EIRP Power Budget Calculator

Enter the radio's transmit power, the losses between the radio and the antenna, and the antenna gain. The calculator shows the power that reaches the antenna and the EIRP that leaves it, in dBm and in mW. You can type a legal limit to be warned when you go over it. All losses and gains are your own datasheet figures; nothing is assumed.

Radio and antenna

Transmit power (dBm)What it is: the power the radio puts out, from its datasheet or settings. How to choose: use dBm; convert from mW with the dBm / mW converter. Example: 20 dBm (100 mW).

Antenna gain (dBi)What it is: how much the antenna concentrates the signal, in dBi. How to choose: take it from the antenna datasheet. Example: 15 dBi.

Losses between the radio and the antenna

Cable loss (dB per 100 m)What it is: the attenuation of the cable per 100 m at the frequency you use. How to choose: read it from the cable datasheet; leave empty for no cable. Example: 20 dB per 100 m.

Cable length (m)What it is: the length of the cable from the radio to the antenna. How to choose: measure the real run, not the straight distance. Example: 10 m.

Number of connectorsWhat it is: how many connectors the signal passes through. How to choose: count each plug-socket joint once. Example: 2.

Loss per connector (dB)What it is: the loss of one connector, in dB. How to choose: use the connector datasheet; leave empty for none. Example: 0.2 dB.

Other losses (dB)What it is: any other loss, such as a lightning arrestor or a splitter. How to choose: add the datasheet values together; leave empty for none. Example: 1 dB.

EIRP limit (dBm, optional)What it is: the legal maximum EIRP for your band and country. How to choose: type the figure from your regulator; leave empty to skip the check. Example: 30 dBm.

EIRP

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Transmit power—

Cable loss—

Connector loss—

Other losses—

Total loss—

Power at the antenna—

Antenna gain—

Free-space and other path loss are not included here; for a whole link use a link budget. The limit is the figure you type and is only compared, not looked up. Check the rules of your country before you transmit.

The formulas

Step	Formula
Cable loss	dB per 100 m × length in m / 100
Connector loss	number of connectors × dB per connector
Power at the antenna	Tx − cable loss − connector loss − other loss
EIRP	power at the antenna + antenna gain (dBi)
EIRP in mW	10^{(EIRP in dBm / 10)}

EIRP is the power a perfect omnidirectional antenna would need to radiate to give the same signal in the strongest direction of your antenna. Cable loss is taken as proportional to cable length.

Worked example

Transmit power 20 dBm, cable 20 dB per 100 m over 10 m (2 dB), two connectors at 0.2 dB (0.4 dB), 1 dB other loss, antenna gain 15 dBi. Total loss is 3.4 dB, so the power at the antenna is 16.6 dBm and the EIRP is 31.6 dBm, which is about 1445 mW.

Common mistakes

Using the radio's maximum power when the radio is set lower.
Reading the cable loss at the wrong frequency. Loss grows with frequency.
Mixing dBi (antenna gain over an isotropic radiator) with dBd (over a dipole). This tool expects dBi.
Treating the EIRP limit as a limit on radio power. Many rules limit EIRP, so a higher-gain antenna can put you over.

Frequently asked questions

What is EIRP?

EIRP is the equivalent isotropically radiated power: the transmit power, minus losses before the antenna, plus the antenna gain. It describes how strong the signal is in the antenna's main direction.

How do I calculate EIRP?

Subtract the cable, connector and other losses from the transmit power in dBm, then add the antenna gain in dBi.

Does a longer cable lower the EIRP?

Yes. Cable loss grows with length, so more cable means less power reaches the antenna and a lower EIRP.

Is a higher EIRP always better?

No. Rules in most countries cap the EIRP for each band, and going over it can be illegal and can cause interference.

Does this include the distance to the other radio?

No. This is the transmit side only. Path loss over distance is a separate step in a link budget.

Related tools

Convert levels with the dBm / mW converter. For the cable side of a camera system, see the PoE Budget Calculator and the CCTV Voltage Drop Calculator.

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