EIRP Calculator

An EIRP Calculator helps you determine the Effective Isotropic Radiated Power of an RF transmission system using transmitter power, antenna gain, and feedline loss. Instead of calculating each value manually, you can enter the three system parameters and quickly obtain EIRP in dBm, watts, and milliwatts, along with ERP and net system gain.

The core calculation is straightforward:

EIRP(dBm) = Transmitter Power(dBm) + Antenna Gain(dBi) − Feedline Loss(dB)

This makes the calculator useful for wireless communication, antenna systems, RF link budgets, point-to-point radio links, Wi-Fi deployments, satellite communication, amateur radio, and RF engineering work.

What Is an EIRP Calculator?

An EIRP Calculator is an RF engineering tool used to calculate the equivalent isotropic radiated power of a transmitting antenna system.

The calculator requires three inputs:

• Transmitter Power: Entered in dBm
• Antenna Gain: Entered in dBi
• Feedline Loss: Entered in dB

It then calculates:

• Net System Gain
• EIRP in dBm
• EIRP in watts
• EIRP in milliwatts
• ERP in dBm
• ERP in watts

The main advantage is that the calculator combines transmitter output, antenna gain, and feedline losses into a single RF power figure.

For example, increasing antenna gain increases calculated EIRP, while increasing feedline loss decreases it.

What does EIRP tell you?

EIRP describes the equivalent power that an ideal isotropic radiator would need to radiate to produce the same maximum power density as the actual transmitting antenna system.

It is therefore different from the transmitter's output power. A radio may produce a particular amount of power at its RF output, but the antenna and feedline change the effective radiated power of the complete system.

What Is EIRP?

EIRP stands for Effective Isotropic Radiated Power.

It is a way of expressing the radiated power of an antenna system relative to an ideal isotropic radiator. An isotropic radiator is a theoretical antenna that radiates equally in every direction.

In an actual RF system, the transmitter supplies power to an antenna through a feedline. The antenna has gain that concentrates radiation in particular directions, while the feedline and other components can introduce losses.

The simplified relationship is:

EIRP = Transmitter Power + Antenna Gain − Feedline Loss

When using logarithmic RF units, the calculation becomes:

EIRP(dBm) = Pt(dBm) + Gt(dBi) − L(dB)

Where:

• Pt = transmitter power
• Gt = antenna gain
• L = feedline loss

EIRP is particularly useful because it provides a standardized way to describe the effective transmitting power of an antenna system.

It is commonly encountered when analyzing wireless communication systems, directional antennas, RF links, and link-budget calculations.

Why is EIRP important?

Transmitter power alone does not describe the complete transmitting system.

Consider two systems with identical 20 dBm transmitters:

• System A uses a 2 dBi antenna.
• System B uses a 12 dBi antenna.

If their losses are identical, System B has a substantially higher calculated EIRP because its antenna provides greater directional gain.

This is why engineers consider transmitter power, antenna gain, and transmission-line losses together.

EIRP Formula

The EIRP Calculator uses the following formula:

EIRP(dBm) = Pt(dBm) + Gt(dBi) − L(dB)

Where:

• Pt(dBm) = transmitter power
• Gt(dBi) = antenna gain relative to an isotropic radiator
• L(dB) = feedline loss

The calculator also determines net system gain:

Net System Gain = Antenna Gain − Feedline Loss

This means the EIRP calculation can also be written as:

EIRP = Transmitter Power + Net System Gain

For example, if an antenna has 12 dBi gain and the feedline introduces 2 dB of loss:

Net System Gain = 12 − 2 = 10 dB

If transmitter power is 20 dBm:

EIRP = 20 + 10 = 30 dBm

So the system's calculated EIRP is 30 dBm.

Because RF power in dBm is logarithmic, adding and subtracting gains and losses is convenient for link-budget calculations.

How the EIRP Calculator Works

Using the calculator involves three basic inputs.

Step 1: Enter Transmitter Power

Enter the transmitter's RF output power in dBm.

dBm is a logarithmic power unit referenced to 1 milliwatt. It is widely used for RF and wireless communication measurements.

