EIRP Calculator: Tx Power, Antenna Gain and Feedline Loss

EIRP Calculator

Compute Effective Isotropic Radiated Power from transmitter output, antenna gain, cable and connector losses. EIRP equals Tx power plus antenna gain minus feedline loss, reported in dBm and watts, with ERP referenced to a half-wave dipole and the net system gain of your RF chain.

📡Real RF System Presets

🔧RF Chain Inputs

Transmitter output power

Power at the radio port before the feedline.

Transmitter power unit

Watts are converted with 10 log10(P x 1000).

Antenna gain

Peak gain of the antenna in its main lobe.

Antenna gain unit

dBi = dBd + 2.15, converted internally.

Cable / feedline loss (dB)

Total coax or waveguide loss between radio and antenna.

Loss per connector (dB)

Insertion loss of each connector or jumper joint.

Number of connectors

Count of mated connector pairs in the path.

Frequency (MHz, reference)

Shown for context; does not change EIRP math.

Result decimal places

Controls rounding on every result card.

EIRP 0 dBm isotropic radiated power

EIRP in watts 0 W linear power equivalent

ERP (dipole ref) 0 dBm EIRP minus 2.15 dB

Net system gain 0 dB antenna gain minus losses

🔢Formula Snapshot

EIRPP + G - L

Watts10^((dBm-30)/10)

ERPEIRP - 2.15

2.15dBi minus dBd

📡EIRP From Tx Power and Net Gain

Tx Power	Net Gain (G - L)	EIRP (dBm)	EIRP (Watts)
20 dBm	6 dB	26 dBm	0.398 W
30 dBm	9 dB	39 dBm	7.94 W
20 dBm	3 dB	23 dBm	0.200 W
24 dBm	5 dB	29 dBm	0.794 W
27 dBm	12 dB	39 dBm	7.94 W
23 dBm	8 dB	31 dBm	1.26 W
33 dBm	15 dB	48 dBm	63.1 W
36 dBm	6 dB	42 dBm	15.8 W

📏dBm to Watts Reference

Power (dBm)	Power (Watts)	Power (mW)	Common Use
0 dBm	0.001 W	1 mW	Reference level
10 dBm	0.01 W	10 mW	Low-power beacon
20 dBm	0.1 W	100 mW	WiFi client radio
23 dBm	0.2 W	200 mW	EU 2.4 GHz limit
30 dBm	1 W	1000 mW	Max WiFi conducted
36 dBm	4 W	4000 mW	US PtMP EIRP cap
43 dBm	20 W	20000 mW	Small cell PA

⚖Regulatory EIRP Limits (Informational)

Band	Region	Application	EIRP Limit dBm	EIRP Limit W	Note
2.4 GHz	US (FCC)	Point-to-multipoint	36 dBm	4 W	30 dBm conducted max
2.4 GHz	EU (ETSI)	WiFi / BLE	20 dBm	0.1 W	100 mW EIRP ceiling
2.4 GHz	US (FCC)	Point-to-point	Up to 158 dBm	Very high	1 dB PA cut per 3 dBi
5.8 GHz	US (FCC)	PtP UNII-3	53 dBm	200 W	No PA reduction rule
5.15-5.25 GHz	EU (ETSI)	Indoor WiFi	23 dBm	0.2 W	200 mW indoor only
915 MHz	US (FCC)	ISM / LoRa	36 dBm	4 W	Part 15.247 spread
868 MHz	EU (ETSI)	ISM / LoRa	14 dBm	0.025 W	25 mW ERP duty cycle
60 GHz	US (FCC)	WiGig / mmWave	82 dBm	Very high	High-gain arrays

📡dBi and dBd Gain Conversion

Antenna Type	Gain (dBi)	Gain (dBd)	Typical Pattern
Isotropic ideal	0 dBi	-2.15 dBd	Perfect sphere
Half-wave dipole	2.15 dBi	0 dBd	Omni doughnut
Quarter-wave whip	5.15 dBi	3 dBd	Vertical omni
Collinear omni	8.15 dBi	6 dBd	Tall vertical
Panel sector	14 dBi	11.85 dBd	Directional 90 deg
Yagi beam	16 dBi	13.85 dBd	Narrow forward
Parabolic dish	30 dBi	27.85 dBd	Pencil beam

⚙Formula Breakdown

EIRP = P + G - LEffective isotropic radiated power in dBm equals transmitter power plus antenna gain in dBi minus total feedline loss. Example: 20 + 6 - 2 = 24 dBm.

Total loss LAdd cable loss to connector loss times the connector count. So 1.5 dB cable plus 2 x 0.25 dB connectors gives L = 2.0 dB.

Net gain = G - LThe system gain the antenna adds after feedline losses. With 6 dBi and 2.0 dB loss the net gain is 4.0 dB.

EIRP (W) = 10^((dBm - 30)/10)Convert EIRP in dBm to watts. 24 dBm gives 10^((24 - 30)/10) = 10^(-0.6) = 0.251 W.

