EIRP Calculator - Effective Isotropic Radiated Power
Find the effective isotropic radiated power of a transmitter, compare it with ERP, and estimate the field it creates at a distance.
📡 What is EIRP (Effective Isotropic Radiated Power)?
EIRP, or effective isotropic radiated power, is the power an ideal antenna that radiates equally in every direction would have to emit to produce the same signal strength as a real transmitter does in the direction of its strongest beam. It is found by adding the antenna gain to the transmitter power and subtracting the losses in the cable between them. Because it folds the radio, the feed line and the antenna into a single number, EIRP is the standard way to describe how strong a transmitter looks from the outside.
You meet EIRP wherever radio links are designed or regulated. Wi-Fi and LoRa installers use it to stay within the limits on their band, satellite engineers use it to describe an uplink or a downlink beam, and cellular planners use it to compare base station sectors. It is also the first term of a link budget, where received power equals EIRP minus path loss plus the receive antenna gain, and it is the input for estimating the field strength and power density at a given distance.
Several distinctions cause confusion. EIRP is not the power the antenna actually radiates: the antenna cannot create power, it only redistributes it, so a high gain antenna radiates the same total power as a low gain one while concentrating it in a narrower beam. EIRP is also different from ERP (effective radiated power), which uses a half-wave dipole as its reference. A dipole has 2.15 dBi of gain, so ERP in dBm is EIRP in dBm minus 2.15, and in watts EIRP is about 1.64 times ERP. Finally, dBi and dBd are different references for antenna gain, so always check which one a datasheet quotes.
This calculator covers three tasks. EIRP and ERP mode converts a transmit power in dBm, dBW, watts or milliwatts into EIRP and ERP. Power density mode estimates the far-field density and electric field strength at a distance, and Link budget mode carries the EIRP through free-space path loss to a received power and a margin against the receiver sensitivity. This page does not list regulatory limits, because they vary by country and band, so look up the rule for your own situation.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — A Wi-Fi Access Point
20 dBm transmitter, 5 dBi antenna, 1 dB cable loss
Example 2 — A 2 W Radio with a Directional Antenna
2 W transmitter, 9 dBi antenna, 2 dB cable loss
Example 3 — Power Density 10 m from a 100 W EIRP Antenna
10 W transmitter, 10 dBi antenna, no cable loss, 10 m away
Example 4 — A 2.4 GHz Link over 1 km
EIRP 24 dBm, 2400 MHz, 1 km, receive antenna 2 dBi, receive loss 0.5 dB, sensitivity −90 dBm
❓ Frequently Asked Questions
🔗 Related Calculators
What is the formula for EIRP?
EIRP in dBm equals transmitter power in dBm plus antenna gain in dBi minus cable and connector losses in dB. In watts, EIRP = P x G / L, where G and L are linear ratios. A 20 dBm radio with a 5 dBi antenna and 1 dB loss gives 24 dBm, or 0.251 W.
What is the difference between EIRP and ERP?
ERP is referenced to a half-wave dipole and EIRP to an isotropic radiator. A dipole has 2.15 dBi gain, so EIRP in dBm equals ERP in dBm plus 2.15, and EIRP in watts is ERP times 1.64.
How do I convert EIRP in dBm to watts?
Use P(W) = 10^((dBm - 30) / 10). For example 24 dBm is 10^(-0.6) = 0.251 W, and 36 dBm is about 4 W. Each 10 dB is a factor of 10 in power.
How do I convert dBm to dBW?
Subtract 30. A level of 24 dBm is -6 dBW, and 40 dBm is 10 dBW. The offset exists because 1 W equals 1000 mW and 10 log10(1000) = 30.
Does cable loss reduce EIRP?
Yes. Cable, connector and filter losses between the transmitter and the antenna subtract from the power that reaches the antenna, so they subtract from EIRP in dB. A 2 dB loss cuts the radiated power by about 37 percent.
Is EIRP the actual power radiated by the antenna?
No. The antenna radiates only the power it receives. EIRP is the power an ideal isotropic antenna would need to produce the same signal strength in the direction of the main beam. A high gain antenna focuses power, so its EIRP is much larger than the power it radiates in total.
How do I calculate power density from EIRP?
In the far field, S = EIRP / (4 pi d squared) in W/m2. An EIRP of 100 W at 10 m gives 0.0796 W/m2, which is 79.58 mW/m2 or 7.96 microwatts per cm2. Doubling the distance cuts the density to a quarter.
What is the electric field strength at a distance from an EIRP?
In free space E = sqrt(30 x EIRP) / d, in volts per metre with EIRP in watts and d in metres. For 100 W at 10 m, E = 5.48 V/m. This follows from S = E squared over 377 ohms.
How do I find received power from EIRP?
Received power in dBm equals EIRP minus free-space path loss plus receive antenna gain minus receive losses. Path loss is 20 log10(4 pi d f / c). At 2400 MHz and 1 km the path loss is 100.05 dB.
What is isotropic radiator gain in dBi?
An isotropic radiator has 0 dBi by definition, because it radiates equally in every direction. Real antennas have positive dBi gain in some directions by radiating less in others. dBi is always measured against this ideal reference.
Can I use this to check a regulatory EIRP limit?
You can compare your calculated EIRP with a limit, but limits differ by country, band and rule. Some are written as EIRP and some as ERP, and they may include power spectral density terms. Look up the rule for your band and region directly.