For example:

• 0 dBm = 1 mW
• 10 dBm = 10 mW
• 20 dBm = 100 mW
• 30 dBm = 1 W

Step 2: Enter Antenna Gain

Enter the antenna gain in dBi.

dBi expresses antenna gain relative to an ideal isotropic radiator.

Directional antennas can have substantial gain because they concentrate RF energy into particular directions rather than distributing it uniformly.

Step 3: Enter Feedline Loss

Enter feedline loss in dB.

This represents the RF power lost between the transmitter and antenna through the transmission path. Depending on the system, this may include the effect of cable and associated passive components.

Step 4: Calculate EIRP

The calculator applies:

EIRP = Transmitter Power + Antenna Gain − Feedline Loss

Step 5: Review the Results

The calculator provides six outputs:

1. Net System Gain in dB
2. EIRP in dBm
3. EIRP in watts
4. EIRP in milliwatts
5. ERP in dBm
6. ERP in watts

This makes it easy to use the result in different RF calculations or compare power levels in different units.

EIRP Calculation Example

Consider a point-to-point wireless radio system with these specifications:

• Transmitter Power: 20 dBm
• Antenna Gain: 12 dBi
• Feedline Loss: 2 dB

Calculate Net System Gain

First subtract the feedline loss from the antenna gain:

Net System Gain = 12 dBi − 2 dB

Net System Gain = 10 dB

Calculate EIRP

Now add the net system gain to transmitter power:

EIRP = 20 dBm + 10 dB

EIRP = 30 dBm

Therefore, the calculated EIRP is 30 dBm.

Convert EIRP to watts

The conversion from dBm to watts is:

P(W) = 10^((P(dBm) − 30) / 10)

For 30 dBm:

P(W) = 10^((30 − 30) / 10)

P(W) = 1 W

So:

30 dBm = 1 W

Convert EIRP to milliwatts

The conversion is:

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

Therefore:

30 dBm = 1,000 mW

Calculate ERP

The calculator uses:

ERP(dBm) = EIRP(dBm) − 2.15

Therefore:

ERP = 30 − 2.15

ERP = 27.85 dBm

This example shows why antenna gain and feedline loss need to be considered when evaluating an RF transmitting system.

EIRP Units: dBm, Watts, and Milliwatts

The calculator reports EIRP using three power representations.

EIRP in dBm

dBm is a logarithmic unit referenced to 1 mW.

The general conversion from dBm to milliwatts is:

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

dBm is particularly useful in RF engineering because gains and losses can be added or subtracted directly.

For example, a 20 dBm transmitter combined with 10 dB of net system gain results in 30 dBm EIRP.

EIRP in Watts

For conversion to watts:

P(W) = 10^((P(dBm) − 30) / 10)

Some useful reference points include:

EIRP in Milliwatts

The calculator also provides EIRP in milliwatts, which can be convenient for lower-power wireless systems.

For example:

20 dBm = 100 mW

30 dBm = 1,000 mW

Understanding Antenna Gain

Antenna gain describes how effectively an antenna concentrates radiated energy in a particular direction compared with a reference antenna.

The calculator expects antenna gain in dBi.

The important point is that antenna gain does not mean an antenna creates additional energy. Instead, an antenna's radiation pattern determines how the available RF energy is distributed spatially.

A directional antenna can concentrate energy more strongly in some directions. This produces higher gain in those directions.

Because antenna gain is included in the EIRP formula:

Higher antenna gain → Higher EIRP

assuming transmitter power and losses remain unchanged.

For example:

Transmitter Power = 20 dBm

Antenna Gain = 8 dBi

Feedline Loss = 2 dB

Then:

EIRP = 20 + 8 − 2

EIRP = 26 dBm

If the antenna gain increases to 14 dBi while everything else stays the same:

EIRP = 20 + 14 − 2

EIRP = 32 dBm

The calculated EIRP increases by 6 dB.

Understanding Feedline Loss

Feedline loss represents power lost between the transmitter and antenna.

Transmission-line losses can become an important consideration when the antenna is physically separated from the radio or when a system uses a relatively long RF cable.

