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EIRP Calculator and dBm to Watt Converter (dBi / dBd Included)

Convert between dBm, watt and milliwatt, switch antenna gain between dBi and dBd, and get EIRP and ERP in one step — with the cable loss applied where it actually sits, between the radio and the antenna. Power density and field strength at a given distance are calculated as well.

dBm ↔ W ↔ mW dBi ↔ dBd (2.15 dB) EIRP, ERP, mW/cm², V/m

Transmitter and antenna

Pick the unit you already have; the rest is converted.

Enter conducted power at the radio port, before the feeder.
dB
Between the radio port and the antenna port. Use a positive number.
m
On-axis, far-field estimate only.

Quick reference

The conversions people ask for most.

dBmWattTypical use
01 mWReference point
1010 mWShort-range telemetry
20100 mWWi-Fi access point
27500 mWHandheld radio
301 WOutdoor CPE, small cell
364 W2.4 GHz EIRP ceiling in FCC rules
4010 WBase station
4640 WMacro radio unit

3 dB is a factor of two in power: 30 dBm is 1 W, 33 dBm is 2 W, 36 dBm is 4 W.

Effective isotropic radiated power
52.5dBm
= 178 W EIRP
Conducted power30.0 dBm / 1.00 W
Antenna gain24.0 dBi / 21.9 dBd
Feeder loss−1.5 dB
EIRP52.5 dBm / 178 W
ERP (referenced to a half-wave dipole)50.4 dBm / 108 W
Power density at 100 m0.14 µW/cm²
Equivalent field strength at that distance0.73 V/m
EIRP with 3 dB more antenna gain55.5 dBm / 355 W
Converted and applied: feeder loss is subtracted before EIRP is calculated.

EIRP is a regulatory and link-budget figure, not a measurement of how far the signal reaches. Two antennas with the same EIRP behave very differently depending on beamwidth.

The arithmetic

P(dBm) = 10 · log10( P(mW) )
P(mW) = 10^( P(dBm) / 10 )

dBd = dBi − 2.15
EIRP(dBm) = Pconducted(dBm) + G(dBi) − Lfeeder(dB)
ERP(dBm) = EIRP(dBm) − 2.15

S(mW/cm²) = EIRP(mW) / ( 4π · dcm² )
E(V/m) = √( 30 · EIRP(W) ) / dm

The power-density and field-strength expressions assume free-space, far-field conditions and a single radiating element. They are engineering estimates, not a compliance assessment.

EIRP, ERP and conducted power are three different numbers

Conducted power is what leaves the radio connector. Add the antenna gain and subtract the feeder loss and you have EIRP, the power that would have to be fed into a theoretical isotropic radiator to produce the same peak field in the main beam. ERP is the same idea referenced to a half-wave dipole instead, which is 2.15 dB lower for the same hardware. A radio sold as “1 W” with a 24 dBi antenna and 1.5 dB of feeder loss is radiating 52.5 dBm EIRP — about 178 W in that direction.

Why the unit matters more than the number

A 30 dBm radio and a 2 W radio are the same thing, but a 30 dBi antenna and a 30 dBd antenna are not: the second one is 2.15 dB stronger. Datasheets switch between the two without warning, and the error is small enough to survive review and large enough to break a link budget.

Power density, and what it does not tell you

The density figure assumes the energy spreads evenly over a sphere. A 24 dBi panel does not do that: it concentrates the same EIRP into a beam a few degrees wide. The on-axis number from this calculator is therefore an upper bound at that distance, and the off-axis level will be tens of decibels lower. For exposure or coexistence work, use the antenna pattern, not a single figure.

BandRegionTypical EIRP limitNote
2.4 GHz ISMUS, FCC Part 15.24736 dBm1 W conducted with a 6 dBi antenna
2.4 GHzEU, ETSI EN 300 32820 dBmAdditional restrictions apply to duty cycle and band edges
5.15–5.25 GHzUS, FCC Part 15.40730 dBmIndoor use, plus power spectral density limits
5.725–5.85 GHzUS, FCC Part 15.24736 dBm1 W conducted with a 6 dBi antenna
5.725–5.875 GHzEU, ETSI30 dBmSome sub-bands are restricted further

These are the values most often quoted in design reviews, not legal advice. Rules change, and national implementations differ — check the current text of the applicable regulation, and the antenna gain assumption it uses, before shipping.

Frequently asked questions

What exactly is EIRP?
EIRP is the transmit power that a theoretical isotropic radiator would need in order to produce the same peak power density in its main direction as your real antenna produces in its main beam. It combines conducted power, antenna gain and feeder loss into one number, which is why regulators and link budgets both use it.
What is the difference between EIRP and ERP?
They use different reference antennas. EIRP is referenced to an isotropic radiator, ERP to a half-wave dipole. A dipole has 2.15 dBi of gain over isotropic, so for the same hardware ERP is always 2.15 dB lower than EIRP. FCC rules for some bands are written in ERP, which is where most of the confusion comes from.
Why is a dBd figure always smaller than the dBi figure?
Because dBd is measured against a dipole, which is already 2.15 dB better than isotropic. The same antenna that measures 24 dBi measures 21.85 dBd. The physical performance is identical; only the reference has changed.
Does feeder loss really reduce EIRP?
Yes, and it does so before the antenna. Every decibel lost in the coax, connectors and lightning arrestor is a decibel off the EIRP, so 3 dB of loss costs half your radiated power. This is why moving the radio to the mast often gains more than buying a larger antenna.
Can I use the power density figure for safety compliance?
Not on its own. The calculation assumes free space, far-field conditions and uniform radiation, while a real antenna concentrates energy in a narrow beam and has near-field behaviour close to the aperture. Compliance work uses the measured antenna pattern and the specific exposure standard that applies in your country.

Related tools and pages

Link Budget CalculatorTake this EIRP into a full path calculation with fade margin. Fresnel Zone CalculatorClearance and earth bulge for the path you are planning. Antenna SelectorMatch gain and beamwidth to the link you actually have. Low PIM Antennas150 dBc rated panel and ceiling antennas for high-density sites. 5G Antennasn78 panel, feedhorn and ceiling formats. Ask an engineerSend the radio and antenna data; we will check EIRP and margin.

Buying gain or buying beamwidth?

Two antennas with the same EIRP can behave very differently on site. Tell us the band, the coverage shape and the mounting point, and we will match the pattern to the job instead of chasing the highest dBi on the page.

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