The EIRP calculator above computes effective isotropic radiated power from the three quantities that determine it: the power leaving the transmitter, the losses between the transmitter and the antenna, and the gain of the antenna itself. EIRP is the number that describes what a receiver in the main beam actually experiences, and it is the quantity radio regulations are written in terms of.
Arb Digital publishes free engineering calculators, and this one comes with a warning that is not decoration. Transmit power limits are legal limits, not guidelines. The maximum permitted EIRP for any transmission is set by the national spectrum regulator in your country, and it depends on the frequency band, the service, the antenna, the location and the licence you hold. This page computes arithmetic. It does not tell you what you are permitted to do, and it cannot.
What This EIRP Calculator Does
It applies the standard link-budget relation, converting whatever unit you enter the transmitter power in to a common decibel scale, subtracting your stated losses and adding your stated antenna gain. It returns EIRP in dBm, in dBW and in watts, gives the equivalent ERP referenced to a half-wave dipole, and shows the power actually reaching the antenna input after the feeder.
It also compares the result against a limit you type in yourself. That limit is not supplied by this page and never will be, because only your regulator can tell you what applies to your band, your service and your licence. The comparison exists so you can see the margin in your own arithmetic, using your own figure.
How to Use It
- Enter the transmitter output power at the antenna port, in dBm, dBW, watts or milliwatts. This is the RF output, not the DC consumption.
- Enter the feeder loss in decibels — cable length multiplied by the loss per metre at your operating frequency, which is a figure from the cable datasheet, not a constant.
- Add connector, jumper, filter and arrestor losses in the second loss field.
- Enter the antenna gain and say which reference it uses. dBi and dBd differ by 2.15 dB and manufacturers quote both.
- Enter the limit your own regulator publishes for the band and service you are working in, and read the margin.
The Formula: How EIRP Is Calculated
In decibels the relation is simply additive, which is why link budgets are done in decibels at all:
EIRP (dBm) = Pt (dBm) − Ltotal (dB) + G (dBi)
Everything hangs on the reference definitions. dBm is power relative to one milliwatt, so P(dBm) = 10 log10(P in mW). dBW is relative to one watt, so it is always 30 less than the same power in dBm. dBi is antenna gain relative to a theoretical isotropic radiator that spreads power equally in every direction, which is the reference EIRP is defined against. dBd is relative to a half-wave dipole, and since a lossless half-wave dipole itself has 2.15 dBi of gain, the conversion is G(dBi) = G(dBd) + 2.15.
The word "effective" carries real meaning. Nothing is actually radiating that power in every direction. EIRP is the power an ideal isotropic radiator would have to transmit to produce the same field strength as your directional antenna produces along its boresight. Off the main beam, the radiated power is much lower.
Work the defaults through by hand. A transmitter at 30 dBm, which is 1 watt, feeding 2.5 dB of cable and 0.5 dB of connector loss, into a 12 dBi antenna, gives EIRP = 30 − 3.0 + 12 = 39 dBm. Converting, that is 1039/10 = 7,943 milliwatts, or 7.94 watts, and 9 dBW. The ERP referenced to a dipole is 39 − 2.15 = 36.85 dBm, about 4.84 watts. The power actually reaching the antenna connector is 30 − 3.0 = 27 dBm, or 501 milliwatts — half the transmitter output has gone into heating the feeder.
EIRP Limits Are Legal Limits, Set by Your Regulator
This is the part that matters more than the arithmetic. Radio spectrum is a regulated resource everywhere in the world. The maximum EIRP permitted for a given transmission is published by the national spectrum regulator — the Federal Communications Commission in the United States, Ofcom in the United Kingdom, the Australian Communications and Media Authority, the Canadian department responsible for spectrum, the national administration in each European country, and so on — and it varies by band, by service, by channel bandwidth, by antenna type and sometimes by location.
Where you see a figure like 36 dBm quoted for a particular unlicensed band, that is an example of what a regulator publishes for one specific band, in one country, under one set of conditions. It is not a general permission and it is not transferable to your situation. Many bands also require a licence before you may transmit at all, and some impose additional conditions on antenna type, out-of-band emissions, duty cycle or dynamic frequency selection.
Transmitting above a permitted limit, or transmitting at all in a band that requires a licence you do not hold, is an offence in most jurisdictions and can carry fines, equipment seizure and prosecution. The obligation to know the limit is yours, and the only reliable source is your own regulator's published conditions. The ITU Radiocommunication Sector maintains the international Radio Regulations that national frameworks derive from, and its Fixed and Mobile Services Division handles the frequency management and coordination side, but national rules are what bind you.
RF Exposure Is a Separate Obligation Entirely
Meeting an EIRP limit does not mean an installation is compliant. Human exposure to radiofrequency electromagnetic fields is governed by its own separate set of limits, based on exposure guidelines adopted into national regulation, and it constrains how close people may be to a radiating antenna rather than how much power it radiates.