The calculator subtracts the entered loss:

EIRP = Transmitter Power + Antenna Gain − Feedline Loss

For example:

• Transmitter Power = 25 dBm
• Antenna Gain = 10 dBi
• Feedline Loss = 3 dB

Then:

EIRP = 25 + 10 − 3

EIRP = 32 dBm

If there were no feedline loss:

EIRP = 25 + 10

EIRP = 35 dBm

The 3 dB loss therefore reduces the calculated EIRP by 3 dB.

When designing an RF system, it is important to use an appropriate feedline-loss value rather than simply assuming the cable has zero loss.

Understanding Net System Gain

Net System Gain is one of the outputs of this EIRP Calculator.

The formula is:

Net System Gain = Antenna Gain − Feedline Loss

It provides a quick view of how antenna gain and feedline loss affect the system as a whole.

Positive Net System Gain

If antenna gain exceeds feedline loss, the result is positive.

Example:

15 dBi − 2 dB = +13 dB

Zero Net System Gain

If antenna gain equals feedline loss:

5 dBi − 5 dB = 0 dB

Negative Net System Gain

If feedline loss exceeds antenna gain:

3 dBi − 6 dB = −3 dB

The relationship to EIRP is:

EIRP = Transmitter Power + Net System Gain

This is useful when quickly comparing different antenna and feedline configurations.

EIRP vs ERP

EIRP and ERP are related but use different antenna references.

The calculator uses:

ERP(dBm) = EIRP(dBm) − 2.15

Therefore:

ERP = EIRP − 2.15 dB

Conversely:

EIRP = ERP + 2.15 dB

The 2.15 dB difference comes from the relationship between isotropic and half-wave-dipole reference levels.

For example, if EIRP is 30 dBm:

ERP = 30 − 2.15

ERP = 27.85 dBm

It is important not to treat EIRP and ERP as identical measurements. They describe equivalent radiated power using different antenna reference standards.

Real-Life EIRP Calculator Use Cases

EIRP calculations appear across many types of RF systems.

Wi-Fi Access Point Deployment

Network engineers may need to evaluate the effective radiated power of an access point when using an external antenna.

The calculation can incorporate:

• Radio output power
• Antenna gain
• Feedline loss

This provides a more complete picture than transmitter power alone.

Point-to-Point Wireless Links

Directional wireless bridges often use higher-gain antennas to establish links between locations.

An engineer can calculate EIRP and then use it as part of a larger link-budget analysis involving path loss, receiving antenna gain, and other system factors.

Cellular and Distributed RF Systems

EIRP can be used to characterize the transmitting side of RF infrastructure where antenna characteristics and transmission-line losses affect the effective radiated power.

Amateur Radio

Radio operators can use EIRP calculations to understand the relationship between transmitter output, antenna gain, and feedline losses.

Satellite Communication

Satellite communication systems rely heavily on RF link-budget calculations. EIRP can represent the transmitting-side power level used as an input to broader propagation and link calculations.

RF Testing and Design

Engineers can use EIRP calculations during:

• Antenna selection
• Prototype development
• RF system configuration
• Cable selection
• Link-budget preparation
• System troubleshooting

Real-Life Example: Wireless Bridge Between Two Buildings

Imagine a business needs to connect two buildings using a directional wireless radio link.

The radio has an output power of:

23 dBm

The directional antenna provides:

17 dBi

The RF feedline introduces:

2.5 dB

Step 1: Calculate Net System Gain

Net System Gain = 17 − 2.5

Net System Gain = 14.5 dB

Step 2: Calculate EIRP

EIRP = 23 + 17 − 2.5

EIRP = 37.5 dBm

Step 3: Convert to Watts

Using:

P(W) = 10^((37.5 − 30) / 10)

The result is approximately:

5.62 W

So the system's calculated EIRP is approximately 37.5 dBm, or 5.62 W.

This does not mean the radio itself is producing 5.62 W. The transmitter output is only 23 dBm. The higher EIRP results from the antenna's directional gain, offset by feedline loss.

The engineer can then use the EIRP as part of a larger RF link-budget calculation.

