
  4. EU 2.4 GHz limit is 20 dBm EIRP → 27 − 20 = 7 dB over
  5. → fine under the US point-to-multipoint rules, over the limit in the EU

FAQ

What is the difference between EIRP and ERP?
Only the reference antenna. EIRP compares against an isotropic radiator, ERP against a half-wave dipole, and a dipole already has 2.15 dBi of gain — so ERP = EIRP − 2.15 dB. Broadcast and land-mobile rules are usually written in ERP, while Wi-Fi and most modern spectrum rules use EIRP. Check which one a limit means before claiming compliance.
Why do regulators limit EIRP instead of transmitter power?
Because interference depends on the field strength arriving at someone else’s receiver, and that follows the radiated power in their direction, not what the transmitter pushes into a cable. A 100 mW radio behind a 24 dBi dish reaches 44 dBm EIRP in the beam — as much as 25 W fed to an isotropic antenna.
Is antenna gain free power?
No — it is focus. A passive antenna radiates the same total power it is fed, minus its own losses; gain only describes how much of that power is concentrated into the main beam instead of spread in every direction. Whatever you gain in the beam comes off the sides and the back.
What is the FCC point-to-point exception at 2.4 GHz?
For fixed point-to-point links in the 2400–2483.5 MHz band, 47 CFR §15.247 relaxes the usual rule: instead of cutting transmitter power dB for dB above 6 dBi of antenna gain, you cut it by 1 dB for every 3 dB of extra gain. A 24 dBi dish therefore still allows 30 − (24 − 6)/3 = 24 dBm at the radio, which is 48 dBm EIRP.

EIRP is what leaves the mast. Where it actually lands is terrain — see your real footprint on the map.

Map your coverage →

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