EIRP / ERP Calculator

EIRP and ERP are the quickest way to understand how much RF power your setup actually radiates after antenna gain and real-world feedline losses. This matters for both range planning and compliance checks, because the same transmitter power can produce very different results with different antennas, cables, and connectors. Use the calculator below to enter your TX power (W/mW/dBm/dBW), antenna gain (dBi/dBd), and typical losses. It will instantly output EIRP (isotropic reference) and ERP (dipole reference) in multiple units, so you can compare configurations and validate your RF chain quickly.

EIRP vs. ERP: the practical guide for accurate RF power calculations

Knowing your transmitter’s “rated power” is only the starting point. What really matters in the air is radiated power—after you account for antenna gain and all the losses between your radio and the antenna. That’s why EIRP and ERP are used everywhere from Wi-Fi and LoRa to point-to-point microwave links, ham radio stations, cellular small cells, and RF lab setups.

This guide explains what EIRP/ERP mean, how to calculate them correctly, how to convert W, mW, dBm, dBW, and how to avoid common mistakes that can cause wrong compliance checks or unrealistic coverage expectations. You’ll also find multiple worked examples and an expanded FAQ.

What is EIRP?

EIRP stands for Effective Isotropic Radiated Power. It represents the power a hypothetical isotropic radiator (an ideal antenna that radiates equally in all directions) would need to emit to match the peak power density produced by your real system in its strongest direction.

Put simply: EIRP tells you how “strong” your transmitter + antenna system is in the direction the antenna focuses energy.

Why EIRP matters

• It’s widely used in regulatory limits (many bands specify EIRP).

• It’s the number you plug into a link budget as the transmit-side “output.”

• It lets you compare different antennas and feed systems on equal footing.

What is ERP?

ERP stands for Effective Radiated Power and is referenced to a half-wave dipole, not an isotropic radiator.

A half-wave dipole has 2.15 dB gain compared to isotropic. So:

• ERP (dBm) = EIRP (dBm) − 2.15 dB

• ERP (dBW) = EIRP (dBW) − 2.15 dB

When ERP is used

ERP appears often in broadcasting and in some regional or legacy regulations. If your limit is stated in ERP, always convert before comparing.

The core formulas you actually use

EIRP in dB units

The most practical form is in dBm or dBW:

EIRP (dBm) = Ptx (dBm) + Antenna Gain (dBi) − Total Loss (dB)

Where “Total Loss” commonly includes:

• coax/feedline loss

• connector/adapter loss

• filters, duplexers, combiners, splitters

• lightning arrestors (insertion loss)

• any inline attenuators

ERP from EIRP

ERP (dBm) = EIRP (dBm) − 2.15 dB

dBi vs dBd (and why it changes the answer)

Antenna gain is usually specified as:

• dBi: gain relative to isotropic

• dBd: gain relative to a half-wave dipole

Conversion:

• dBi = dBd + 2.15

• dBd = dBi − 2.15

Common mistake: plugging dBd into an EIRP formula (which expects dBi) makes your EIRP 2.15 dB too low.

Power unit conversions (W, mW, dBm, dBW)

Quick relationships

• dBm = dBW + 30

• dBW = dBm − 30

Convert dBm to linear power

• W = 10^((dBm − 30) / 10)

• mW = 10^(dBm / 10)

Useful anchors (great for sanity-checking):

• 0 dBm = 1 mW

• 10 dBm = 10 mW

• 20 dBm = 100 mW

• 30 dBm = 1 W

• 33 dBm ≈ 2 W

• 36 dBm ≈ 4 W

• 40 dBm = 10 W

• 43 dBm ≈ 20 W

• 50 dBm = 100 W

Understanding losses: why “small” dB values matter

In RF, 3 dB is roughly a half/double effect in power (depending on direction), so a couple of dB can be meaningful.

Typical real-world loss sources

• Coaxial cable loss: often the biggest loss, grows with frequency and length

• Connectors/adapters: each adds a bit; poor quality adds more

• Filters/duplexers/splitters: can add noticeable insertion loss

• Lightning arrestor: small but real insertion loss

• Feedthroughs and patch panels: often overlooked

Practical takeaway: If you’re building a clean RF chain, you can gain “free performance” by reducing unnecessary adapters and using lower-loss cable, especially at UHF and above.