This distinction is critical: EIRP is an equivalent radiated-power measurement, not simply the electrical output power of the transmitter.

EIRP in RF Link Budgets

EIRP is commonly used as a transmitting-side quantity in RF link-budget analysis.

A simplified conceptual relationship for received power is:

Received Power = EIRP − Path Loss + Receive Antenna Gain − Additional Losses

The EIRP Calculator does not calculate the entire link budget. Instead, it provides the transmitting-side EIRP that can be used as an input to additional calculations.

A complete link analysis may also need to consider:

• Frequency
• Distance
• Free-space path loss
• Receiving antenna gain
• Receiver sensitivity
• Cable losses
• Connector losses
• Polarization effects
• Atmospheric effects
• Fading
• Other propagation losses

For that reason, a high EIRP value does not automatically guarantee a successful communication link.

EIRP is one component of the overall RF system analysis.

How to Convert EIRP From dBm to Watts

To convert EIRP from dBm to watts, use:

P(W) = 10^((P(dBm) − 30) / 10)

For example, suppose the calculated EIRP is:

36 dBm

Then:

P(W) = 10^((36 − 30) / 10)

P(W) = 10^0.6

P(W) ≈ 3.981 W

Therefore:

36 dBm ≈ 3.98 W

The calculator performs this conversion automatically, eliminating the need to manually calculate the logarithmic conversion.

How to Convert EIRP From dBm to Milliwatts

The conversion formula is:

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

For example:

30 dBm = 10^(30/10)

30 dBm = 1,000 mW

Therefore:

30 dBm = 1 W = 1,000 mW

This conversion is useful when comparing EIRP with wireless equipment specifications expressed in milliwatts.

Common EIRP Calculation Mistakes

Mistake 1: Adding Feedline Loss

A common error is to add cable loss to the system power.

Incorrect:

EIRP = Pt + Gt + L

Correct:

EIRP = Pt + Gt − L

Losses reduce the resulting EIRP.

Mistake 2: Confusing dBi and dBd

dBi and dBd use different antenna references. Make sure the antenna gain entered into this calculator is expressed in dBi.

Mistake 3: Entering Watts Instead of dBm

The calculator's transmitter-power input expects dBm.

If your transmitter specification is provided in watts, convert it to dBm before entering the value.

Mistake 4: Ignoring Feedline Loss

Antenna gain does not tell you the complete system gain. Feedline losses should be considered when calculating the effective radiated power.

Mistake 5: Confusing EIRP With Transmitter Power

EIRP includes antenna gain and feedline loss. It is not simply the transmitter's RF output.

Mistake 6: Confusing EIRP With Received Power

EIRP describes the transmitting system. It does not represent the actual power arriving at a receiving antenna.

EIRP Calculator Input Guide

Transmitter Power

Enter the transmitter's RF output in dBm.

Antenna Gain

Enter the antenna's gain in dBi.

Feedline Loss

Enter the total feedline loss in dB.

Once these values are entered, the calculator determines the net system gain and EIRP.

How to Get More Accurate EIRP Results

The accuracy of an EIRP calculation depends on the quality of the input values.

Use the Appropriate Transmitter Power

Use the transmitter power corresponding to the actual operating configuration rather than automatically using a maximum advertised value.

Account for Feedline Loss

Use a feedline-loss value appropriate for the cable, length, connectors, and operating conditions being evaluated.

Verify Antenna Gain

Use the antenna gain specified for the frequency and configuration relevant to your system.

Include Relevant Passive Losses

Where appropriate, account for losses introduced by connectors, adapters, and other passive RF components within the feedline-loss figure.

Keep Units Consistent

The calculator expects:

• Transmitter power in dBm
• Antenna gain in dBi
• Feedline loss in dB

Mixing units can produce incorrect results.

EIRP Calculator vs Manual Calculation

You can calculate EIRP manually, but the process becomes repetitive when comparing several system configurations.

A manual workflow requires you to:

1. Determine antenna gain.
2. Determine feedline loss.
3. Calculate net system gain.
4. Add net gain to transmitter power.
5. Convert EIRP from dBm to watts.
6. Convert EIRP from dBm to milliwatts.
7. Convert EIRP to ERP.
8. Convert ERP to watts.

