EIRP Calculator (effective isotropic radiated power, dBm, W and ERP)

Understanding the EIRP calculator — concept and principle

This calculator finds the effective isotropic radiated power (EIRP) of a radio transmitter from its output power, antenna gain and the losses in between, and also gives the effective radiated power (ERP). Radio amateurs, Wi-Fi and IoT installers and RF engineers use it for link budgets and to compare a set-up with regulatory power limits.

In decibels, gains add and losses subtract: 24.5 dBm EIRP, about 0.28 W.

How it is calculated

EIRP (dBm) = P_TX (dBm) + G (dBi) − L (dB)

EIRP (W) = 10^((EIRP dBm − 30) ÷ 10); dBW = dBm − 30

ERP = EIRP − 2.15 dB

Transmitter power in W or mW is first converted: dBm = 10 log₁₀(P ÷ 1 mW).
G is the antenna gain relative to an isotropic radiator; L includes cable, connector and filter losses.

The principle

An isotropic antenna is an imaginary source that radiates equally in all directions. A real antenna concentrates power in some directions, and its gain in dBi says how much stronger its peak is than that ideal source. EIRP is the power an isotropic source would need to produce the same peak signal. In decibels, multiplying powers becomes adding numbers, so gains add and losses subtract. ERP uses a half-wave dipole as the reference instead; a dipole has 2.15 dBi of gain, so ERP is always 2.15 dB below EIRP.

Worked example

Default inputs: 20 dBm transmitter, 6 dBi antenna, 1.5 dB of losses.

Power into the antenna = 20 − 1.5 = 18.5 dBm.
EIRP = 20 + 6 − 1.5 = 24.5 dBm, which is 24.5 − 30 = −5.5 dBW.
In watts: 10^((24.5 − 30) ÷ 10) = 0.2818 W.
ERP = 24.5 − 2.15 = 22.35 dBm = 0.1718 W.

Each point is the calculator re-run with only “Transmitter output power” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Transmitter output power	EIRP (dBm)
5	9.5
10	14.5
15	19.5
20	24.5
25	29.5
30	34.5
40	44.5

How to read the result

Every 3 dB doubles the power and every 10 dB multiplies it by ten, so a 6 dBi antenna gives four times the peak power of an isotropic source. A higher EIRP means a stronger signal in the main beam, not more total power: the antenna only redistributes it. When checking a legal limit, read whether it is written as EIRP or ERP and compare like with like.

Common mistakes

Adding dBd gain as if it were dBi. Convert with dBi = dBd + 2.15.
Forgetting cable loss, which can be several dB at microwave frequencies.
Comparing an ERP limit with an EIRP result.
Adding watts and decibels together.

Keep in mind: Allowed power depends on band, country and equipment rules. Check the regulations that apply to you before transmitting.

Frequently asked questions

What is the difference between dBm and dBi?

dBm is an absolute power relative to 1 mW; dBi is a ratio, the gain of an antenna relative to an isotropic radiator.

Does a high-gain antenna create more power?

No. It focuses the same power into a narrower beam, so the signal is stronger in some directions and weaker in others.

How many watts is 30 dBm?

1 W. Each further 10 dB multiplies by ten.

Sources: Standard decibel link-budget arithmetic (EIRP = P_T + G_i − L; half-wave dipole gain 2.15 dBi)

Checked 9 Nov 2026. Standard decibel link-budget arithmetic: EIRP = P_T + G_i − L, dBm = 10 log₁₀(P/1 mW), and the 2.15 dB gain of a half-wave dipole relative to an isotropic radiator (ERP = EIRP − 2.15 dB).

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