Worked examples (realistic scenarios)

Example 1: LoRa / ISM device (sub-GHz) with modest antenna

Assume:

• TX power: 100 mW

• Antenna gain: 2 dBi

• Cable loss: 0.7 dB

• Connector loss: 0.3 dB

• Other loss: 0.0 dB

Convert 100 mW to dBm:

• 100 mW = 20 dBm

Total loss:

• 0.7 + 0.3 + 0.0 = 1.0 dB

EIRP:

• 20 + 2 − 1 = 21 dBm
  That’s about 126 mW EIRP (since 21 dBm ≈ 125.9 mW)

ERP:

• 21 − 2.15 = 18.85 dBm (≈ 76.7 mW ERP)

Why this is useful: you can immediately see that even with 100 mW at the radio, losses and gain shift your effective radiated power. This is exactly what many ISM band limits care about.

Example 2: Wi-Fi router with external antenna and cable run

Assume:

• Conducted TX power: 20 dBm (100 mW)

• Antenna gain: 5 dBi

• Cable loss: 2.0 dB

• Connector loss: 0.5 dB

• Other loss: 0.0 dB

Total loss = 2.5 dB
EIRP = 20 + 5 − 2.5 = 22.5 dBm (≈ 177 mW EIRP)

Key insight: a “bigger antenna” doesn’t automatically mean huge EIRP if your cable run is long or lossy. Cutting cable loss by 1–2 dB can be as impactful as upgrading antenna gain.

Example 3: Ham VHF/UHF mobile setup

Assume:

• TX power: 50 W

• Antenna gain: 3 dBd (note: dBd!)

• Cable loss: 1.5 dB

• Connector loss: 0.5 dB

• Other loss: 0.5 dB

Convert power to dBm:

• 50 W = 50,000 mW

• dBm = 10 log10(50,000) ≈ 46.99 dBm (≈ 47 dBm)

Convert gain to dBi:

• 3 dBd = 5.15 dBi

Total loss:

• 1.5 + 0.5 + 0.5 = 2.5 dB

EIRP:

• 47 + 5.15 − 2.5 = 49.65 dBm
  That’s ≈ 92.3 W EIRP

ERP:

• 49.65 − 2.15 = 47.5 dBm ≈ 56.2 W ERP

Why this matters: If you forget the dBd → dBi conversion, you understate EIRP by 2.15 dB, which is not small.

Example 4: Point-to-point directional link (high-gain antenna)

Assume:

• TX power: 1 W (30 dBm)

• Antenna gain: 24 dBi (dish/panel)

• Cable loss: 1.0 dB

• Connector loss: 0.5 dB

• Other loss: 1.0 dB (filter/duplexer)

Total loss = 2.5 dB
EIRP = 30 + 24 − 2.5 = 51.5 dBm ≈ 141 W EIRP

Reality check: Directional antennas can push EIRP very high even at modest transmitter power. This is why directional installations often require careful compliance checks and good engineering margins.

Example 5: Same radio, two cable options (why cable choice matters)

Assume:

• TX power: 500 mW = 27 dBm

• Antenna gain: 8 dBi

• Connectors: 0.6 dB

• Other losses: 0.4 dB

Case A: low-loss cable = 1.0 dB
Total loss = 1.0 + 0.6 + 0.4 = 2.0 dB
EIRP = 27 + 8 − 2 = 33 dBm (≈ 2 W)

Case B: higher-loss cable = 3.0 dB
Total loss = 3.0 + 0.6 + 0.4 = 4.0 dB
EIRP = 27 + 8 − 4 = 31 dBm (≈ 1.26 W)

A 2 dB cable difference cut EIRP from ~2 W to ~1.26 W. That can show up as less reliable links, lower throughput, or reduced range.

EIRP in link budgets (how this calculator fits into “range” thinking)

A simplified receive power estimate in dB terms:

Prx (dBm) ≈ EIRP (dBm) − Path Loss (dB) + Rx Antenna Gain (dBi) − Rx Losses (dB)

This is why EIRP is so valuable: it’s your transmit-side “starting point.” From there, you subtract propagation loss (free-space, obstacles, fading margin) and add the receiver-side antenna system.

If you expand this calculator later, the most popular next step is adding:

• FSPL (free-space path loss) from distance + frequency

• Rx gain/loss

• Receiver sensitivity estimate (optional) to compute link margin

Common mistakes and how to avoid them

Mistake 1: mixing dBi and dBd

Always convert before calculation. If the datasheet says dBd, convert to dBi by adding 2.15.

Mistake 2: forgetting the “hidden losses”

Patch leads, bulkhead connectors, lightning arrestors, splitters, duplexers—these add up. If you don’t know exact values, estimate conservatively and keep margin.

