EIRP Calculator – Effective Isotropic Radiated Power
Welcome to the EIRP calculator! This tool calculates the effective isotropic radiated power (EIRP) for a system consisting of a transmitter, a cable, and an antenna. In this article you will learn what the EIRP is, how to calculate it, and the difference between the effective radiated power and the effective isotropic radiated power — ERP vs. EIRP.
Definition: What is the EIRP?
EIRP stands for effective isotropic radiated power (or equivalent isotropic radiated power). This term is often used in telecommunications, mainly in link budgets. EIRP is defined as the amount of power that a theoretical isotropic antenna — one that distributes power uniformly in all directions — would radiate to produce the peak power density observed in the direction of maximum antenna gain.
Radio communication systems must comply with rules established by regulatory bodies such as the Federal Communications Commission (FCC) in the US or the European Telecommunications Standards Institute (ETSI) in Europe. One such rule is that radio equipment must not exceed certain EIRP values.
The effective isotropic radiated power is calculated with the following formula:
EIRP = Tx − Lc + Ga
where:
- Tx — Output power of the transmitter (dBmW);
- Lc — Sum of cable and connector losses, if present (dB); and
- Ga — Antenna gain (dBi).
As you can see, the EIRP considers losses in the transmission cable and connectors and includes the antenna gain. It is often expressed in decibels relative to the reference power emitted by an isotropic antenna with equivalent signal power.
EIRP calculation example: How to use the antenna EIRP calculator
Let's see a concrete example. Here is the data that you have:
- The transmitter output power: 21 dBmW;
- The total cable loss: 3 dB;
- The connector loss: 0.5 dB per connector (you have 2); and
- The antenna gain: 11 dBi.
Follow these step-by-step instructions:
- Select the cable-loss method Total cable loss and enter 3 dB.
- Enter the transmitter output power, 21 dBmW. Your input is in decibel-milliwatts (dBmW or dBm), but you can switch the unit to watts instead.
- Enter the antenna gain in the corresponding field: 11 dBi.
- Check the box I have connectors. The default number of connectors is already 2 — next to it, enter the connector loss of 0.5 dB.
Here is your result: the EIRP equals 28 dBmW or about 0.631 W. By hand:
EIRP = 21 dBmW − (3 dB + 2 × 0.5 dB) + 11 dBi = 21 − 4 + 11 = 28 dBmW
The calculator offers additional cable options. If you know the type of cable you want to use at a specific frequency, select Cable type & frequency, choose your cable from the list, enter the frequency and the cable length. If your cable is not listed, select Cable loss per unit of length and enter the known loss per 100 m (or 100 ft) together with the length.
Isotropic antennas vs. practical antennas
An isotropic antenna is a theoretical, idealized point source that radiates power equally in every direction — a perfect sphere of radiation. It cannot be built in practice, but it is an extremely useful reference because its gain is exactly 1 (0 dBi) in all directions.
A practical antenna (a dipole, a Yagi, a parabolic dish, etc.) always concentrates energy in some directions at the expense of others. Its gain, measured in dBi, tells you how much stronger the signal is in the direction of maximum radiation compared with the isotropic reference. EIRP combines the real transmitter power with this directional gain to express the equivalent power an isotropic source would need to match the antenna's peak.
Decibels: dB vs. dBmW
Decibels make it easy to multiply and divide large ranges of power by simply adding and subtracting:
- dB is a relative unit — a ratio between two powers. Cable loss, connector loss, and antenna gain (dBi, dBd) are all relative quantities.
- dBmW (often written dBm) is an absolute unit referenced to 1 milliwatt:
P(dBmW) = 10 × log₁₀(P / 1 mW). So 0 dBmW = 1 mW, 30 dBmW = 1 W, and 60 dBmW = 1 kW.
Because EIRP adds an absolute power (dBmW) to relative quantities (dB, dBi), the result is again an absolute power expressed in dBmW — which the calculator also converts to watts.
ERP vs. EIRP
Both quantities describe radiated power, but they use a different reference antenna:
- EIRP is referenced to an isotropic antenna (gain in dBi).
- ERP (effective radiated power) is referenced to a half-wave dipole (gain in dBd).
A half-wave dipole has a gain of about 2.15 dBi over an isotropic source, so the two figures differ by that constant:
EIRP (dBmW) = ERP (dBmW) + 2.15 dB
FAQs
What does EIRP stand for?
EIRP stands for effective isotropic radiated power (also called equivalent isotropic radiated power) — the power a perfect isotropic antenna would have to radiate to match the peak signal produced by your real antenna.
How do I calculate EIRP?
Add the transmitter output power (dBmW) and the antenna gain (dBi), then subtract all cable and connector losses (dB): EIRP = Tx − Lc + Ga.
What is the difference between dBm and dBmW?
They are the same unit. "dBm" is simply the common shorthand for "dBmW" — decibels relative to one milliwatt.
How do I convert EIRP from dBmW to watts?
Use P(W) = 10^((EIRP − 30) / 10). For example, 28 dBmW becomes 10^((28−30)/10) ≈ 0.631 W.
Why does EIRP matter?
Regulators cap the maximum EIRP for licence-free bands (for example Wi-Fi) to limit interference. Knowing your EIRP ensures your installation stays legal and helps you size a link budget correctly.