EIRP Calculator - Transmitter Power Plus Antenna Gain

An EIRP calculator combines the transmitter output power, the cable and connector loss along the feed line, and the gain of the transmit antenna into the effective isotropic radiated power of a radio system. The result is the power a theoretical isotropic radiator would have to emit to match the peak signal your real antenna produces. That single number is what regulators and link budgets use to describe a transmitter's on-air level, so the EIRP calculator is a standard part of any RF or wireless workflow.

• • Link budget planning: Work out the radiated power at one end of a fixed wireless, Wi-Fi, or cellular link before checking the receive side.
• • Regulatory compliance: Compare the calculated EIRP to FCC, ETSI, or other regional limits before deploying a transmitter.
• • Antenna swap analysis: Compare the EIRP produced by different antennas or cable runs without redoing the entire link budget by hand.
• • Classroom and lab exercises: Give physics, communications, and RF students a quick numerical check of textbook EIRP examples.

Three numbers set the EIRP for a working link: how much power the transmitter delivers to the antenna, how much the cabling and connectors waste, and how much the antenna focuses its energy toward the receiver. Because the formula is a simple decibel sum, the result is also useful for sanity checks on a link budget.

When you need to confirm the power unit conversion behind the formula, the Watt Converter is a quick reference for switching between milliwatts, watts, kilowatts, and related units.

The result is a single dB-domain sum: transmitter power, minus total loss, plus antenna gain. The answer is reported in dBmW, watts, dBW, and ERP so it can be compared to datasheets, regulations, and link budgets directly.

• Tx: Transmitter output power in dBmW, the power delivered to the antenna feed line.
• L_total: Total cable and connector loss in dB, the combined insertion loss of the feed line.
• G_antenna: Antenna gain in dBi, the peak gain of the transmit antenna relative to an isotropic radiator.

Every term is a decibel value, so the formula tolerates the addition or removal of loss and gain terms as long as the units match. A common mistake is to mix dBi and dBd gains, which differ by 2.15 dB. The watts and dBW outputs let the result drop into path-loss formulas that expect a linear power value.

According to Omni Calculator EIRP, EIRP = Tx (dBmW) - cable and connector losses (dB) + antenna gain (dBi).

To see how dB, dBmW, and dBi relate as power ratios, the Decibel (dB) Calculator shows the same operations in raw form.

EIRP combines four ideas from RF engineering. Knowing them keeps the result from being misused when it is plugged into a propagation or coverage model.

The EIRP value is the right number to compare against a regulatory limit, but the actual signal at a receiver is shaped by path loss, antenna patterns, and the receive antenna. The on-air signal is not the same as the EIRP in watts. As published by IEEE Standard 145-2013, antenna gain expressed in dBi is referenced to an isotropic radiator, while dBd is referenced to a half-wave dipole.

To convert the EIRP calculator output from dBmW into watts for a propagation formula, the dBm to Watts Calculator handles the same conversion directly.

A complete run takes three numbers and a single click. The form above shows the three common inputs and the four outputs you get back.

1. 1 Enter the transmitter output power: Look up the transmit power at the antenna port on the radio datasheet, in dBmW. Convert from watts to dBmW first if needed.
2. 2 Add up the total cable and connector loss: Use the coax loss per metre at the operating frequency times the run length, then add the insertion loss of any inline connectors.
3. 3 Enter the antenna gain in dBi: Use the peak gain at the boresight direction. If the datasheet only lists gain in dBd, add 2.15 to convert it to dBi before entering it.
4. 4 Read the EIRP and ERP results: The result returns the effective isotropic radiated power in dBmW, watts, and dBW, plus ERP in dBmW for dipole-based regulations.
5. 5 Compare to a regulatory limit: Check the result against the relevant FCC, ETSI, or local rule. If the EIRP is over the limit, reduce transmitter power, switch to a lower-gain antenna, or shorten the cable run.
6. 6 Save the inputs for a link budget: Use the same numbers as the start of a complete link budget, where the EIRP result is the transmit term on the left side of the path-loss equation.

A 2.4 GHz point-to-point link uses a 20 dBmW radio, 5 m of LMR-400 cable, and a 12 dBi patch antenna. LMR-400 loses about 0.22 dB per metre at 2.4 GHz, so the 5 m run adds roughly 1.1 dB, and the inline connectors add another 0.6 dB. The EIRP calculator returns 20 - 1.7 + 12 ≈ 30.3 dBmW, below the 36 dBmW Part 15.247 ceiling for the 2.4 GHz band.

When you need to translate a frequency into a wavelength for a path-length estimate, the Harmonic Wave Equation Calculator handles the wave-equation side of the link budget.

EIRP shows up in regulations, datasheets, and link budgets, so a dedicated tool keeps the convention consistent across teams and projects.

• • Regulatory checks in seconds: Compare the result to an FCC, ETSI, or regional limit without re-deriving the formula, and flag non-compliant designs before ordering hardware.
• • Antenna swap comparisons: Run the tool with two candidate antennas and see the dBmW difference directly, faster than rebuilding a link budget.
• • Loss-aware link planning: Use the same three inputs to see how a longer cable run or extra connector changes the EIRP, and pick the lowest-loss option for the install.
• • Teaching and lab support: Give students a quick way to check textbook EIRP examples against the same formula their textbook uses.
• • Cross-team consistency: RF engineers, planners, and field technicians can share the same output in dBmW, watts, dBW, and ERP without converting units by hand.

The main benefit is consistency: the same decibel-domain formula is applied every time, so the result is comparable across sites and teams. A doubling of transmitter power always adds 3 dB.

For sites where the radio is powered from mains AC, the AC Wattage Calculator can size the supply and confirm the radio stays inside the budget for the rest of the rack.

The result is only as good as the three numbers that go into it. Several real-world factors shift those numbers and change the on-air power in ways the simple formula does not show.

• • The formula assumes a lossless isotropic reference and a single boresight direction. Real antenna patterns have sidelobes and back lobes that the result does not capture.
• • Connector and cable losses vary with frequency, temperature, and bending radius. A lab-measured loss at one frequency may not match the value at the deployed frequency.
• • The formula does not account for transmit power backoff, duty cycle, or spectral mask compliance, which are checked separately when a device is type-approved.

The biggest practical risk is treating the result as a definite prediction of on-air power. The model assumes a clean feed line, a well-matched antenna, and a boresight direction. Real deployments should be checked with a spectrum analyzer or power meter at the antenna port when compliance matters.

According to FCC Part 15.247 (Cornell LII), unlicensed 2.4 GHz ISM spread-spectrum transmitters are generally limited to 36 dBmW EIRP. That is the right number to compare the EIRP calculator result against; the calculator itself just returns EIRP for the inputs and does not enforce the ceiling.

When the result needs to be combined with a different unit, such as horsepower or BTU/h on a remote-site generator, the Power Converter handles the broader power conversion.