Mistake 3: adding watts and dB

Do all the chain math in dB space (dBm/dBW). Only convert to watts at the end.

Mistake 4: assuming EIRP is “all directions”

It’s typically the peak direction (main lobe). Side and rear radiation can be much lower.

Mistake 5: ignoring installation realities

Antenna placement, polarization mismatch, and obstructions can matter more than 1–2 dB of computed EIRP. EIRP is crucial, but it’s not the whole story.

Practical optimization tips (performance without guesswork)

If you want more reliable links

• Use lower-loss cable or shorten the run

• Avoid unnecessary adapters and cheap connectors

• Choose antennas with credible gain and published patterns

• Confirm polarization matches between link endpoints

If you need to reduce EIRP to meet a limit

• Reduce conducted TX power

• Use a lower-gain antenna

• Add a small attenuator (predictable and stable)

• Document the configuration for repeatability

Extended FAQ (GYIK)

What is EIRP in simple words?

EIRP is the effective radiated power of your system referenced to an isotropic antenna. It combines your transmitter power, antenna gain, and subtracts all losses.

What is ERP and why does it exist?

ERP is effective radiated power referenced to a half-wave dipole. It’s used in some regulations and broadcasting standards. It’s not “more correct” than EIRP—just a different reference.

How do I convert EIRP to ERP?

In dB units: ERP = EIRP − 2.15 dB.

Why is the difference 2.15 dB?

Because a half-wave dipole has 2.15 dB gain over an isotropic radiator.

My antenna gain is in dBd. What do I do?

Convert it: dBi = dBd + 2.15. Then use the dBi value for EIRP calculations.

Can EIRP be higher than transmitter power?

Yes. Antenna gain concentrates power in certain directions, so EIRP can be much higher than conducted transmitter power.

Does antenna gain create extra power?

No. It redistributes energy. You get more in the main direction and less elsewhere.

Do cable and connector losses really matter?

Absolutely. A few dB can change EIRP noticeably. Cable loss is often the biggest “silent killer,” especially at higher frequencies.

Is 3 dB a big deal?

Yes. 3 dB is about a factor of 2 in power. Losing 3 dB means roughly half the power delivered (or radiated), depending on where it occurs.

How do I convert dBm to watts quickly?

Use anchors: 30 dBm = 1 W, 33 dBm ≈ 2 W, 40 dBm = 10 W. For exact: W = 10^((dBm − 30)/10).

What’s the difference between dBm and dBW?

dBm references 1 mW; dBW references 1 W. They differ by 30 dB: dBm = dBW + 30.

What is “conducted power”?

That’s the power measured at the transmitter output (before the antenna), typically at a connector port into a 50-ohm load.

Should I include duplexers, filters, and splitters as losses?

Yes. Any device with insertion loss reduces power delivered to the antenna and reduces EIRP/ERP.

Does SWR/VSWR affect EIRP?

Mismatch can reduce delivered power and increase reflected power. Many practical calculators treat mismatch as part of “loss,” but accurate modeling requires return loss/mismatch loss. If you want, this can be a good calculator add-on.

Is EIRP the same as “radiated power”?

EIRP is an equivalent peak radiated power referenced to isotropic. Real radiated power depends on direction; EIRP is a standardized way to express the peak direction.

Can EIRP be negative?

Yes, in dBm. Very low-power transmitters or high-loss chains can produce negative dBm EIRP. Negative dBm just means less than 1 mW.

Why do my results look “too high” with a directional antenna?

High-gain antennas can create very large EIRP even with modest transmit power. This is normal, and it’s why compliance checks often use EIRP.

What’s a good engineering margin for compliance?

Not a legal rule, but in engineering practice it’s common to keep headroom (e.g., a few dB) because real components vary, measurement uncertainty exists, and installations change.

Does duty cycle affect EIRP/ERP?

EIRP/ERP as computed here reflects the instantaneous equivalent radiated power. Some regulations also consider time-averaged power or duty cycle. Adding an “average power mode” is a useful extension for burst systems.

What should I enter for “other loss”?

Anything inline not covered by cable/connectors: filters, duplexers, splitters, combiners, lightning arrestors, attenuators, RF switches—anything with insertion loss.

Is this calculator enough to predict range?

It’s the correct starting point on the transmit side. Range also depends on frequency, path loss, antenna height, obstructions, noise floor, receiver sensitivity, bandwidth, and fading margins. For better range estimates, add FSPL and a receiver-side section.

Image(s) used in this article are either AI-generated or sourced from royalty-free platforms like Pixabay or Pexels.