Those two things are not the same question. A high-gain antenna concentrating a modest EIRP into a narrow beam can produce field strengths near the aperture that require a compliance boundary, signage and access control, even where the EIRP itself is well within limits. Exposure assessment depends on power density at distance, antenna pattern, mounting height, occupancy and whether the exposure is occupational or general public. Nothing on this page addresses it. If you are installing a transmitting antenna, the exposure assessment is a separate piece of work with its own methodology and its own legal requirements.
Why Feeder Loss Deserves More Attention Than It Gets
Cable loss is where link budgets quietly fall apart, because it is frequency-dependent and people reuse a figure from a different band. A coaxial cable that loses a fraction of a decibel per ten metres at VHF can lose several decibels over the same run in the microwave bands. Every 3 dB of feeder loss halves the power reaching the antenna.
Two practical consequences follow. First, always take loss per unit length from the cable datasheet at your actual operating frequency, and add the connectors separately — they are not negligible at microwave. Second, on a long run it is often better to mount the amplifier at the antenna than to push more power up a lossy cable, because the loss applies to what you send and to what you receive alike. The dBm to watts converter is useful for seeing what a few decibels of loss actually costs in power terms.
Where This Sits Next to the Other RF Tools
This page gives you the transmit end of a link budget. The free space path loss calculator gives you what the signal loses on the way, so subtracting one from the other and adding the receive antenna gain gives the received power. The dipole antenna calculator handles the physical antenna dimensions rather than its gain, which is a different question from the one answered here.
For the decibel arithmetic itself, the decibel calculator and the dBm to watts converter cover conversions and ratios, and the power converter moves between power units generally. The wavelength calculator and the frequency converter handle the frequency side that determines both antenna size and cable loss.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Mixing dBi and dBd — they differ by 2.15 dB, and using a dBd figure as though it were dBi understates your EIRP.
- Using a cable loss figure from the wrong frequency — loss rises steeply with frequency, and a VHF figure applied at microwave is badly optimistic.
- Assuming a limit you read somewhere applies to you — limits are national, per-band and per-service, and only your own regulator's published conditions govern what you may do.
- Treating an EIRP check as an exposure assessment — RF exposure limits are a separate legal obligation with a separate methodology.
- Entering DC input power instead of RF output power — a transmitter's efficiency is well below 100 per cent, so the two are very different numbers.
Related Free Tools From Arb Digital
Pair this with the free space path loss calculator to complete a link budget, and the dipole antenna calculator for the antenna's physical dimensions. Handle the decibel arithmetic with the decibel calculator and the dBm to watts converter, convert power with the power converter, and cover the frequency side with the wavelength calculator and the frequency converter. Everything Arb Digital publishes sits on the free online tools hub.
Frequently Asked Questions
Only your national spectrum regulator can answer that. The permitted maximum depends on the frequency band, the service, the channel bandwidth, the antenna, sometimes the location, and the licence you hold. Figures quoted online are examples of what one regulator publishes for one band in one country, and they are not a permission that transfers to your situation.
The reference antenna. EIRP is referenced to a theoretical isotropic radiator; ERP is referenced to a half-wave dipole, which itself has 2.15 dBi of gain. The same transmission is therefore always 2.15 dB higher expressed as EIRP than as ERP. Which one a regulation uses varies, so check before comparing a figure against a limit.
It can, because the limit is on radiated power rather than transmitter power. Gain adds directly to EIRP, so replacing an antenna with a higher-gain one raises EIRP by the difference in gain even though the transmitter has not changed. Some regulations handle this by requiring transmitter power to be reduced as antenna gain rises.
That depends entirely on the band, the service and the country. Some bands are licence-exempt subject to strict conditions on power, antenna and emissions; many others require an individual or class licence before you may transmit at all. Transmitting without a required licence is an offence in most jurisdictions, and the regulator's published conditions are the only reliable source.
No. RF exposure compliance is a separate obligation with its own limits and its own assessment method, based on field strength and power density at distance rather than on radiated power. A directional antenna can require a compliance boundary and access control even where its EIRP is comfortably within limits. This page does not address exposure at all.
Before, because it happens before. The signal leaves the transmitter, loses power in the cable and connectors, and only then reaches the antenna that applies gain. In decibels the order does not change the arithmetic, but it matters for understanding the intermediate figure the tool shows as the power at the antenna input.
Because antenna gain is not amplification. A directional antenna does not create power; it concentrates the same power into a narrower beam, so the field strength along the boresight is higher than an isotropic radiator would produce. EIRP describes that concentrated beam, which is why it can exceed the transmitter output.
The arithmetic is the same, and EIRP is the standard way satellite uplinks are specified. The regulatory position is not the same at all: satellite services involve coordination, filings and operator requirements well beyond a terrestrial licence, and the figures come from the operator's access procedures rather than from a general calculation.
This tool is provided for educational and preliminary engineering use. It performs a link-budget calculation only and does not determine what you are permitted to transmit. Maximum permitted EIRP, licensing requirements and emission conditions are set by your national spectrum regulator for each band and service, and exceeding a permitted limit or transmitting without a required licence is an offence in most jurisdictions. RF exposure compliance is a separate legal obligation not addressed here. Confirm every limit with your national spectrum regulator and, where an installation is involved, with a qualified RF engineer.
