EIRP Calculator
Effective Isotropic Radiated Power (EIRP) is the power a perfect isotropic antenna would need to radiate to match the peak power your real antenna concentrates in its main lobe.
Enter Values
How to use this calculator
- Enter the transmitter's output power in dBm (30 dBm = 1 W). Convert from watts first if your radio is rated in W or mW.
- Enter the antenna gain in dBi (an isotropic reference). If your antenna is rated in dBd, add 2.15 to get dBi.
- Optionally enter total cable and connector loss in dB between the radio and the antenna; leave it blank for a direct connection. Read off EIRP in dBm, watts and dBW.
How it works
Effective Isotropic Radiated Power (EIRP) is the power a perfect isotropic antenna would need to radiate to match the peak power your real antenna concentrates in its main lobe. It combines everything in the transmit chain into one figure: EIRP (dBm) = transmitter power (dBm) − feed-line and connector loss (dB) + antenna gain (dBi). Because all terms are already in decibels, the maths is a simple add and subtract.
To express EIRP as power, convert out of the decibel scale: watts = 10^((EIRP_dBm − 30) / 10), and dBW = dBm − 30. Regulators specify maximum EIRP (not just transmitter power) because a high-gain antenna can push a legal transmitter over the limit. Antenna gain in dBd (relative to a half-wave dipole) is 2.15 dB lower than the same gain in dBi, so always work in dBi here.
Worked example
Wi-Fi access point with a directional antenna. A transmitter puts out 20 dBm (100 mW). The feed line and connectors lose 2 dB, and the antenna has 6 dBi of gain. EIRP = 20 − 2 + 6 = 24 dBm. In watts that is 10^((24 − 30) / 10) = 0.251 W (251 mW) — comfortably under a 36 dBm (4 W) regulatory cap.
Common mistakes
- Entering cable loss as a negative number. It is already a loss — enter it as a positive dB value and the calculator subtracts it.
- Mixing up dBi and dBd. This tool expects gain in dBi; if your antenna is spec'd in dBd, add 2.15 dB first, or your EIRP will read about 2 dB low.
- Confusing dBm with dBW or plain watts. 30 dBm = 0 dBW = 1 W. Feeding watts straight into the dBm field will be wildly wrong.
Frequently asked questions
What is EIRP and why does it matter?
EIRP (Effective Isotropic Radiated Power) is the total effective power radiated in the direction of maximum antenna gain, referenced to an ideal isotropic antenna. Regulators cap EIRP rather than transmitter power alone, because a high-gain antenna can make a modest radio radiate far more energy in one direction — so EIRP is the number that determines legal compliance and real-world range.
How do I convert EIRP from dBm to watts?
Use watts = 10^((EIRP_dBm − 30) / 10). For example 36 dBm is 10^((36 − 30)/10) = 10^0.6 ≈ 4 W. To go from dBm to milliwatts, drop the −30: mW = 10^(dBm / 10). This tool shows all three (dBm, W/mW and dBW) at once.
Should I use antenna gain in dBi or dBd?
Enter it in dBi (gain relative to an isotropic radiator), which is what EIRP is defined against. If your antenna is rated in dBd (relative to a half-wave dipole), add 2.15 dB to convert: dBi = dBd + 2.15.
What is the difference between EIRP and ERP?
EIRP references an isotropic antenna (dBi), while ERP references a half-wave dipole (dBd). They describe the same radiated power but on different baselines: EIRP is 2.15 dB higher than ERP for the same setup. This calculator reports EIRP; subtract 2.15 dB from the dBm result if you need ERP.
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- dBm to Watts Converter
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- Antenna Gain Unit Converter (dBi dBd)
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Cite this tool
Zerdly. (2026). EIRP Calculator. Retrieved from https://www.zerdly.com/tools/eirp-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.