ERP = EIRP - 2.15ERP is referenced to a half-wave dipole instead of an isotropic source, so it is 2.15 dB lower. 24 dBm EIRP equals 21.85 dBm ERP.

dBi = dBd + 2.15Dipole-referenced gain converts to isotropic gain by adding 2.15. A 6 dBd antenna is 8.15 dBi.

Tx dBm from wattsIf power is in watts, dBm = 10 x log10(P x 1000). So 0.1 W is 10 x log10(100) = 20 dBm.

💡RF Link Planning Tips

Watch the regulatory ceiling: A 30 dBm radio on a 12 dBi antenna with 2 dB feedline loss yields 40 dBm EIRP, which exceeds the US 36 dBm point-to-multipoint cap by 4 dB. Cut power to 26 dBm, or use a lower-gain antenna, so the EIRP lands at or below 36 dBm (4 W).

Feedline loss is real EIRP loss: At 2.4 GHz, 30 meters of RG-58 loses roughly 20 dB, turning a 30 dBm radio into just 10 dBm at the antenna. Swapping to low-loss LMR-400 drops that to about 6.4 dB, recovering nearly 14 dB and quadrupling effective range. Keep cable runs short and use quality coax.

You have an antenna specifically designed to take energy from your radio, which outputs exactly one watt, and direct that energy into a narrow beam. The signal should be plenty strong enough to span length of town on paper. But in practice? Most of the power gets absorbed by coaxial cable running up pole. That’s where Effective Isotropic Radiated Power (EIRP) come in.

It’s not just another word to remember. It’s the one and only number that will tell you if your link work or fail. And your neighbor cares about it too, because EIRP is what regulators care about and it tell them the truth about how far your system can reach.

What Is EIRP and Why It Matters

EIRP is an acronym for Effective Isotropic Radiated Power. It’s the amount of power your system would seem to have if it were a perfect point source radiating out of its best direction. That means actual antennas don’t radiate equally in every direction; instead, they concentrate their energy in some sort of beam or doughnut pattern, which causes signal to look stronger then the raw transmitter output indicates.

For example, imagine you have a one watt radio and feed that into a six dBi antenna with no cable loss. Your system would then behave like a four hundred milliwatt source coming from an isotropic source (a perfect point source) at the feed point. Therefore, when it comes to planning for coverage and interfering potential, EIRP is importanter than conducted power.

Decibel math remain easy, it’s addition instead of multiplication. It’s basically Transmit Power + Antenna Gain. Total Feedline Loss. People tend to remember the addition (the gain) and overlook subtraction (loss). That’s where the math becomes expensive.

This page has a calculator that does the math for you right now, but knowing what each number mean helps keep you from shooting yourself in the foot down the road. “Transmitter power” refers to output of your radio port before any cables has been added. Antenna gain should also point to something specific like a reference antenna. Gain is typically specified either as dBd (gain relative to a half-wave dipole) or dBi (gain relative to an isotropic source).

Gain of 8.15 dBi. 6 dBd. It is 12 dBd. The two are not interchangeable; mixing them up gets you a quiet mistake which reduce your range by close to a third. The tool will convert between units internally so you don’t need to do any head-math with specs.

RF links have a quiet killer: feedline loss. As cable lengths increases and frequency increases, so does the attenuation of the cable. At 2.4 GHz, twenty decibels can be eaten by long run of thin RG-58 and you’ve gone from your one watt radio to a few milliwatts before your signal ever reaches antenna.

Upgrade that run to low-loss LMR-400 and you might get back over ten decibels, more than doubling your effective range. Each tenth of a decibel improvement in the feedline is a tenth of a decibel improvement on your final EIRP. Good connectors and short runs pays off directly in radiated power.

That net system gain card in the results tell you just how many decibels of antenna boost remain after subtracting out all those losses. Watts and dBm are regulatory limits and engineering specs that convert back and forth. Rules is usually expressed in watts, but dBm is more convenient for calculating gain and loss. Compliance checking gets easy when you see it in both formats at once.

One other thing: make sure you are comparing apples-to-apples, EIRP vs. ERP. Effective Radiated Power is measured against a half-wave dipole (not an isotropic source). 15 dB less then EIRP. WiFi specs express their limits in terms of EIRP; broadcast and land-mobile rules often gives limits in ERP.

Both are reported on the calculator so you have what you need to comply with whatever standard governs your service, no more guessing about the offset. Realistic presets load parameters for popular systems such as ham repeaters or LoRa sensors. They visualize how antenna gain, feedline losses, and transmitter power trade off by band. You can use them to test out various scenario before committing to hardware.

Confirm results against regulatory reference tables included and tweak power/gain until operating on the correct side of the law for your jurisdiction. If you’re certifying an IoT product or aligning a backhaul dish, the difference between static and a robust connection is getting these numbers right. You should of checked this before.

Begin with the basics, be sure to account for each decibel of loss, and let the apparent power do the talking.