The EIRP Calculator automates these steps.

This is especially useful when evaluating multiple combinations of transmitter power, antenna gain, and feedline loss.

For RF design work, the ability to quickly test different scenarios can make preliminary system analysis much more efficient.

Frequently Asked Questions

What is EIRP?

EIRP, or Effective Isotropic Radiated Power, is a measure of equivalent radiated power referenced to an ideal isotropic radiator. It combines transmitter power, antenna gain, and feedline loss.

What is the EIRP formula?

The formula used by this calculator is:

EIRP(dBm) = Transmitter Power(dBm) + Antenna Gain(dBi) − Feedline Loss(dB)

How do you calculate EIRP?

Add transmitter power and antenna gain in their appropriate logarithmic units, then subtract feedline loss.

How do you convert EIRP from dBm to watts?

Use:

P(W) = 10^((P(dBm) − 30) / 10)

How do you convert EIRP from dBm to milliwatts?

Use:

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

What is net system gain?

Net system gain is antenna gain minus feedline loss:

Net System Gain = Antenna Gain − Feedline Loss

Does antenna gain increase EIRP?

Yes. In the calculator's equation, antenna gain is added to transmitter power, increasing calculated EIRP when other values remain constant.

Does feedline loss reduce EIRP?

Yes. Feedline loss is subtracted from the EIRP calculation.

Is EIRP the same as transmitter power?

No. Transmitter power is the RF output from the transmitter, while EIRP accounts for antenna gain and feedline losses.

What is the difference between EIRP and ERP?

EIRP is referenced to an isotropic radiator, while ERP is referenced to a half-wave dipole. The calculator uses:

ERP(dBm) = EIRP(dBm) − 2.15

What is 30 dBm EIRP in watts?

30 dBm corresponds to 1 watt.

Can EIRP be negative in dBm?

Yes. Because dBm is a logarithmic unit referenced to 1 mW, power levels below 1 mW have negative dBm values.

Practical EIRP Calculation Checklist

Before using your calculated EIRP in an RF design, check the following:

• Confirm transmitter power is in dBm.
• Confirm antenna gain is in dBi.
• Determine the relevant feedline loss in dB.
• Calculate antenna gain minus feedline loss.
• Add the resulting net system gain to transmitter power.
• Check EIRP in dBm.
• Convert EIRP to watts when required.
• Convert EIRP to milliwatts when required.
• Calculate ERP when a dipole-referenced value is needed.
• Use EIRP as the transmitting-side value in a broader link-budget calculation.
• Verify any applicable technical or regulatory requirements separately.

Limitations and Engineering Considerations

This EIRP Calculator performs a simplified calculation based on the three supplied inputs: transmitter power, antenna gain, and feedline loss.

It does not independently verify whether those inputs accurately represent a particular physical RF installation.

The calculation does not directly account for factors such as:

• Free-space path loss
• Receiver antenna gain
• Receiver sensitivity
• Atmospheric attenuation
• Polarization mismatch
• Multipath propagation
• Fading
• Detailed antenna radiation patterns
• Frequency-dependent propagation conditions

It also does not determine whether a particular RF configuration complies with a specific country's regulatory requirements.

For a real deployment, the calculated EIRP should therefore be treated as one component of the engineering analysis rather than a complete prediction of communication performance.

Conclusion

The EIRP Calculator provides a fast way to determine the effective isotropic radiated power of an RF transmitting system.

The core equation is:

EIRP(dBm) = Transmitter Power(dBm) + Antenna Gain(dBi) − Feedline Loss(dB)

By entering transmitter power, antenna gain, and feedline loss, you can calculate net system gain, EIRP in dBm, EIRP in watts, EIRP in milliwatts, ERP in dBm, and ERP in watts.

For wireless links, antenna installations, RF design, satellite communication, Wi-Fi deployments, and other RF applications, these values provide a useful starting point for understanding the transmitting side of a system.

Enter your transmitter power, antenna gain, and feedline loss into the EIRP Calculator to calculate your system's EIRP and ERP instantly.