EIRP Calculator converts transmitter power, cable loss, and antenna gain into effective isotropic radiated power for RF link budgets and FCC/ETSI compliance evaluations onsite.
Calculate EIRP: Effective Isotropic Radiated Power for RF Link Budgets
This EIRP calculator converts transmitter power, cable loss, and antenna gain into effective isotropic radiated power, the standardized figure regulators and link-budget engineers use to compare radio systems on equal terms. RF engineers, wireless ISPs, and FCC/ETSI compliance testers use it to check a transmitter-cable-antenna chain before certification or deployment.
How to Use
Enter transmitter power (PT) in dBm, dBW, W, or mW; cable loss (LC) in dB; and antenna gain (GTX) in dBi or dBd. The calculator returns EIRP in dBm and watts, the ERP equivalent, and how much power actually reaches the antenna after cable loss.
Formula
Per the FCC’s Guidelines for Determining the Effective Radiated Power (ERP) and Equivalent Isotropic Radiated Power (EIRP) (KDB 412172), EIRP is the linear product of the power delivered to the antenna and the antenna’s gain relative to a theoretical isotropic radiator. In logarithmic form:
$$EIRP_{(dBm)} = P_{T(dBm)} – L_{C(dB)} + G_{TX(dBi)}$$
If gain is specified in dBd (relative to a half-wave dipole) instead of dBi, convert it first, since EIRP is always referenced to an isotropic radiator:
$$G_{(dBi)} = G_{(dBd)} + 2.15$$
Worked example: a 100 mW (20 dBm) transmitter feeding a 5 dBi antenna through a 2 dB lossy cable gives $EIRP = 20 – 2 + 5 = 23$ dBm, about 200 mW. The most common input mistake is entering only a cable’s per-length attenuation rating (e.g., dB per 100 ft) as $L_C$ without adding the fixed loss from connectors and adapters, which understates total cable loss and reports an EIRP higher than what actually leaves the antenna.
Diagram
Reference Table
| Parameter | Value | Basis (47 CFR §15.247) |
|---|---|---|
| Max conducted transmitter output power (digitally modulated, ≥500 kHz bandwidth) | 30 dBm (1 W) | §15.247(b) |
| Antenna gain permitted without any power reduction | up to 6 dBi | §15.247(b)(4) |
| Resulting maximum EIRP (unity-gain-offset case) | 36 dBm (4 W) | §15.247(b)(4) |
| Power reduction required above 6 dBi antenna gain | 1 dB conducted-power reduction per 3 dB of additional gain | §15.247(b)(4) |
These limits apply to intentional radiators in the 2.4–2.4835 GHz ISM band under U.S. FCC Part 15. ETSI, ISED, and other regulators set different limits for their bands.
FAQ
What does EIRP measure?
EIRP is the power a theoretical isotropic antenna would need to radiate uniformly in every direction to match the peak signal strength your real antenna produces in its strongest direction. It combines transmitter power, cable loss, and antenna gain into one comparable number.
What’s the difference between EIRP and ERP?
EIRP references power to a theoretical isotropic antenna; ERP references it to a real half-wave dipole. ERP is always 2.15 dB lower than EIRP for the same physical system, per the FCC’s KDB 412172 guidance.
Does the 2.15 dB dBi-to-dBd offset change with frequency?
No. The 2.15 dB gap comes from the dipole’s fixed radiation-pattern geometry, not its operating frequency, so it applies the same way at 900 MHz or 6 GHz — a detail most calculators leave unexplained.
Why does my calculated EIRP look higher than what I measure on-site?
Cable loss entered as a flat per-length spec often omits connector and adapter loss, and antenna gain in dBd sometimes gets used as if it were dBi without adding 2.15 dB — both inflate the calculated result above reality.
Why do regulators limit EIRP instead of just transmitter power?
EIRP reflects the actual radiated power a receiver or bystander experiences, regardless of how it was produced. A low-power transmitter paired with a high-gain antenna can radiate as much as a high-power transmitter on a simple antenna.