EIRP Calculator (eirp calculator)
Effective Isotropic Radiated Power: EIRP (dBm) = Pt (dBm) + Gt (dBi) − L (dB). Four-way solver, ERP↔EIRP converter, regulatory limit table, and Friis distance estimation — 100% in your browser.
EIRP solver — all directions
ERP ↔ EIRP converter
ERP uses a half-wave dipole reference; EIRP uses isotropic. The difference is fixed: ±2.15 dB (EIRP ≈ 1.64× ERP in watts).
Power & gain unit conversion
EIRP regulatory limits (common examples)
Distance estimation — Friis equation (free-space approximation)
Quick example
A 1 W (30 dBm) transmitter, 6 dBi antenna, 3 dB cable loss → EIRP = 30 + 6 − 3 = 33 dBm = 1,995.26 mW ≈ 1.9953 W. Equivalent ERP = 33 − 2.15 = 30.85 dBm ≈ 1.2162 W.
Frequently asked questions (FAQ)
What is EIRP?
EIRP (Effective Isotropic Radiated Power) is the total power a radio system radiates toward the antenna's strongest beam direction, stated as if radiated by an ideal isotropic antenna. The formula: EIRP (dBm) = transmitter power P_t (dBm) + antenna gain G_t (dBi) − total line loss L (dB). Example: a 1 W (30 dBm) transmitter, 6 dBi antenna, 3 dB cable loss gives EIRP = 30 + 6 − 3 = 33 dBm, or 1,995.26 mW ≈ 1.9953 W.
How do I calculate EIRP?
Convert all quantities to dB units first: transmitter power to dBm, antenna gain to dBi (add 2.15 if known in dBd), and line loss in dB. Then add them: EIRP = P_t + G_t − L. The calculator on this page does it automatically from any units (W, mW, dBm, dBW for power; dBi or dBd for gain).
What is the difference between EIRP and ERP?
Both measure effective radiated power, but use different reference antennas: EIRP uses an ideal isotropic antenna, while ERP uses a half-wave dipole. Because the dipole itself has 2.15 dB gain over isotropic, EIRP is always 2.15 dB larger than ERP for the same system: ERP (dBm) = EIRP (dBm) − 2.15. In watts, EIRP ≈ 1.64 × ERP — don't subtract 2.15 directly from a watt value. Example: EIRP 33 dBm (1.9953 W) equals ERP 30.85 dBm (1.2162 W).
What is the legal Wi-Fi EIRP limit?
Commonly used example limits: Wi-Fi 2.4 GHz FCC (US) max 36 dBm EIRP (≈3.98 W), Wi-Fi 5 GHz U-NII-2A/2C FCC max 30 dBm EIRP (1 W), while ETSI (Europe) limits Wi-Fi 2.4 GHz to only 20 dBm EIRP (100 mW) and U-NII-1 to 23 dBm EIRP (≈200 mW). The full table is in the Regulatory Limits section. This is general information, not legal advice — always check the applicable local regulations.
Can EIRP be solved backwards — e.g. finding the required transmitter power?
Yes. The EIRP formula can be reversed for all four variables: P_t = EIRP − G_t + L (transmitter power from an EIRP target), G_t = EIRP − P_t + L (required antenna gain), and L = P_t + G_t − EIRP (maximum allowable line loss). Pick the calculation mode on the Solver tab to compute in any direction.
Why is antenna gain added, not subtracted?
Antenna gain focuses energy in a particular direction, so the power density in the strongest beam direction is larger than from an isotropic antenna with the same input power. EIRP measures the combined effect of input power and that focusing, so gain adds to EIRP while cable loss reduces the power reaching the antenna and is therefore subtracted.
What is the difference between dBi and dBd?
dBi is gain relative to an ideal isotropic antenna, while dBd is gain relative to a half-wave dipole. Because the dipole itself has 2.15 dB gain over isotropic, the conversion is: dBi = dBd + 2.15. The EIRP formula uses dBi; if your antenna spec is in dBd, the calculator converts it automatically.
How do I convert dBm to watts?
dBm is power relative to 1 mW: P(dBm) = 10 × log10(P / 1 mW). For watts: P(W) = 10^((dBm − 30)/10). Reference values worth memorizing: 0 dBm = 1 mW, 20 dBm = 100 mW, 30 dBm = 1 W, 36 dBm ≈ 3.98 W. dBW is the same but relative to 1 W: dBW = dBm − 30.
What is the Friis distance estimation mode?
This mode uses the free-space Friis transmission equation: d = λ/(4π) × √(EIRP × G_r / P_min), with λ wavelength, G_r receive antenna gain, and P_min receiver sensitivity. Example: EIRP 36 dBm (≈3.98 W) at 2400 MHz with a 0 dBi receive antenna and −90 dBm sensitivity gives a theoretical distance of ≈19.83 km. This is a free-space approximation with no obstructions — real-world distance is always shorter because of buildings, trees, and multipath.
Does this calculator replace link budget calculations?
Not entirely. This calculator accurately computes the transmitter-side EIRP, which is one component of a link budget. A full link budget also needs free-space loss, receiver gain, sensitivity, and fading margin. The Friis mode gives an order-of-magnitude distance picture, not a replacement for professional RF planning.
How is this calculator different from feedline-lab (#142)?
This #1599 is a dedicated EIRP calculator: four-way solver (EIRP, power, gain, loss), ERP↔EIRP converter, full unit conversion, regulatory limit table, and Friis distance estimation. #142 feedline-lab is a coaxial line-loss suite (13 cable types, SWR formulas, etc.) whose one output happens to be ERP/EIRP. If you need detailed cable loss calculation, use feedline-lab.
Are this calculator's results accurate enough for regulatory purposes?
The formulas and conversions are mathematically exact, but regulatory compliance depends on many factors beyond formulas: actual measured power (not brochure figures), real antenna gain including efficiency, connector losses, and country-specific rules that can change. Use it as an initial aid, and verify with measurements and official regulatory documents before operating equipment.
Why does EIRP use dBm instead of watts?
The dB scale turns multiplication into addition, so EIRP = P_t + G_t − L can be computed directly without a calculator, and huge value ranges (microwatts to kilowatts) are represented compactly. This calculator shows results in dBm, dBW, W, and mW at once so you can choose freely.
Is my data sent to a server?
No. All calculations run 100% in your browser (client-side JavaScript) — no input is sent to any server.
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