EIRP Calculator

EIRP (dBm): Effective isotropic radiated power. It is the radiated power referenced to an ideal isotropic antenna, which spreads energy equally in all directions.[4]

ERP (dBm): Effective radiated power. It is a similar idea, but referenced to a half-wave dipole instead of an isotropic antenna; some rules and bands use ERP limits instead of EIRP.[4]

dBm: A power level compared with 1 milliwatt. For example, 0 dBm means 1 mW.

dBW: A power level compared with 1 watt. Because 1 W = 1000 mW, dBm is 30 dB higher than dBW for the same power.

dBi: Antenna gain compared with an isotropic antenna, which is the reference used for EIRP.

dBd: Antenna gain compared with a half-wave dipole. To use it in the EIRP formula, convert it to dBi by adding 2.15 dB.

dB loss: A reduction term such as coax, connector, or attenuator loss. In the calculator, losses are subtracted from transmitter power plus antenna gain.[2]

Transmitter power: The power coming out of the radio before antenna gain is applied. Real-world limits are often written in terms of transmitter power and EIRP together.[2]

Mistake: Typing an EIRP target into "Transmitter power (selected unit)" instead of the radio's actual output.
Fix: In EIRP mode, enter only the radio output in "Transmitter power (selected unit)" and let the calculator produce "EIRP (dBm)" and "EIRP (W)".

Mistake: Entering cable loss as a negative number in "Total losses between radio and antenna (dB)".
Fix: Loss should normally be a positive dB number because the formula subtracts it already. Re-enter "Total losses between radio and antenna (dB)" as 0 or a positive value.

Mistake: Leaving "Antenna gain (dBi)" as a dBi label when your antenna spec is actually in dBd.
Fix: Keep the gain number in "Antenna gain (dBi)" but change "Antenna gain reference" to dBd so the calculator adds the 2.15 dB conversion before computing "EIRP (dBm)".

Mistake: Using 0 or a negative value with "Transmitter power unit" set to W or mW.
Fix: If the unit is W or mW, "Transmitter power (selected unit)" must be greater than 0 because logarithms cannot convert zero or negative power into dBm.

Mistake: Filling in both power sections and expecting both to affect the math.
Fix: The active mode decides which field matters. In "Max Tx power" mode, the calculator uses "EIRP limit (selected unit)" and ignores the transmitter power inputs for the result "Max transmitter power to meet limit (dBm)".

Mistake: Thinking "ERP (dBm)" and "EIRP (dBm)" are the same quantity with different names.
Fix: They use different antenna references. Turn on "Also compute ERP (optional)" and compare both outputs so you can report the one your rule or datasheet actually asks for.

• The calculator uses the standard strongest-direction dB method: transmitter power plus antenna gain minus total losses. It does not model a full antenna pattern, beam shape, polarization mismatch, or fading.
• "Antenna gain (dBi)" is treated as the gain in the direction of maximum radiation. If your antenna data is off-axis or averaged, real field strength can differ.
• "Total losses between radio and antenna (dB)" is a single combined value. If your system has variable cable loss, duplexers, filters, or frequency-dependent parts, actual EIRP may change by channel.
• When using W or mW inputs, the power must be greater than 0 for conversion into dB units. Zero or negative physical power is not valid for log conversion.
• The calculator can show negative dBm results, and that is valid. Negative dBm does not mean negative physical power; it means less than 1 mW.
• ERP is shown only when enabled, and it is derived from EIRP using the 2.15 dB reference difference. If a rule uses a different convention, follow that rule's wording.
• Regulatory compliance depends on the exact service, band, country, averaging rule, and equipment setup. Use this tool as a math helper, then compare the result with the actual rule that applies to your system.

Core method

The calculator first converts the active power input into dBm, because dB-based RF math is easiest when everything is in matching logarithmic units. Then it applies antenna gain and subtracts losses.

EIRP_dBm = TxPower_dBm + AntennaGain_dBi - TotalLoss_dB

This is the standard practical EIRP relationship used in radio system setup guides.[2]

Power conversions

If you enter power in W, mW, or dBW, the calculator converts it to dBm before using the main formula. Results are then converted back into W and mW for easier reading.

P_mW = 10^(P_dBm/10)

P_W = 10^((P_dBm - 30)/10)

P_dBm = 10 x log10(P_mW)

P_dBm = 10 x log10(P_W) + 30

P_dBW = P_dBm - 30

These conversions require watts or milliwatts to be greater than zero.

Max transmitter power mode

When you choose the mode for finding the largest allowed radio output, the calculator rearranges the same EIRP equation to solve for transmitter power.

TxMax_dBm = EIRPLimit_dBm - AntennaGain_dBi + TotalLoss_dB

This is useful because many rules cap EIRP, while your device settings are usually entered as transmitter output power.[2]

Gain reference conversion

If your antenna spec is given in dBd instead of dBi, the calculator converts it before applying the EIRP formula.

Gain_dBi = Gain_dBd + 2.15

That 2.15 dB difference comes from the half-wave dipole reference being higher than the isotropic reference.

ERP option

If you enable ERP, the calculator also reports power referenced to a half-wave dipole.

ERP_dBm = EIRP_dBm - 2.15

Rules may use either EIRP or ERP depending on the band and service, so the output labels matter.[2][2]

Worked mini-example

Suppose "Transmitter power (selected unit)" is 20 dBm, "Antenna gain (dBi)" is 6, and "Total losses between radio and antenna (dB)" is 1.5. Then:

EIRP_dBm = 20 + 6 - 1.5 = 24.5 dBm

EIRP_W = 10^((24.5 - 30)/10) = 0.2818 W

So the strongest-direction radiated power is 24.5 dBm, which is about 0.282 W.

Assumptions used by the calculator

The math assumes one total loss value, one antenna gain value in the strongest direction, and correct unit/reference selection by the user. Real systems can differ because of mismatch, frequency-dependent loss, mounting effects, or service-specific legal wording.[4]